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Paleoethnobotany in the neotropics from microfossils new insights into ancient plant use and agricultural origins in the tropical forest

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Paleoethnobotany in the neotropics from microfossils new insights into ancient plant use and agricultural origins in the tropical forest Paleoethnobotany in the neotropics from microfossils new insights into ancient plant use and agricultural origins in the tropical forest Presentation Transcript

  • Journal of World Prehistory, Vol. 12, No. 4, 1998 Paleoethnobotany in the Neotropics from Microfossils: New Insights into Ancient Plant Use and Agricultural Origins in the Tropical Forest Analysis of plant microfossils (phytoliths, pollen, and starch grains) from archaeological andpaleoecological sediments in thehumid Neotropical forest canprovide information on some formerly intractableproblems in American paleoethnobotany and archaeology. Each technique has strengths thatredress the other's shortcomings, and all three microfossils can be recovered from early sites, securely identified, and dated. Agricultural origins, Pleistocene/ Holocene environmental changes, and the evolution of slash-and-burn agri- culture are three important issues that yield substantial results to phytolith, pollen, and starch grain study. Microfossils of a number of domesticates, including maize, manioc, squash, bottle gourd, arrowroot, and leren, have been identified in contexts dating from 9000 to 7000 radiocarbon years B.P. The scope and methodology of traditional paleoethnobotany should be ex- panded to routinely include microfossil study. INTRODUCTION Paleoethnobotany is normally defined as the study of the interactions between past human societies and the plant world through the analysisof archaeobotanical remains (Ford, 1985; Pearsall, 1989; Gremillion, 1997). 1Smithsonian Tropical Research Institute, Box 2072, Balboa, Panama. 2To whom correspondence should be addressed at STRI, Unit 0948, APO AA 34002-0948. KEY WORDS: Paleoethnobotany; Neotropics; phytoliths; pollen; starch grains. 393 0892-7537/98/1200-0393$15.00/0 © 1998 Plenum Publishing Corporation Dolores R. Piperno1,2
  • As this definitionsuggests, most studies characterized by their practitioners as paleoethnobotanical focus on the archaeological record. The meaning and scope of paleoethnobotany should be expanded to include plant fossils from nonarchaeological (paleoecological) contexts such as lake and bog sediments, because these fossils provide information on human/plant rela- tionships as surely as do seeds retrieved from ancient hearths around which people sat and prepared meals. Many researchers, recognizing that no single technique can possibly identify the range of seeds, fruits, and underground plant organs exploited by ancient people, stress the importance of a database consisting of multiproxy indicators ofplant use and manipulation(e.g.,Ford, 1988; Hather, 1994; Pearsall, 1989; Watson 1997). The reasons why multiple lines of evi- dence are needed to studycultural uses of plants are many,and they are well discussed in the literature. For example, corms, tubers, and other commonly eaten plant structures, includingmany seeds, are not sufficiently hard to en- dure the carbonization process; economically important plants often do not produce enough pollen to be recovered and identified; more humidclimates tend to quickly destroy organic remains of plants; phytoliths are well pre- served in many contexts, but they cannot identify some important economic plants; carbonized plant remains may not survive for very long in heavy, clayey sediments typical of the humidtropics, and so on (e.g., Pearsall, 1989). This article reviews how plant microfossil analysis (phytoliths, pollen, and starch grains) contributes to paleoethnobotanical research in the low- land American tropical forest. Since the author last undertook such a review for the phytolith record of the Neotropics (Piperno, 1991), developments in plant microfossil studies have accelerated. Previously unstudied regions are under scrutiny; new techniques have been introduced and older tech- niques refined. As a result, considerable light is being shed on important issues pertaining to early plant use and domestication, together with the ecological contexts in which plant subsistence strategies evolved through time. Such developments in plant microfossil studies are by no means confined to the lower latitudes of the New World, but also encompass equatorial Africa (Alexandre et al., 1997; Mercader et al., 1998; Mworia- Maitima, 1997; Runge and Runge 1997), southern China (Zhao, 1998; Zhao et al., 1998; Jiang and Piperno, 1998), mainland southeast Asia (Kealhofer and Piperno, 1994,1998; Maloney, 1994; Kealhofer and Graves, 1996; Keal- hofer and Penny, 1998), New Guinea (Haberle, 1994; Therin et al., 1997, 1998; Torrence and Fullager, 1997), Island Melanesia (Loy et al., 1992; Loy 1994; Athens et al., 1996), and Polynesia (Athens and Ward, 1993; Flenley, 1994). In the Neotropics, economic and paleoecological reconstruction through phytolith and pollen analysis are about to enter their third decade. Piperno394
  • This makes it possible to consider the current state of knowledge in these fields in light of earlier applications of the techniques, when much basic research was still being undertaken. Some important points concerning the present state of the art in plant microfossil analysis that are discussed in detail later can be summarized as follows: (1) phytoliths from a number of domesticated species—for example, maize (Zea mays L.), squash (Cucur- bita spp.), bottle gourd (Lagenaria siceraria),arrowroot (Maranta arundina- cea), and leren (Calathea allouia)—can be confidently identified; (2) pollen from a number of domesticated species—for example, maize, manioc (Man- ihot esculenta Crantz), squash, and chile pepper (Capsicum spp.)—can be confidently identified,particularlyin regions outside the range of their wild ancestors; (3) starch grains from maize, manioc, squash, arrowroot, yams (Dioscorea spp.), and achira (Canna edulis) can currently be distinguished and there is significant potential for more precise identifications in this emerging field of research; (4) forests and grassland vegetationalformations and vegetation disturbed by humans leave characteristic phytolith and pol- len signatures; (5) direct AMS dating of phytolith and pollen assemblages is a reliable way of establishingtheir antiquityand starch grains are potentially dateable by accelerator mass spectroscopy (AMS) as well; and (6) combined microbotanical evidence indicates that human manipulation of Neotropical plant species, includingmaize, squash, arrowroot, leren, and, likely,manioc, began in the early Holocene (10,000-7000 B.P.) in a region from lower Central America to northwestern South America. PHYTOLITH ANALYSIS Phytolith Production A great many tropical angiosperms and gymnosperms heavily silicify their vegetative and reproductive organs, resulting in the production of copious amounts and many kinds of phytoliths that are ultimatelydeposited as discrete particles into tropical sediments. The notion prevalent in much of the English-language botanical literature for much of the 20th century that only the Poaceae and a few other monocotyledonous families would leave an informative phytolith record can be put to rest. As ongoing Old World tropical research also demonstrates, phytolith production and mor- phological patterns are nonrandom, predictable, and now well understood (Pearsall et al, 1995; Runge, 1995; Runge and Runge, 1997; Kealhofer and Piperno, 1998), Temperate and Boreal zone representatives of the families and genera studied in the Neotropics follow patterns almost identical to their low latitude relatives (e.g., Bozarth 1987, 1992; Hodson et al, 1997; Tropical Paleoethnobotany Through Microfossil Analysis 395
  • Ollendorf, 1992; Sangster et al, 1997). Phytolith analysts know in which families and genera phytoliths are most likely to be found and in which they are least likely to be found, and they have essentially replicated the extensive research on phytolith production that was carried out by German botanists during the first third of the 20th century (see Piperno, 1988, for a review of this literature). Another strong and consistent pattern of phytolith production is that angiosperms that contribute high numbers of vegetative structure phytoliths may sometimes, but not always, heavily silicify their seeds and fruits. Con- versely, angiosperms that do not heavily silicify their vegetative structures seldom produce reproductive organ phytoliths.The demonstrated patterns of phytolithproduction allow researchers to effectively build large modern- type collections for a diverse flora because we know which plants and individual structures should reward our efforts. The patterns also facilitate the development of identification procedures for domesticated plants, be- cause we can be confident that diagnostic forms produced in cultivars will not also be made in unrelated species that have not been analyzed. Table I contains summaries of phytolith production in Neotropical plants. They are little changed from those given in Piperno (1991) by the results of research in the 1990s, even though more than 200 additional nongrass species, includingabout 170 trees and shrubs, from a larger number of families and a broader geographic area were incorporated into the au- thor's modern reference collection. This collection now includes more than 2000 species from 108 families. Another large (ca. 1000 species) collection of Neotropical species is the one housed in Pearsall's laboratory at the University of Missouri. In many plants, we analyzed replicate samples of the same species from different populations in order to judge what kind of infraspecific variability existed in phytolith production. The numerous types of phytoliths referred to here were produced with a high degree of fidelity, regardless of the location where the plant was sampled. Indeed, similarities in the production and localization of solid silica in the same species and among closely related ones are such that we can begin to talk with some confidence about plants of all kinds havingspecialized phytolith- making cells or, perhaps, enzymes that are turned on to deposit solid silica. Our pre-1991 efforts to build a modern reference collection concen- trated on accumulating a very good sample of phytoliths from the markedly seasonal deciduous and semi-evergreen forests of Central America and northern South America, where most archaeological sites investigated to that point were located. In the 1990s, we set out to improve the collection for the wetter and more speciose forests that still dominate the Darien province of Panama, where many trees important in the South American forest reach their northernmost limits (Piperno, 1994a). We also began to Piperno396
  • Tropical Paleoethnobotany Through Microfossil Analysis 397 Table I. Patterns of Phytolith Production and Taxonomic Significance among Tropical Plants Families in which production is high, phytoliths minimally specific to family are common, and subfamily and genus-specific forms occur, sometimes widely in the family Monocotyledons: Arecaceae,a Cyperaceae,a Heliconiaceae,ab Marantaceae,ab Musaceae,a Orchidaceae, Poaceae,a Zingiberaceaea Dicotyledons: Acanthaceae, Annonaceae, Burseraceae,a Chrysobalanaceae,a Compositae, Cucurbitaceae,a Dilleniaceae, Magnoliaceae, Moraceae, Podostemaceae, Ulmaceae,a Urticaceae Pteridophytes: Equisetaceae, Hymenophyllaceae, Selaginellaceae Families in which production is not high in many species, but where family and genus- specific forms are present Pinaceae, Euphorbiaceaea Examples of important families in which phytoliths have not been observed, or where pro- duction is rare and nondiagnostic Amaranthaceae, Araceae, Cactaceae, Chenopodiaceae, Dioscoreaceae, Liliaceae, Melas- tomataceae, Myrtaceae, Podocarpaceae, Rubiaceae, Sapindaceae, Solanaceae Examples of subfamily and genus-specific forms with their cultural and paleoecological sig- nificance I. Herbaceous Plants Zea luxurians, Tripsacum, Arundinella, Polypogon; Bambusoideae—Chusquea, Pharus, Streptochaeta, Neurolepis, Maclurolyra; Cyperus/ Kyllinga and other sedge genera; Hell- coma; Trichomanes, Mendoncia, Podostemum, Calathea, Stromanthe, and other Maranta- ceae genera Explanation: Zea luxurians (Race Guatemala), a teosinte endemic to the Guatemalanlow- lands, has species-specific fruitcase phytoliths. Tripsacum is a near relative of maize. Its phytoliths are more easily separable from those of teosinte than are its pollen grains be- cause morphologic differences in phytoliths from these taxa are considerable. Arundinella is a tall grass whose cross-bodies are confusable with maize, but it contributes other genus- specific phytoliths. Podostemum attaches to rocks in swiftly flowing rivers. Trichomanes is a fern that grows attached to tree trunks in humid, lowland forest. Mendoncia is a good in- dicator of a humid tropical forest. Calathea and other Marantaceae grow in the moist un- derstory of a tropical forest. Bamboos are commonly used for construction, bedding, and a myriad of other purposes in the American tropics. Some (Chusquea) are excellent indica- tors of mature forest. Trichomanes, Mendoncia, Marantaceae, and bamboos like Chusquea can be used to document older growth forest, and to follow the course of forest clearing by humans. Sedges and Heliconia are major indicator taxa for early successional growthfol- lowing human disturbance. II. Trees and Other Woody Plants Magnolia, Talauma, Protium, Bursera, Trema, Poulsenia, Celtis, Hedyosmum, Pinus spp., Unonopsis/Oxandra, Trema, Curatella Explanation: These taxa are common trees and shrubs of various kinds of mature tropical forest (montane, lowland, semi-evergreen, evergreen) and successional growth. Curatella is a major indicator taxon for serious land degradation and anthropogenic savanna in the tropics. a Reproductive structures (fruits and seeds) also produce high amounts of diagnostic phytoliths. b Underground organs (roots, tubers, and corms) may contribute high amounts of diagnos- tic phytoliths.
  • seriously consider the terra firme (dry, upland) and floodplain Amazonian forests (Piperno and Becker, 1996), and we incorporated more trees and grasses typical of the Central and South American cool and mountainous tropics—from elevations above 1200 m. Finally, we investigatedphytoliths in a large number of close relatives of some major plant domesticates in order to develop identificationcriteria for the crop plants where possible. We do not yet have an answer to the question of why some plants and not others make phytoliths,but the facts that (1) families and generaexhibit a strong tendency either to silicify or not to silicify their organs and (2) production is often localized in particular kinds of tissues (e.g., leaf and seed epidermis, fruit pericarp) indicate a functional significance for phyto- liths.Two nonmutually exclusivehypothesesfor why mechanismsto deposit solid silica were selectively favored by many plants head the list that should be tested. One is that phytoliths, like lignin and many chemical substances made by plants, are antiherbivorous and antipathogenicin design. Investiga- tors have long been aware that depositing large amounts of solid silica benefits certain crop grasses by increasing their resistance to fungi and stem-boring insects (e.g., Sangster and Hodson, 1997). Although there is a growing literature on tropical plant/herbivore and pathogen interactions (e.g., Coley and Barone, 1996), the role that phytoliths may play in plant defense has not been considered. The following points illustrate that this is an area worthy of study. There are thousands of species of "herbivores" in a tropical forest, most of them being small insects such as ants, caterpillars, and beatles that feast on the leaves of many species and may cause severe defoliation and decreased fitness. By incorporating high amounts of solid silica, plants can better defend themselves because phytoliths toughen plant structures and make them harder to eat and digest. Toughness, in fact, has been called the most effective defense that can be marshaled by plants (Coley and Barone, 1996). However, toughness is a less viable option when leaves are young and need to expand, so young leaves fortify themselves largely through chemical means, leading to the well-documented greater produc- tion of toxic compounds in young as compared to mature leaves (Coley and Barone, 1996). Very little is known about the time of solid silica formation in most tropical plants, but if it is anythinglike that documented for some temperate and boreal zone grasses and trees (Hodson etal., 1997; Sangster and Hodson, 1997; Sangster et ai, 1997), then it would increase with age, perhaps taking place mainly after the leaf has expanded and could begin to mechanically defend itself. This might suggest a two-pronged approach to plant defense, with phytoliths and other compounds such as lignin assuming a more impor- tant role after the leaf has matured. Once plants deposit solid silica, it can Piperno398
  • be carried unaltered until the leaf drops, negating or lessening the need for the expensive turnover of chemical compounds that might be particularly troublesome for long-lived (average 3-14 yr) leaves of tropical species. These points are largely speculative at this time and do not directly address the question of why some plants do not make phytoliths.However, evolutionary strategies are often nothing if not trade-offs relating to the costs and benefits of resource allocation. Hence, questions relating to the whys and why nots of phytolith production should benefit from studies of the relative costs and advantages to plants of making phytolithsat any stage of their development vs. manufacturing chemicalcompounds or investingin other kinds of mechanical defenses like lignin. Another good reason why plants might want to make phytoliths is that silicon dioxide may ameliorate the toxic effects of aluminum (Hodson and Evans, 1995). Aluminum oxides are common components of highly weathered tropical soils, and are ingested by plants through the transpira- tion stream along with other soluble substances in groundwater. Because silicon and aluminum have been shown to co-occur in plant tissues of some plants, the mechanism for detoxification may be a sequestration of aluminum by the silicon (Hodson et al., 1997). Phytolith Taxonomy and Identification Discussions of phytolith taxonomy in the botanical and archaeological literature of the 1970s and early 1980s were sometimes dominated by what came to be known as the "redundancy" and "multiplicity" issues (Rovner and Russ, 1992). It was well understood from the many studies of grass leaf phytoliths carried out by botanists to that point that any single genus and species of the Poaceae might contain several different types of phytol- iths, and that any one of these could be found in another grass (Piperno, 1988). Although a limited number of other plant families had been closely studied, there were concerns that phytoliths might never provide the taxo- nomic precision necessary for subsistence and environmental reconstruc- tions. With the initiation of the modern period of archaeological phytolith research in the late 1970s and 1980s and the increased attention it brought to a wider spectrum of plants and plant structures, it started to become clear that taxonomically significant forms were common in the plant kingdom and that genus-level identifications were possible, even in some grasses (Bozarth, 1987; Pearsall, 1978, 1989; Piperno, 1984, 1985, 1988, 1989). Research undertaken in the 1990s further demonstrates that phytolith redundancy and multiplicityare not serious issues in many tropical plants (Piperno, 1993; Piperno and Pearsall, 1993a,b, 1998a; Veintimilla, 1998). Tropical Paleoethnobotany Through Microfossil Analysis 399
  • Phytolith morphology exhibits a strong correspondence to a plant's taxo- nomic affiliation, indicating a strong genetic influence controlling plant silicification. Many families produce phytoliths with diagnostic shapes (Ta- ble I). Genus-level discriminationof some important trees and herbaceous plants is possible (Table I; Figs. 1-4). The fruits and seeds of many angio- sperms produce single or very few types of phytoliths often diagnostic to genus and also commonly distributed into archaeological and paleoecologi- cal sediments (Figs. 4-6) (Piperno and Pearsall, 1993a,b; Piperno and Becker, 1996). Species-level discrimination is possible with some plants that have been artificially altered in a fundamental way from their wild states—that is, crop domesticates (see later). A survey of silica bodies (dumbbells, cross-shapes, saddle shapes) in more than 300 Neotropical grass species revealed that some contribute individual, diagnostic phytoliths,including maize's wild ancestor, teosinte, Fig. 1. Center, a genus-specific phytolith from the leaves of Talauma (Magnoliaceae) from modern soils underneath tropical montane forest in Panama. 400 Piperno
  • Tropical Paleoethnobotany Through Microfossil Analysis Fig. 2. Center, a genus-specific phytolith from the leaves of Magnolia (Magnoliaceae) from Late Pleistocene-age lake sediment from Panama. Palynologists are unable to distinguish the pollen of the closely related taxa Talauma and Magnolia. and its near relative, Tripsacum (later this chapter) (Piperno and Pearsall, 1998a) (Figs. 7, 8). The fact that these two grasses contribute distinctive phytoliths is not really surprising because they are the only New World grasses that enclose their grains with a hard, cupulate fruitcase in which the diagnostic forms originate. Also, bamboos, which are often informative indicators of past climate and vegetation and obviously were well used in prehistoric economic systems, often contribute genus and tribal-specific shapes (Table I). Most other wild grasses produce less specific types, but it remains that phytoliths offer a much better interpretation of past grass communities and cultural/grass interactions than pollen because they, un- like pollen, can be identified below the family level. As is the case for phytolithproduction, phytolithmorphology in family, genera, and species is congruent across the world (Bozarth, 1992;Ollendorf, 1992; Runge, 1995; Runge and Runge, 1997; Chen and Jiang, 1997; Houyuan 401
  • Fig. 3. A probable genus-specific phytolith from the leaves of Perebea xanthochyma (Mora- ceae). Palynologists often cannot discriminate genera in this family. et al., 1997; Kealhofer and Piperno, 1998; Zhao et al, 1998). In other words, like pollen grains, phytoliths from sedges, composites, pines, grasses, Magnolias, elms, and other cosmopolitan plants have the same basic form regardless of whether they are from an arctic tundra or a tropical rain forest. Such patterns further indicate a strong genetic influence on phytolith morphology and indicate that phytolith shapes are not likely to be subject to the whims of local environmental variability. We presently have little understanding of why tropical plants make so many kinds of phytoliths. Some phytolith shapes (e.g., those from dicotyle- don hair cell and hair bases and seed epidermes) are simply mineralized replicas of the former livingcell, this occurring when the cell is completely silicified or when the cell wall becomes encrusted with silica. Many others, however (e.g., from the Marantaceae, Palmae, and Poaceae) cannot be so easily understood because they involve a silicification of the interior of the cell, resulting in diverse forms unlike that of the cell. 402 Piperno
  • Fig. 4. A genus-specificphytolith from the fruit of Trema micrantha (Moraceae). It is possible that as with pollen (Dajoz et al., 1998), phytolith morpho- logical variation is associated with increased fitness, but there are no data at present to indicate what the selective pressures on plants may have been to induce morphological change. Major global climatic changes and extinction events relating to vertebrate and other herbivore evolution are among the candidates that perhaps should be considered. It is, at least, clear that phytolith morphology has changed through time in certain plant families. For example, the most primitive living grasses, such as bamboos and species in the subfamily Arundinoideae, have taller, thicker, and more angled silica bodies than grasses derived later in time, such as mainstream species in the Chloridoideae and Panicoideae (Piperno and Pearsall, 1998a). Phytoliths in Plant Domesticates and Other Important Economic Plants The sheer abundance of different species in the tropical flora has given pause to archaeologists who would like to consider the availablephytolith Tropical Paleoethnobotany Through Microfossil Analysis 403
  • Fig. 5. Genus-specific phytolith from the seeds of Mendoncia spp. recovered from modern soils underneath tropical montane forest in Panama. record with regard to its relevance to early plant domestication. This is a legitimate concern, particularly when applications of a technique are new and basic reference collections are being constructed for a complex flora. However, with the accumulation of the substantial amount of information on phytolith production and taxonomic patterns just described, phytolith investigators have been able to move onto more tightly focused and re- stricted sets of taxonomic problems, involving studies of those important crop plant species that produce high amounts of phytoliths in promising shapes. In the Neotropics, sixcrop plants stand out in such a manner: maize, three members of the Cucurbitaceae family [Cucurbita spp. (squashes and gourds), Lagenaria siceraria (bottle gourd), and Sicana odorifera (cassaba- nana)] and the Marantaceae tubers Maranta arundinacea (arrowroot) and Calathea allouia (leren) (Table II). Identification criteria in most of them relies heavily on phytoliths found in the reproductive structures of the 404 Piperno
  • Fig. 6. A family-specific phytolith from the fruit of Altis spinosa (Ulmaceae). plants, an advantageous factor because, as discussed, such phytoliths can be particularly diagnostic. I begin with maize. Considerable effort during the 1990s has been invested in developing reliable identification criteria to differentiate maize from wild Neotropical species, including its putative wild ancestor, teosinte. Research on maize phytoliths in archaeological contexts began with a study by Pearsall (1978) that focused on the size of a type of grass phytolith called "cross-shaped" (hereafter also called "cross-bodies"), produced predominantly in leaves. Subsequent research by the author (1984, 1988) explored morphological differences in cross-bodies and found them also to be useful indicators of maize presence. Cross-bodies were divided into eight different morphologi- cal "variants" based on their three-dimensional structures. The combination of size and three-dimensional shape resulted in a significant statistical sepa- ration of cross-bodies from maize and wild grasses using a multivariate (discriminant function) analysis (Piperno, 1988). Tropical Paleoethnobotany Through Microfossil Analysis 405
  • Fig. 7. A Tripsacum-specific fruitcase phytolith from T. lanceolatum. It has markedly serrated edges and possesses ridges on the top. In order to test the viability of the technique in maize's homeland, Mexico, expand the applications to regions such as the Amazon Basin, and bring the research to a point of closure, the author and Pearsall completed a study in which the leaves, other vegetative structures, and inflorescences of numerous previously untested tropical grasses were analyzed using the same size and shape criteria. This brings the total number of wild species investigated to more than 350 (see Piperno and Pearsall, 1998a, for a list). These were compared with 25 modern Latin American maize races and all fiveextant races of teosinte, includingmaize's putative wild ancestor Balsas teosinte. The non-Zea wild grasses were chosen on the basis that they were the most likely to possess cross-bodies because of taxonomic affiliation and were the most common reported in forest, mangrove, anthropogenic, and other Neotropical habitats. Replicates of every species of maize's close relative Tripsacum and of every genus and most species of bamboos re- ported from the Neotropics were analyzed (Table II). Piperno406
  • Fig. 8. A fruitcase phytolith from Balsas teosinte. It is produced in the same tissue as phytolith illustrated from Tripsacum but lacks serrated edges and ridges along the top. The overwhelming majority of the wild grasses have cross-shaped phy- toliths significantly smaller and/or structured differently from those of maize. A new discriminant function analysis incorporating all wild grasses along with maize separated phytoliths into two distinct groups, maize and wild, with a high degree of statistical confidence (D. R. Piperno and D. M. Pearsall, unpublished data). We identified a few non-Zee wild grasses [Arundinella deppeana (leaves), Maclurolyra tecta (leaves), and Tripsacum spp. (fruitcases); none used today in indigenous tropical economies] that contributed high percent- ages of large-sized Variant 1-type cross-bodies, as does maize. (The leaves of Tripsacum spp. do not possess maize confusers.) However, these grasses contribute several distinctive types of cross-body and other phytoliths not found in maize that are easily recognized in soils (Fig. 7) (see also Piperno and Pearsall, 1993a, 1998a, for illustrations). This means that a completely secure maize identification using cross-bodies rests on a verification using Tropical Paleoethnobotany Through Microfossil Analysis 407
  • these distinctive phytoliths that these grasses are not present. There are other ways to identify maize in archaeological samples that can be applied independently or in place of the discriminant function analysis.For example, if large-sized (greater than 16 micrometer) cross-bodies constitute more than 10% of all cross-bodies counted, and if they are predominantly (more than 50%) Variant 1, then maize is almost certainly present because wild grasses with the exception of the three noted lack these characteristics. Importantly, this grass study informed us that in maize's homeland, southwestern Mexico, identification of maize based on cross-body study requires an assemblage approach because nearlyall races ofteosinte,includ- ing Race Balsas, likely maize's direct ancestor (Doebley, 1990), contribute cross-shaped phytoliths that are maize mimics (Piperno and Pearsall, 1993a). The confuser phytoliths, like those of Tripsacum, originate largely from the fruitcases and ear sheaths of the plant, although Nobogame teo- sinte, presently endemic to higher and drier elevations in Mexico, also produces maizelike cross-bodies in its leaves. This situation is made less problematic bythe fact that other unique phytoliths are produced in teosinte fruitcases that should signal teosinte presence archaeologically (Piperno and Pearsall, 1993a). In contrast to fruitcase cross-bodies, those from the 408 Piperno Table II. Crops Plants with Diagnostic Phytoliths Zea mays (maize), Cucurbita moschata, C. ficifolia (squashes), Lagenaria siceraria (bottle gourd), Sicana odorifera (cassabanana), Calathea allouia (leren), Maranta arundinacea (arrowroot) Number of species in the crop plants' families studied 1. Maize was compared with about 350 species of Neotropical grasses, includingmultiple replicates of leaves, fruitcases, and tassels from every race of teosinte and replicates of these structures from every known species of Tripsacum; 25 different maize races were evaluated. 2. Squash, bottle gourd, and cassabanana were compared with replicates of 41 species from 22 different genera in the Cucurbitaceae. Domesticated squashes were compared with 7 wild and 1 semidomesticated species of Cucurbita, including 11 different popula- tions of C. ecuadorensis from Ecuador, and multiple replicates of five different populations of C. argyrosperma ssp. sororia from Panama. 3. Leren and arrowroot were compared with 22 species comprising 8 different genera in the Marantaceae. Comments 1. Maize's wild ancestor, teosinte, produces diagnostic fruitcase phytoliths. Cross-bodies from the leaves of Balsas teosinte, maize's likely direct ancestor, are much smaller than and structured differently from those of many races of maize. 2. There is infraspecific variation among domesticated species of Cucurbita that needs fur- ther study. Spherical, scalloped phytoliths appear to be confined to the tribe Cucurbiteae, which also includes the Old World melon Benincasa.
  • leaves of Balsas teosinte can be readily separated from most maize races (Piperno, 1988; Piperno and Pearsall, 1993a). Marked differences are also apparent between phytolith assemblages from teosinte fruitcases and maize cobs, which produce siliceous bodies primarily in the cupules and glumes (Mulholland, 1993; Piperno and Pear- sall, 1993a). For example, teosinte possesses high numbers of decorated epidermal and decorated circular phytoliths, whereas in maize,mostcircular bodies are undecorated, even in the most primitive races of maize known today from Mexico and Peru (Piperno and Pearsall, 1993a, unpublished data) (Figs. 9, 10). In summary, results to date on phytolith production in maize, teosinte, and other grasses allow a number of observations to be made about the phytolith record of southwestern Mexico. If teosinte seeds were used as a common food item, they should be detectable using the phytolithsthat derive from their fruitcases. Conversely, if the strategy of wild maize utiliza- tion was the consumption of immature ears as vegetables and the seeds with their hard fruitcases were never commonly collected and processed, Fig. 9. A decorated circular phytolith from the fruitcase of Balsas teosinte. Tropical Paleoethnobotany Through Microfossil Analysis 409
  • Fig. 10. Undecorated circular phytoliths from a cob of maize, Race Maiz Ancho from Mexico. as several students of maize origins have proposed (e.g., Harlan, 1992), fruitcase phytoliths should be missing from sites dating to the early Archaic period. Neither teosinte nor maize should be confused with their near relative Tripsacum using fruitcase and cob phytoliths, and phytoliths from Tripsacum leaves are also differentiablefrom maize. Phytolith assemblages from teosinte fruitcases and maize cobs are differentiable. Finally, leaf cross-bodies may play an important role in detection of early cultivated maize in maize's hearth if teosinte and Tripsacum fruitcase phytoliths can be ruled out from representation. We should not be deterred by the fact that we sometimes have to use an assemblage approach to identify maize. Bone chemistry studies employ a similar methodology. For example, when high delta I3C values are found in archaeological bone samples suggesting a diet heavy in C4 plants like maize, nitrogen isotope ratios must also at times be examined to rule out the possibility of heavymarine food contribution to the diet, because marine fauna have carbon isotope signatures that may mimic maize (Norr, 1995). 410 Piperno
  • Treating the diversity of phytolith shapes in grasses as independent lines of evidence can only improve the accuracy of archaeological phytolithiden- tification. Cucurbita and Other Cucurbitaceae Although maize has taken center stage in discussions of phytoliths in archaeological sediments, other important, domesticated plants can be identified with phytoliths. Cucurbita and other important Cucurbitaceae contribute large numbers of distinctive phytoliths in the rinds of their fruits (Piperno et al., 1998) (Table II) (Figs. 11-16). Rinds produce single to very few types of phytoliths. Cucurbita phytoliths are large (ranging from about 56 to 120 micrometers in mean length), more or less spherical, and have deeply "scalloped" (Bozarth, 1987) surfaces (Figs. 11 and 12). They were Fig. 11. A spherical, scalloped phytolith from the rind of Cucurbita moschata. The scallops are round, deep, and regularly distributed. Tropical Paleoethnobotany Through Microfossil Analysis 411
  • Fig. 12.A phytolith from Cucurbita lundelliana like the one in Fig. 11,turned on itsside to reveal the different pattern of scallop shape and size on the hemispheres of Cucurbita phytoliths. compared with multiple replicates of phytoliths from 41 other species in the Cucurbitaceae (Piperno et al, 1998) revealing that they are distinguish- able on the basis of both morphology and size from other genera in the family (Figs. 13-16). Phytolithsfrom wild Cucurbita are significantly smaller than those from domesticated species (Piperno and Pearsall, 1998b, p. 191; Piperno et al., 1998). Bottle gourd and a cucurbit fruit called cassabanana (Sicana odorifera), which was domesticated in the lowland Neotropics, also contribute distinctive, large rind phytoliths (Figs. 13 and 14), although in bottle gourd phytolithproduction can be spotty. In all of the Cucurbitaceae, phytolith size appears to be highly correlated with fruit and seed size, indicating that, as with archaeological seed analysis (Smith, 1997), increase in size through time probably indicates manipulation and genetic change under human cultivation (Piperno et al., 1998) (Fig. 17). Another interesting feature of phytolith production in Cucurbita is that domesticated populations often contribute far fewer phytolithsthan do wild ones. This may be a case where silicifying the outermost part of 412 Piperno
  • Fig. 13. A phytolith from Lagenaria siceraria (bottle gourd). It is distinctive because it has large, often elongated, and irregularly distributed scallops on one side of the phytolith, and is hemispherical. the fruit is more beneficial when the plant is wild and commonly preyed on by herbivores than when the plant is brought under human control. Also, humans probably consciously selected for softer rinds, which would mean an emphasis on those mutant wild gourds that produced fewer phy- toliths. Tuber Crops Very few Neotropical plants that were domesticated for their subterra- nean organs produce a valuablephytolith record. Starch analysis will proba- bly become the primary source of information for this area. Exceptions are two now-minor crops of the lowland tropical forest, leren (Calathea allouia) and arrowroot (Maranta arundinacea). In these plants, distinctive phytoliths 413Tropical Paleoethnobotany Through Microfossil Analysis
  • Fig. 14. A phytolith from Sicana odorifera. It has one hemisphere that is markedly conical and faintly decorated (compare with Cucurbita and Lagenaria). are formed in the seed epidermes, which, like fruit rinds, produce single to very few types of phytoliths (Fig. 18). Comparison of phytoliths from these cultigens with wild progenitors is not possible because wild ancestors are unknown for both of them. However, analysis of large numbers of species in their family indicates that a species-specific identification in ar- chaeological contexts is likely a secure one (Table II). Other tree and root crops such as pineapples (Ananus comosus) and achira (Canna edulis) produce phytolithsthat also occur in other members of their family, and here identification must rest on the inferred absence of these possible confuser plants in the prehistoric regional flora. A few crop plants have phytoliths in need of further study before their diagnostic potential is known. One is the common bean, Phaseolus vulgaris, which produces a type of pod phytolith (Bozarth, 1990) that may be larger than those in other Neotropical taxa. It should be noted that this phytolith appears to be less resistant to destruction in tropical sediment than others 414 Piperno
  • Fig. 15. Scalloped phytoliths from a wild species in the Cucurbitaceae, Peponopsis adhaerens. They differ from domesticated Cucurbitaceae in having small and indistinct scallops, little difference in the decoration of the hemispheres, and in overall size. Also, they often are not spherical, but are flattish (see also Fig. 16). discussed here, possibly because it is formed by cell-wall silicification. It has not been observed in any sediment sample studied to date by the author. In summary, paleoethnobotanistsin possession of large comparative collections should be able to identify phytolithsfrom maize, squash, bottle gourd, cassabanana, leren, and arrowroot with a high degree of confidence. Identification of most other crop plants will likely have to rely on other methods. Phytoliths in Sediments The 1990s saw the growthand refinement of phytolith analysis through the retrievaland identification of phytoliths from "modern" archaeological, Tropical Paleoethnobotuny Through Microfossil Analysis 415
  • Fig. 16. Scalloped phytolith from a wild species in the Cucurbitaceae, Cionosicys macrantha (see caption for Fig. 15). and paleoecological sediments from previously unstudied regions (the Bra- zilian Amazon, the Colombian Cauca Valley and Amazon, and Belize) and contexts (natural soils underneath modern forest; deep sea sediments). Phytoliths sampled from the uppermost soils underneath various extant vegetational formations along temperature and precipitation gradients indi- cate that mature forests of several types (e.g., lowland evergreen and semi- evergreen, montane), along with grasslands and vegetation disturbed by humans at varying scales of intensity, leave readable phytolith signatures (Piperno, 1993; Piperno and Pearsall, 1998b, pp. 177-178; Piperno and Becker, 1996). Archaeological phytolith results are summarized later (see also Piperno and Pearsall, 1998b). Because phytoliths and pollen from paleoecological sequences were often analyzed in tandem and the two microfossil indicators often closely agreed with respect to environmental history, phytolith results can be grouped with those from pollen, described next. Archaeological and paleoecological sediments also contain many un- Piperno416
  • Tropical Paleoethnobotany Through Microfossil Analysis 417 Fig. 17. Graph of the relationship between phytolith length and fruit length in modern wild and domesticated Cucurbita. A highly significant linear correlation exists between the two variables (R2 = 0.894; p < .001). A similarly strong significant correlation exists between phytolith thickness and fruit/seed size. Closed circle: C. argyrosperma ssp. sororia (wild species); triangle: C. pepo ssp. texana (wild species); closed square: C. ecuadorensis (a semido- mesticated species); hexagon: C. ficifolia (domesticated species). (From Piperno and Pear- sail, 1998b). known (and highly distinctive) phytoliths that should be named as the modern reference collection is expanded (Figs. 19 and 20). POLLEN ANALYSIS Introduction Although palynology has a deeper history than phytolith analysis, pollen analysis in the humid tropics is still a young science. Not so long ago, there were real concerns among some practiced palynologists of the temperate zone as to whether the lowland tropical forest would leave a useful pollen record. The worries (e.g., Faegri, 1966) centered around whether a flora that was largely pollinated by insects could produce and disperse enough pollen to be represented in fossil contexts, if pollen could survive under circumstances like high rates of decomposer activity that
  • Fig. 18. Phytoliths from the seeds of Calathea allouia (leren). They are diagnostic because they have flat and undecorated upper bodies, in contrast to all other studied species in the Marantaceae. typified tropical habitats, and whether regional floras that contained thou- sands of species of higher plants were amenable to the development of pollen keys. Nevertheless, spurred by the success of a pollen application in the Maya heartland (Deevey et al, 1979), investigators set out to study the sequence of past vegetational and climatic changes throughout the lowland Neotropics. Old, permanent bodies of water turned out to be much more common in areas originally or still covered by lowland tropical forest than had been believed, pollen preservation in these water bodies was excellent, and, most surprisingly, pollen content (influx) was sometimes comparable with that from lakes in the temperate zone (e.g., Leyden, 1984,1985; Bush and Colinvaux, 1988, 1990; Frost, 1988; Piperno et al., 1990; Bush et al, 1992). Although many taxa could not be identified in these early studies, the furious construction of reference collections that often accompanied this research allowed a broad reconstruction of environmental history, Piperno418
  • Fig. 19. Unknown, diagnostic phytolith recovered from modern soils underneath tropical evergreen forest in the central Amazon basin. including the crucial Pleistocene to Holocene transition, in regions ranging from Guatemala to the interior of the Amazon Basin for the first time. Interestingly, pollen and phytoliths from lakes and swamps located in archaeological study regions even far removed from ancient centers of civilization were revealing systematic interference with, and sometimes destruction of, the natural vegetation thousands of years ago that appeared to reflect early forms of plant manipulation by humansand slash and burn agriculture (Monsalve, 1985; Rue, 1987; Bush et al, 1989; Piperno et al, 419Tropical Paleoethnobotany Through Microfossil Analysis
  • Fig. 20.Unknown,diagnosticphytolith probably from the Burseraceae, from the same context as Fig. 19. 1991). The popular notion of the noble tropical savage who lived in harmony with the forest and minimally disrupted its natural processes was imperiled. Often, too, the evidence for a human presence and interference with the vegetation predated existing archaeological records, not a radical finding when one considered that aceramic people who fashioned their bowls and tools out of wood would not be so easily traced with an archaeological record. On the whole, the paleoecological evidence indicated that societies already fully organized as shifting agriculturalists were widespread in the Piperno420
  • lowland tropical forest by 7000 B.P.-5000 B.P. It also pointed to lower- level human interference with the vegetation during the early Holocene. Pollen Research Since the Late 1980s Environmental Reconstruction As with phytolith analysis,tropical palynology's most significant devel- opments since the late 1980s with regard to putting to rest earlier concerns about its viability, substantiating the results generated during the 1980s and early 1990s, and refining archaeological subsistence and environmental reconstructions, involved three areas of research: (1) building large, modern reference collections for the natural and domesticated flora; (2)systemati- cally sampling the modern pollen deposited into different extant vegeta- tional formations; and (3) further investigatingpollen occurrence and age in a variety of ancient sedimentary contexts, including archaeological sites, lakes, and deep sea sediments. Very large (at least 2000 and up to 10,000 species) modern collections of pollen are now available for a considerable number of study regions, including the Brazilian Amazon (Colinvaux et al., 1998), Cauca and Magda- lena valleys of Colombia and the western Amazon Basin (Herrera and Urrego, 1996), and the Pacific and Caribbean watersheds of Central America (Roubik and Moreno, 1992; J. Jones, B. Leyden, and M.B. Bush, personal communication, 1998). The number of identifiable taxa in pollen diagrams typicallyexceeds 200now, genus-levelidentifications are common, and the proportion of unknownsin some regions has been effectively halved (E. Moreno and M.B. Bush, personal communication, 1998). Accompanying these studies have been detailed evaluations of the relationship of the modern pollen rain to the regional vegetation in terms of pollen representation and dispersal, taxon abundance, and the identifica- tion of indicator forest taxa for various temperature and rainfall gradients (Bush, 1991,1992,1998; Bush and Rivera, 1998a; Rodgers and Horn, 1996). Such analyses refine the interpretation of fossil pollen in relation to both natural and human-induced changes in the vegetation. Bush and Rivera's studies concentrated on mature, well-described forests in permanent plots from Panama, Ecuador, Brazil, and Costa Rica under little or no human pressure. Their results substantiated the interpretations of the magnitude of climatic and vegetational change that accompanied the Pleistocene to Holocene transition in lower Central America (Bush and Colinvaux, 1990; Piperno et al., 1990; Bush et al., 1992). If anything, these studies had some- what underestimated the degree of cooling, and, therefore, probably of Tropical Paleoethnobotany Through Microfossil Analysis 421
  • drying that marked the late glacial period. The massive environmental changes that accompanied the end of the last ice age in the lowland tropical forest have been recorded in many other pollen sequences published since the late 1980s (e.g., Leyden et al, 1993; Van der Hammen and Absy, 1994; Behling, 1996; Colinvaux et al., 1996a,b; Salgaldo-Labouriau et al, 1997; Athens and Ward, 1998; see also Piperno and Pearsall, 1998b, pp. 90-107). It is increasingly clear that the low latitudes of the New World experienced environmental oscillations at the close of the Pleistocene no less profound that those that occurred elsewhere on the globe and that have long been associated with major economic transitions. Why the lowland tropical forest produces and disperses much more pollen than was once believed possible, making a viable paleoecological record possible, has also been elucidated by studies of the modern pollen rain. It seems that the reproductive characteristics of tropical plants (monoe- cious vs. dioecious), in addition to the particular pollination system they employ, play major roles in determining how much pollen is made and then liberated in a tropical forest (Bush, 1995; Bush and Rivera, 1998b). There are many insect-pollinated plants that are dioecious, which means they outcross, and because it is unlikely they will find conspecifics next to them in a species-rich forest, they require relatively long distance pollen transport and, likely, relatively greater amounts of pollen in their flowers. These taxa, in addition to the fewer wind-dispersed plants, become the potent suppliers of pollen that subsequently becomes amply represented in fossil diagrams (Bush, 1995; Bush and Rivera, 1998b). Although the many lowland forest taxa that are monoecious and pollinated by insects will still be largely blind or dramatically underrepresented in pollen diagrams, knowledge of which plants are likely to enter a pollen record makes the building of reference collections for a complex flora a more feasible and efficient endeavor. The modern pollen studies are also sustaining interpretations relating to early human interference with the forest and its clearance by slash and burn agricultural techniques that were made on the basis of earlier paleoecological research cited previously. Very low levels of pollen from herbaceous and secondary woody taxa that were used as indicators of ancient human disturbance occur in the modern forests, including those from the driest areas, indicating that the effects of human forest clearance and other disturbance can often be disentangled from the effects resulting from natural perturbations. Studies carried out in the 1990s continue to provide evidence for human manipulation of the lowland tropical forest during the early Holocene, together with the emergence of slash and burn agriculture between 7000 B.P. and 4200 B.P., depending on the region (e.g., Mora et al, 1991; Jones, 1994; Behling, 1996; Pohl et al., 1996; Goman and Piperno422
  • Byrne, 1998; Piperno, 1994a; Piperno and Pearsall, 1998b; Pope and Pohl, 1998; Athens and Ward, 1998). Identification of Crop and Other Economic Plants from Pollen Palynologists managing large reference collections that include pollen from many economic taxa are agreed that pollen from domesticates like manioc and Cucurbita are identifiable on the basis of both morphology and size (Herrera and Urrego, 1996; Colinvaux et al, 1998; E. Moreno and J. Jones, personal communication, 1998). Maize pollen is much larger than most other grasses, including bamboos of the tropical forest (Salgado- Labouriau and Rinaldi, 1990; E. Moreno, P. De Oliveira, and M.B. Bush, personal communication, 1998). Identification of maize in its Mesoamerican homeland must proceed with caution because teosinte pollen is sometimes as large. Even so, grass pollen measuring 95 microns or more is almost certainly maize, no matter from where. The investigators cited previously believe they can also identify the pollen of Capsicum (chile pepper), Gos- sypium (cotton), Spondias (hog plum), and Theobroma bicolor (cacao). Caution is still called for in regions where these plants were originally brought under cultivation. The identifications of domesticated manioc, maize, and Cucurbita pollen in preceramic-age (ca. 7000 B.P.-4800 B.P.) paleoecological and archaeological contexts from Belize, Panama, the Co- lombian Amazon and middle Cauca Valley, northern Peru, and the Ecua- dorian Amazon seem secure (Bush et al, 1989; Mora et al., 1991; Pohl et al., 1996; Monsalve, 1985; Piperno and Pearsall, 1998b, p. 207). NEW TECHNIQUES: STARCH GRAIN ANALYSIS Background A cardinal feature of American agriculture is the large number of plants that were taken under cultivation and domesticated for their starch- rich underground organs. These plants have been largely blind in paleoeth- nobotanical records from the humid tropics, seriously hindering attempts to understand the history of tropical forest food production. The analysis of starch grains may help us out of this problem. Starch grain analysis has been well used by Donald Ugent and associates over the years to document or confirm the identity of various tubers preserved at prehistoric sites on the arid Peruvian coast (Ugent et al., 1984, 1986). However, starch grain studies have recently found their first applications in the low and humid Tropical Paleoethnobotany Through Microfossil Analysis 423
  • regions of the New World tropics. Here, as with phytolith studies, the analysis is predicated on the notion that when the macrostructures of tubers decay, some of the starch grains they contain survivein a largely unaltered state and are retrievable for study. Because starch analysis is probably a less-familiar method to many archaeologists, a brief review of some basic aspects of the technique is necessary here. Starch Production, Morphology, and Other Properties Starch grains,which are found in large quantities in most higherplants, are the major form in which plants store their carbohydrates, or energy (Sivak and Preiss, 1998). They can be found in all organs of a higher plant, including roots, rhizomes, tubers, leaves, fruits, and flowers. However, only subterranean organs and seeds commonly make what iscalled reservestarch, which is differentiated from another starch called chloroplast or transitory starch. The latter type is principallyformed in leaves and other vegetative structures and also can be found in pollen (Reichert, 1913; Shannon and Garwood, 1984). An important difference between chloroplast and reserve starch is that transitorystarch granulesdo not have the taxon-specific shapes associated with reserve starch granules. Also, transitory starch, as its name suggests, is formed during the day and utilized at night, whereas reserve starch is stored to be utilized later in the cycle of the plant. Therefore, it is the reserve starch, formed in tiny organdies called amyloplasts, that is most useful for archaeological study. There is a large literature on starch grain properties and morphology that researchers interested in archaeological applications can refer to (e.g., Reichert, 1913; Whistler et al, 1984; Sivak and Preiss, 1998). Any number of atlases and keys of starch grains exist, among the most extensive of which are Reichert's (1913) and Seidemann's (1966), which contain descriptions and photographs of starch from hundreds of economically important tropical and other plants. A dedicated starch journal, Die Starke, also exists. Researchers agree that starch grains are often highly diagnostic of individual taxa. For example, Snyder (1984, p. 662) states: "Most of the common starches are readily and unequivocallyidentifiableunder a polariz- ing microscope, using the criteria of granule size and shape, form and position (centric or eccentric) of the hilum (the botanical center of the granule), and brilliance of the interference cross under polarized light." The surface decoration of the granule (e.g., lamella visible or not visible) also provides identification criteria (Piperno and Hoist, 1998). Although there is a large corpus of literature on starch grain morphology, most of it Piperno424
  • has been compiled by researchers interested in the purely botanical aspects and commercial uses of starches. They understandably paid littleattention to how grain size and morphology in domesticated crops might differ from those in closely related wild species. This will become an area of intense interest for paleoethnobotanists. Based on a reference collection we have accumulated of more than 100 species of economically important Neotropical species (Piperno and Hoist, 1998, unpublished data) and its comparison with the literature refer- enced previously, the following points can be made about starch identifica- tion in the Neotropical archaeological record: (1) maize, manioc, arrowroot, sweet potato (Ipomoea batatas), achira, yams, yautia (Xanthosoma sagitti- folium), squash, legumes (Fabaceae), and palms can be easily differentiated from each other and from other plantsrepresented in our modern collection; (2) there is significant potential for a precise identification of squashes, maize, achira, and arrowroot, for individual genera of legumes, including Phaseolus and Arachis (peanuts), and for some individual species of yams; (3) the specificity of manioc grains needs to be determined, but they are differentiable from other plants currently represented in our collection as well as those documented by other researchers—in other words, manioc starch is not of a common design; (4) there appear to be differences in maize starch from individual races that may correspond in some way to endosperm type; and (5) tree fruits such as avocado (Persea americana), many palms, mamey (Mammea americana), and soursop (Annona spp.) are unlikely to provide informativedata because they largely contain oils, not starch (palm trunks are obvious exceptions—the starch in these is in need of study). The properties of starch grains and their sensitivity to degradation under various conditions are fairly well understood and can be summarized as follows. Starch grain molecules are primarily composed of amylose or amylopectin; many grains are mixtures of the two. They are highly sensitive to heat, strong acids and bases, and oxidizing compounds. Many grains start to geletinize, whereby they melt and lose their diagnostic properties, at temperatures of between 40° and 50°C. This means that they probably will not be identifiable residues of archaeological ceramics (unless the pots had a storage function), although this question needs study. A finding of even a few manioc-type grains on the ceramic "griddles" retrieved from archaeological sites throughout eastern South America, whichmanyinvesti- gators think were used to bake bitter manioc cakes (e.g., Roosevelt, 1980), would be enough to strongly suggest manioc presence on these artifacts, and at the same time unequivocally rule out other tuber crops, palms, maize, legumes, and various other plants that might have been consumed at the site. Tropical Paleoethnobotany Through Microfossil Analysis 425
  • Starch grains can gelatinize at lower temperatures under alkaline condi- tions. Hence, their resistence to destruction in shell middens and other archaeological contexts of high pH is in need of study. They often lose their characteristic extinction crosses under polarized lightwhen desiccated, but this is a process reversible by rehydrating them. The degree to which starch from different species may differentially survive in the varying con- texts associated with human settlement (alkaline shell middens, leached and acidic sediments, locations near the heat of hearths) isunknown. Starch on Stone Tools, in Archaeological Sediments, and Inside Human Teeth The number of starch grain studies carried out to date in the humid Neotropics islimited,but results are promising.Piperno and Hoist (reported in Piperno and Pearsall, 1998b, pg. 200) isolated grains from the surface of grinding stones from an early Holocene age site in the Upper Cauca Valley, Colombia (Gnecco and Mora, 1997). A variety of taxa were repre- sented, including arrowroot, legumes, and grasses. Grinding stones and grinding stone bases from late preceramic and early ceramic contexts (ca. 7000 B.P.-3000 B.P.) in Central Pacific Panama similarly yielded grains from a variety of plants, including arrowroot, yams (probably not D. trifida, the major domesticated yam), and legumes, in addition to grains that in morphology and size were indistinguishable from modern varieties of maize and manioc (Piperno and Hoist, 1998). The latter two types of starch grains were recovered from secure late preceramic contexts (7000 B.P.-5000 B.P.). The starch analysis supported the previously obtained archaeobotanical and paleoecological data in indicating an early development of agriculture in central Panama. The tools that yielded the highest number of starch grains in our studies were typically those that contained small interstitial cracks and crevices that could be sampled with a fine needle. Presumably, these tiny fissures on the tools were places in which the grains lodged and then were protected from the effects of the humid climate over time. Cummings and Magennis (1997) found maizelike starch grains in Mayan teeth calculi,where it was the most abundant remain present. Possi- ble manioc grains were also present on the teeth. Food residue of various types found in tooth calculi, although not commonly studied at present, have considerable potential for the elucidation of dietary trends in the Neo- tropics. A very recent development has been the successful isolation of starch grains from archaeological sediments (Piperno and Hoist, unpublished Piperno426
  • data). These include the sediments that were adhering directly to grinding stones, as well as samples taken from features and profiled walls. Starch, like phytoliths and pollen, can be floated out of sediments from the humid tropics using heavy liquids (Therin et al., 1998). The number of grains is not astoundingly large, but sample sizes of 50 and more starch grains from a 15cc volume of soil are common. This means that out of the tens of thousands of grains that were deposited into sites where people commonly used rhizomes and tubers, a few are surviving, but these few retain the characteristics, including the extinction cross, necessary to make positive identifications of plants and meaningful measurements of grain size. Starch research is just starting in the humid Neotropics, but its potential is clear. OLD AND NEW CONTROVERSIES The Neotropics is probably the most contentious area in which micro- fossil techniquesare being applied to archaeological subsistence reconstruc- tion. Debates concerning New World agriculturalorigins,particularlyabout maize history and the place of tropical forest cultures in the beginningsof the domestication process, have a long and rancorous history.The injection of data from a new technique, phytolith analysis, that indicated people of the lowland tropical forest developed and dispersed domesticated plants, including maize, at an early date (Pearsall, 1978; Piperno, 1988; Piperno et al., 1991) did little to quell the unrest. Predictably, then, many of the disagreements about the use of microfossils in subsistence reconstruction have centered around the author's and Pearsall's belief that maizephytoliths can be identified in the archaeological records of the Neotropics. The phytolith (and pollen) evidence for the use of maize in southern Central America and northern South America between about 7000 B.P. and 5000 B.P., summarized in Pearsall and Piperno (1990) and Piperno and Pearsall (1998b), is controversial because it conflicts with the known evidence from macrobotanical remains and with isotopic measurements on human skeletons. However, these differences are probably more apparent than real, because the various lines of evidence have very different levels of visibility. Carbonized macrobotanical and human skeletal remains are very poorly preserved in pre-3000 B.P. sites from the Neotropics, and bone isotope data are most useful for assessing the status of maize as a staple crop, consumed on a regular basis. One wonders what bone isotope records would look like from modern tropical forest peoples who consume maize in fermented beverages a few times a month during ceremonial and ritual activities. Maize was dispersed quickly through the tropical forest after it was taken under cultivation possibly because it became highly desired as Tropical Paleoethnobotany Through Microfossil Analysis 427
  • a feasting beverage. This isnot the forum in which to discuss such a scenario in detail, but investigations of early maize should consider the cultural context in which it was used. Having identifiedand measured thousands of modern and archaeologi- cal grass phytolithsand completed the additional work on maizediscrimina- tion just described, I am convinced that cross-shaped phytoliths provide a reliable means for maize identification, with the caveats as noted previously. My desire is to move on and study other important problems, not least of which is the history of the root and tuber crop complex of the Neotropical forest. Because the phytolith record likely will continue to provide muchof the primary empirical data on early maize movements through the tropical forest, I offer the following comments about some criticisms that have been directed at maize phytolith identification. Three researchers have expressed serious reservations about the Pearsall/Piperno approach to maize identification:Doolittle and Frederick (1991) and Rovner (1996) (also Rovner, 1990 and Rovner and Russ, 1992). It is clear from Doolittle and Frederick's (1991) paper that they had very little practical experience with phytoliths before starting their research. They attempted to determine the diagnostic potential of maize phytoliths from the American southwest using the Pearsall/Pearsall cross-body crite- ria; reported that in the leaves of four modern races of maize from the region they could find no cross-bodies; claimed that they, therefore, could not carry out the intended study; and then made from this nonstudysweep- ing conclusions asserting the problematic nature of phytolith research in the southwest United States and elsewhere. However, the phytolith drawingsin their Fig. 1 demonstrate that they did isolate manycross-bodies from maize (even with the use of a more restricted definition of cross-shaped phytolith preferred by Mulholland, 1993), and could not recognize them. Doolittle and Frederick (1991) also claimed that: (1) teosinte and maize phytoliths could not be differentiated, citing not empirical data they or anyone else generated, but a personal communication from a botanist who has never studied phytoliths;(2) cross-shaped phytolithsof different sizes were being differentially preserved in sites, using a study of Pleistocene-aged geologic sediments in which no cross-bodies and very few phytoliths of any kind were present (differential preservation is highly unlikely because all are solid plugs of silica, and many smaller cross-bodies have been recovered from pre-maize contexts in Latin America); and (3) that phytoliths of all types were of limited taxonomic value, citing a single study in which the researcher involved (Metcalfe, 1971) actuallystressedthe taxonomic poten- tial of the phytoliths being studied (Cyperaceae). Doolittle is a well-knowninvestigator who has carried out important studies dealing with agricultural evolution, but who was obviously ill- Piperno428
  • acquainted with phytolith research at the time of his analysis. Surprising, in addition to the numerous factual errors in the article, isthe aggressiveness of its tone. It is as if the goal of the study was to refute rather than evaluate the idea that maize phytoliths could be identified.The phytolith community still awaits a legitimate study of maize in the southwest United States. Rovner's criticisms of our use of cross-bodies in maize identification have to do with the following points: (1) Pearsall's (1978) definition of a cross-shaped phytolith, which used such attributes as indentation number and length to clearly demarcate them from other phytoliths in grass leaves called bilobates, is rigid and arbitrary, goes far beyond definitions made by earlier phytolith researchers, and does not capture a segregated set of phytoliths; (2) phytolith descriptions and measurements made with conven- tional techniques (an eyepiece micrometer and a microscope) are inferior to those made with the assistance of computer-assisted image analysis; (3) variability in phytolith size of a single species caused by local environmental conditions poses significant problems; and (4) phytolith researchers have problems identifying the cross-body three-dimensional variants I described. Points one and two have been answered by Piperno (1992), Pearsall (re- ported in Piperno and Pearsall, 1993b), and Piperno and Pearsall (1993a). To these comments, I add the following. Rovner (e.g., Rovner and Russ, 1992; Rovner, 1996) claims that com- puter-assisted image analysis (CAIA) is a superior technique for maize and other phytolith description and identification.However, he has not carried out the extensive studies of phytoliths in modern plants and archaeological sediments needed to test the approach. In CAIA, phytolith images are captured by the computer, which then makes a variety of measurements of size and shape. CAIA cannot adequately describe three-dimensional structures of particles because it makes measurements in only two dimen- sions, and for this reason it performed poorly in discriminatingwild grasses from maize when Pearsall compared it with conventional microscopy (in Piperno and Pearsall, 1993b). Pearsall also demonstrated that phytolith measurements made with the eyepiece micrometer, importantlyincluding all of the different cross-body variants (Fig. 21), were as accurate as those made with the help of the computer. She found that measuring one type of phytolith was no faster using the computer system than using an eye- piece micrometer. Rovner's single published CAIA study that dealt with a Neotropical archaeological problem and plants (Russ and Rovner, 1989) compared a small number of phytolithsin three races of maize and two races of teosinte. The limited number of phytoliths and taxa studied resulted in erroneous conclusions concerning the diagnostic potential of phytolith size in another grass type, the bilobate, in maize discrimination (see Piperno and Pearsall, 429Tropical Paleoethnobotany Through Microfossil Analysis
  • Fig. 21. Top center and bottom, three cross-shaped phytolithsenclosed in the leaf epidermis of maize. The two Variant 1 cross-bodies at the bottom are wider than the Variant6 cross- body at the top. This difference in the size of different cross-body variantstypifies maize and other grasses, and also contributesto accurate identificationarchaeologically. 1993a, pp. 348-350). Russ and Rovner (1989), like Pearsall, did not find any unique characteristics of maize phytoliths using CAIA parameters. CAIA has shown some promise, and may be especially useful for quickly measuring and quantifying a restricted range of phytoliths in a large data set. The degree to which it might improve current phytolith classification systems, and whether it can effectively and routinely identify phytoliths in large and diverse archaeological and paleoecological assemblages as does conventional microscopy, is unknown. It is unnecessary for many taxo- nomic problems. Concerning Rovner's third point, there undoubtedly is some variation in phytolith size of a single species that is caused, in part, by localenviron- mental conditions (Piperno, 1988). Infraspecific variability is always the case for morphologic traits, be they from phytoliths,pollen, or seeds. Paly- nologists have shown that average infraspecific pollen size in wild grasses Piperno430
  • can vary by 10 to 15% (E. Moreno, personal communication, 1998). The real question is, does this variabilityconflate interspecific comparisons? The answer with regard to wild grass phytolith size and maize seems to be no. For example, infraspecific variability in the wild grass cross-bodies replicated by Piperno (1988, pp. 75-79), to which Rovner (1996) refers as a demonstration of problematic and uncontrolled variation,is actuallyquite limited. A Mexican population of a wild grass, Hymenache amplexicaulis, had a mean cross-body width of 11.7 micrometers, compared with mean widths ranging between 10.5 and 11.3 micrometers for cross-bodies from four populations from Panama (Piperno, 1988, p. 76). The Mexican replicate falls outside of the range for Panamanian grasses, as Rovner (1996) notes, but by only 0.4 micrometers, which amounts to a difference of 4%. More to the point, cross-bodies from all five populations of this species are small, and all are significantly smaller than those of maize. This pattern, in which infraspecific variation in wild grass cross-body size was unremarkable, and in which cross-bodies were separable from those of maize, or not, characterized my replicate study. In a Belizean population of a wild grass, Cenchrus echinatus, mean cross-body width was 15.1 micrometers, which is 8% larger than the maximum value for mean width in three Panamanian populations (range: 13.3-14.0 micrometers). It was obvious from the initial studies of this grass (Piperno, 1984) that it was unusual among wild species in that it produced cross-bodies as large as maize (mean cross-body size in Latin American races of maize usually varies between 12.8 and 15.9 micrometers) (Piperno, 1988, p. 74). The fact that a Belizean population produces even larger phytoliths than Panama- nian populations does not indicatethat environmentalvariablesinflict chaos on its size distributions, but rather that the cross-bodies are large, no matter where they grow. Cross-body three-dimensional morphology is needed to separate C. echinatus from maize, which it does quite effectively (Piperno, 1988, pp. 68-79). In the replicate study, cross-body morphology of the wild grasses was also consistent (Piperno, 1988, p. 76). Additional studies of infraspecific phytolith variability were carried out by the author as part of the large analysis of maize and wild grass phytoliths described previously. Three different populations of one of the wild grasses with cross-bodies as large as maize, Maclurolyra tecta, contributed Variant 1 cross-bodies with mean widths of 16.7, 15.6, and 13.8 micrometers. Simply put, these phytoliths, like those of C. echinatus, are large. Replicates of wild grasses that did not present size problems for maize identification all produced small cross-bodies (D. R. Piperno, unpublished data). Contra Rovner's claims, these data indicate that phytolith size variability in wild species is not problematic for maize phytolith identification. Tropical Paleoethnobotany Through Microfossil Analysis 431
  • I can find no basis for Rovner's claim that my identificationof different three-dimensional cross-body forms is coming under serious criticism from phytolith researchers. There is no doubt that these forms actuallyexist and are not artificial creations of conventional microscopy, as Rovner (1996) argues (see Fig. 21 and SEM and other illustrations in Piperno, 1988, pp. 228-231, and Piperno and Pearsall, 1998a). Calderon and Soderstrom (1973, p. 30) noticed the bamboo-specific Variant 8 cross-bodies in epidermal thin sections of the aforementioned species Maclurolyra tecta (and also noted that they were large), but because the phytoliths were partially obscured by enclosing plant tissue they could not describe them very well and called them "modified and oryzoid" cross-bodies. Other cross-body variants are consistently illustrated in large studies of grass phytoliths (e.g., Brown, 1984; Figueiredo and Handro, 1971). Raising and lowering the focal plane of the microscope, by which Rovner means focusing in and out on the phytolith, could certainly make the top of a phytolith clearer than the bottom, or vice versa, but how can three-dimensional or any aspect of phytoliths be studied if the body is not put into clear focus? A number of published studies have employed the three-dimensional criteria without problem (e.g., Mulholland, 1993; Pearsall, 1989; Umlauf, 1993). I recently took an informal poll of others who have studied cross- bodies and asked them whether they had difficulty recognizing the different forms. The response was uniformly negative (J. Jones, L. Kealhofer, Z. Zhao, and Q. Jiang, personal communication, 1998). Where, then, is the technique being "widely questioned" (Rovner, 1996, p. 431)? Are some of the three-dimensional criteria subtle, and does it take time, practice, and a good reference collection to learn how to recognize all of the them? Yes; identifying phytoliths, like pollen and starch grains, requires studied work and patience, especially where a diverse flora is being investigated. Finally, what Rovner (1996, pg. 431) calls the "failures" of the maize technique (citingstudies by Mulholland, 1993, and Umlauf,1993, and claim- ing that others are being "increasingly reported" from New World sites) actually refers to the fact that in some sites where maize is thought to have been consumed or where macro-remains of the plant are present, cross- shaped phytoliths are not well represented or are not large enough to be discriminated from wild species with a high degree of confidence. Results such as this are commonplace for paleoethnobotanists, who must consider taphonomic as well as taxonomic concerns in their attempts to reconstruct ancient diet (e.g., Pearsall, 1989). Cross-body presence depends largely on whether maize leaves [husks also produce cross-bodies, but in much smaller numbers than leaves (Piperno, 1988)] were discarded at a sampling locale. In fact, Mulholland (1993, p. 143), well aware of these taphonomic issues, cited ethnographic evidence that most vegetative material "was never Piperno432
  • brought to the village," and that "only the cobs were consistently introduced to the villageitself," providingan explanation for the rarityof cross-bodies. She ultimatelydeveloped a method to identify cob phytoliths,which were quite common in the site's sediments, and concluded (p. 145) that the cross- body data "corresponds to the ethnographic pattern reported" for the culture. No failure is detectable here. As I commented in the Phytolitharien Newsletter (Piperno, 1992), if current maize identification procedures were resultingin the misidentifica- tion of wild grasses as maize, or a persistent inability to recognize maize cross-bodies in contexts in which maize leaves were left behind, the follow- ing would not be true of the results to date: (1) maize occurrence at ca. 7000 B.P. and after, but not before, in records from southern Central America and northern South America, despite the abundance of cross- shaped phytoliths at some earlier sites; (2) maize phytolith occurrence in the same contexts in which maize pollen and maize-like starch grains are identified (in other words, maize pollen and starch is also consistently occurring between 7000 B.P. and 5000 B.P., but not before in the records); and (3) absence of maize phytoliths in which maize pollen and starch are not identified (see Piperno and Pearsall, 1998b, for a summary of this information). Maize phytolith procedures obviously cannot identify every race of maize that was used prehistoricallybecause variability in this domes- ticated species is high, and some maize races have smaller phytolithsthan others. Palynology has similar problems. I urge researchers who are not convinced by the maize phytolith results to evaluate other lines of data they may find more compelling (pollen, molecular biology, starch grains) as counterpoint to what I firmly believe is a largely blind and inadequate macrofossil record for early maize in the humid tropics (see later). Eventually,a critical mass of data will be assem- bled from the different kinds of evidence being studied, and a consensus on maize origins and early dispersals will emerge. The degree of harmony between the phytolith evidence and this consensus may be stronger than scholars might believe possible now. Another controversial issue in microfossil studies has been the reluc- tance on the part of some investigators to seriously consider any form of pollen or phytolith evidence for plant cultivationand domestication because of what they perceive to be serious problems with chronological and/or taxonomic control. Smith (1995a,b) and Fritz (1994,1995) have been partic- ularly vocal in this regard. Associated with their belief that directly dated macrofossils provide virtually the only acceptable proof for prehistoric agriculture, what Smith (1995b, p. 176) termed the "new standard of evi- dence," was their radical new chronology for the beginnings of farming in the New World. They proposed that it began during the middle Holocene Tropical Paleoethnobotany Through Microfossil Analysis 433
  • (ca. 5000 B.P.)rather than shortly after the termination of the Pleistocene, and that, as in southwest Asia, it was originally undertaken by sedentary societies with complex forms of social organization. Their revised age for New World agricultural origins was based largely on radiocarbon dates obtained on macrofossils of maize and Phaseolus beans from the Tehuacan Valley in the arid southcentral highlands of Mexico, which scholars agree was probably not a nuclear area of agricultural development. Although their new chronology better fit the earliest evidence for agriculture from the eastern United States, it was at odds with microfossil data from the lowland tropical forest, where most of the staple crops that supported indigenous populations at the European contact, including maize, were originally brought under cultivation and domesticated (Sauer, 1950; Pip- erno, 1994b; Piperno and Pearsall, 1998b). Recent dates on domesticated C.pepo macro-squash remains from the Oaxaca Valley, Mexico (Smith 1997), as well as increasing empirical evi- dence from sites located in the tropical lowlands (later; Piperno and Pearsall, 1998b), are beginningto make it clear that: (1) plantcultivationand domesti- cation in Mesoamerica and South America occurred shortly after the termi- nation of the Pleistocene, as they did in other pristine areas ofagricultural origins; (2) lowland tropical forest cultures played crucial roles in these developments; and (3) models of agricultural origins extrapolated from southwest Asia, where sedentary life and considerable social complexity may have preceded the onset of cultivation and domestication, cannot be shoehorned into the dramatically different American ecosystem. The doctrinaire approach to paleoethnobotany advocated by Smith and Fritz that emphasizes macrofossils to the virtual exclusion of all other evidence is particularly unsuited to the humid tropics, because many seeds and roots simply do not survive for very long, even after carbonization, under the humid environments, alternating conditions of drying and wetting, and dense, clayey depositional contexts typical of the tropical lowlands (Piperno and Pearsall, 1998b, pp. 32-34). Seeing the results from the screening and flotation of gallons of archaeological sediment from Panama and elsewhere, which typically yielded small quantities of a few types of hard seeds and nuts, convinces the author that preservation often is just too poor to provide cogent reconstructions of plant use, and that the older the sites are, the worse the preservation is. Pearsall (in Piperno and Pearsall, 1998b, pp. 33-34) demonstrated a sharp drop-off in the quantity of macrofossils through time even in an Ecuadorean site occupied during the past 3000 years. The root crops that were so essential to tropical forest subsistence obviously are not open to study with macrofossils except under the very best preservational conditions, such as are found in tropical deserts. Piperno434
  • It seems that the key to reconstructing early plant use in the humid tropics will be microfossil evidence from both archaeological and paleoecological contexts. Next, I discuss in more detail the aspect of this evidence that has most troubled some investigators: determining the dates of phytoliths and pollen grains recovered from ancient sites. DEVELOPING RELIABLE CHRONOLOGIES FOR MICROFOSSIL ASSEMBLAGES With the important development of AMS dating, which made it possi- ble to determine the age of single seeds or small, vegetal fragments recov- ered from archaeological sites, came the uneasy feeling on the part of some investigators, discussed in part earlier, that because microfossils could not be dated in the same manner, one could never be certain that an associated date for a phytolith or pollen assemblage was the true age of the plants represented in them. Although concerns about the stratigraphic integrity of plant remains are paramount, the chronology of every remain, be it of macro- or micro-type, should not automatically become subject to serious question without a close consideration of the specific context in which it was found. For example, dry caves are notorious for subjecting botanical materials to significant vertical movement because uncharred food remains are potential foods for burrowing or commensal animals [although a ca. 9000 B.P. age for the domesticated Cucurbita pepo recovered from early Holocene strata at Guila Naquitz has been confirmed with AMS dating (Smith, 1997)]. In contrast, the dense clays of humid tropical sites are probably much less prone to such admixture. Microfossil assemblages that demonstrate persistent trends through time (e.g., increase of Cucurbita phytolith size, presence of phytoliths and pollen typical of more primitive maize races and other cultigens in earlier but not later strata) likely contain few to no intrusive particles from more recent occupations of sites. Single-component sites are inherently unlikely to contain plant remains from occupations that occurred later in time. Remains of plants directly recovered from stone tools may confidently be dated by association with the age of the tool because large artifacts are not expected to have moved downward in sediments after their deposition if no other disturbances are evident in the sediments. Turning to paleoecological contexts, small bits of sediment containing pollen and phytoliths sampled from tropical lake cores have been shown by numerous studies to provide a reliable context for dating (Colinvauxet al., 1996a, b, Piperno and Pearsall, 1998b). Bioturbation is inherently un- likely due to the rarity of organisms that promote sediment mixing in Tropical Paleoethnobotany Through Microfossil Analysis 435
  • anoxic conditions. X-ray and visual analyses of core sections are capable of revealing precise details of sedimentation, such as fine, undistorted laminae. When present, such details indicate continuous and undisturbed sedimenta- tion. A series of internally consistent 14-C determinations on sediments sampled at close and regular intervals from the cores adds to the likelihood that sedimentation was indeed undisturbed. Lakes located in regions with limestone substrates, where ancient geo- logic carbon is introduced into lake water causing anomalous 14-C determi- nations, or situations in which deep root penetration occurs, as happens in mangrove swamps, should be approached cautiously. It is true that age determinations from lake cores cannot provide as precise an estimate for agricultural activity as a date on a domesticated seed, but knowing within a few hundred years when significant land clearance signaling agriculture occurred obviously is important information. These comments are unlikely to satisfy those who insist on having directly dated macrofossils as proof of domestication and agriculture, but even the most hardened microfossil skeptic can now rest easier in light of demonstrations that it is possible to directly date phytoliths and pollen from archaeological and paleoecological sites. When phytoliths form in plant cells, some of the organic material of the cell becomes trapped inside the phytolith and it remains there over long periods of time. Because the carbon is locked within the phytolith,it is immune from the various modes of postdepositional contamination. Dating a single phytolith is obviously impossible, so a phytolith assemblage isolated from a discrete sediment context becomes the basis of the analysis. Wilding (1967) first showed that it was possible to radiocarbon-date phytoliths, and he provided the chemical and physicalanalyses of the carbon inside them needed to show that derived 14-C dates were meaningful. For example, it was demonstrated that this carbon was organic in origin, that it was impervious to a strong oxidation treatment, and that such a treatment removed the extraneous carbon clinging to the outside of phytoliths that was a potential contaminant (Wilding et al, 1967). Dating phytoliths did not become commonplace because many hundreds of grams of sediment had to be processed when conventional beta decay counters were the only means available to determine an age. With the advent of AMS dating,only a handful of phytolith-richsediment, about 35-45 g, usuallyprovides a 1-3 milligram amount of carbon sufficient for an age determination. A method to process archaeological sediment samples for AMS phytolith dating is provided by Mulholland and Prior (1993). It should be noted that stable carbon isotope (12-C/13-C) values can also be calculated directly from phytoliths as part of the routine procedure used to date them (C. Prior, personal communication, 1998). More basic research isneeded in the tropics to explore the precise relationship between Piperno436
  • delta 13-C values derived from phytoliths and from whole vegetative struc- tures of the same plant, but results so far from the temperate zone indicate that phytolith-stable carbon isotope values provide significantinformation on whether primarily C3 or C4 plants contributed the phytoliths in soil assemblages (Kelly et al., 1991; Fredlund, 1993). Such an approach applied to phytoliths from lake sediments may be particularly useful in assessing how the changes in the seasonality of annualprecipitation and temperature during the Pleistocene to Holocene transition, which are strongly predicted by climatic modeling, may have affected the photosynthetic pathways and growth habits (perennial vs. annual) of plants that soon after were brought under systematic cultivation and domesticated. There are currently a small number of phytolith dates available from archaeological contexts. However, they strongly support the viability of the technique in the Neotropics because they accord well with dates from associated cultural materials such as charcoal, shell, and human bone (Piperno and Pearsall, 1998b, Chapter 4). Also, when insufficient dateable material is recovered from particular occupational phases of sites, as often happens with the earliest habitations, phytoliths provide a handy medium for chronological assessment because they are often well pre- served in the most ancient strata (Piperno and Pearsall, 1988b, pp. 214-216). In the case of the early Holocene site Vegas, located in southwest Ecuador, a close correspondence was found between the age of phytolith assemblages and the size of the Cucurbita phytoliths isolated from them. Phytolith sizes and ages indicate that wild Cucurbita spp. were being ex- ploited during the terminal Pleistocene and earliest Holocene, and that a domesticated species was present by 9060 B.P. (Piperno and Pearsall, 1998b, pp. 186-197; Piperno et al, 1998). The possibility of directly dating pollen grains has been discussed by palynologists for some time, and the 1990s witnessed successfulapplications in lake sediments (Long et al., 1992; Goman and Byrne, 1998). Techniques being developed minimize the amount of extraneous organic matter recov- ered along with pollen from lake cores, like cellulose, and ensure that age determinations are based on an almost pure extract of pollen (Prior, 1998). It also seems possible to separate pollen grains from individual taxa for dating by means of density gradients (Prior, 1998). Such an approach may be especially applicable to cultivar taxa (e.g., Cucurbita, Zea mays, Mani- hot), whichoften are much larger, and, hence, more carbon-rich than others. Preliminary studies have shown that C-14 dates derived directly from pollen closely agree with the associated C-14 ages from the core's bulk sediments, indicating that reliable pollen chronologies are being generated (and also that the sediment is not much older or younger than the pollen it contains) (Prior, 1998). Only a 1-cm pollen-rich segment of a core is needed to provide a sufficient quantity of pollen for an AMS date (Prior, 1998). Tropical Paleoethnobotany Through Microfossil Analysis 437
  • The routine AMS dating of phytolith and pollen assemblages from archaeological and paleoecological contexts that now seems possible should dramatically improve the confidence in the results derived from microfossil studies. The likelihood that sufficient starch residue can sometimes be retrieved from archaeological sediment for an AMS date appears to be high. This raises the prospect of having three different classes of microfossil data from a single sediment context, all independently dated by radio- carbon. SUMMARY What Have Microfossil Approaches Told Us About Ancient Plant Use and Domestication in the American Tropical Forest, and What Can We Expect to Learn? This paper attempts to identify the crop plants and general subsistence and paleoenvironmental problems that can be studied with the phytolith, pollen, and starch records in the Neotropics. The complementary nature of these microfossils deserves to be repeatedly stressed. Where one or two of them are weak, as in detection of root crops by pollen and phytoliths, another is strong. Pollen will never tell us if teosinte seeds were regularly exploited and then manipulated into maize cobs. Phytoliths can because female reproductive structures produce highly diagnostic forms. Paleoeco- logical studies have already found substantially improved precision by using phytoliths and pollen in tandem because the strengths and shortcomings of one technique in identifying individual taxa of the tropical forest and different scales of human disturbance are often offset by the other (Piperno, 1993). Table III presents a summary of the expected visibility of major economic taxa of the lowland Neotropical forest in microfossil records. It is far too early to attempt a transtropical comparison of subsistence and dietary trends because few sites dating to before 3000 B.P. have been investigated, particularly in Mesoamerica. However, there are sufficient data from southern Central America and northern South America to iden- tify and evaluate the following patterns. First, a variety of tropical forest tubers, herbaceous seed plants, and tree fruits are present in archaeological pollen, phytolith,and starch assem- blages from early Holocene (ca. 10,000 B.P.-8000 B.P.) contexts in the Upper and Middle Cauca valleys, Colombia, Amazonian Colombia, and central Panama (Herrera et al, 1992; Cavelier et al., 1995; Gnecco and Mora, 1997; Piperno and Pearsall, 1998b, pp. 182-227). The Upper Cauca Valley evidence indicates that people were already developing close rela- Piperno438
  • tionships with the tropical forest flora and processing tubers and a variety of other plants,including legumes,for food duringthe earliest Holocene (by or shortly after 10,000 B.P.). A criticalquestion,in view of the dramatically different ice age environmentsand (probably) resources that the ancestors of some of these people were exploiting before the Holocene began, is when such close relationships began to be formed. Few late and terminal Pleistocene sites have so far been studied,but arrivingat some understand- ing of pre-Holocene plant subsistence is obviously of fundamental impor- tance. Second, Cucurbita phytoliths are present in early Holocene contexts (ca. 10,000 B.P.-8000 B.P.) from archaeological sites in central Panama, the Upper Cauca Valley,Colombia,southwestEcuador, and the Colombian Amazon (Piperno and Pearsall, 1998b, pp. 186-217; Piperno et al, 1998; D. R. Piperno, unpublisheddata). Cucurbitaphytolithsfrom the latter two 439Tropical Paleoethnobotany Through Microfossil Analysis Table III. Projected Levels of Visibility of Some Crop and Other Economically Important Plants in Tropical Plant Microfossil Records Based on Present Knowledge of Production, Taxonomic Specificity, and Survival in Sediments Plant Tubers Manihot esculenta (manioc) Ipomoea batatas (sweet potato) Dioscorea spp. (yams) Xanhossoma sagittifolium (yautia) Canna edulis (achira) Calathea allouia (leren) Maranta arundinacea (arrowroot) Legumes Phaseolus spp. (beans) Arachis hypogaea (peanuts) Canavalia spp. (jackbeans) Cereals and vegetables Zea mays (maize) Cucurbita spp. (squash) Tree fruits Palmae (palms) Persea americana (avocado) Phytoliths 0 0 0 0 1 2 2 0 0 0 2 2 2 0 Pollen 1" 0 0 0 0 0 0 0 0 0 1 0 1' 0 Starch 2 1?b 2 1?b,c 2 1?b 2 2? 1? 9 2? ?'' 0 Note: 0 = low probability of recovery; 1 = fair probability of recovery; 2 = good probability of recovery. a Manioc pollen is probably most visible in sediments recovered from former field areas. b Tubersand other plants that were not processed before being eaten may have less visibility, since starch recovery will have to be from sediment. c Yautia starch may be identifiable but the grains are tiny and many will have to be recovered. d Cucurbita starch seems diagnostic but survivability in sediment is unknown. e Some palms have diagnostic pollen, but recovery is often poor.
  • areas have sizes indicating domestication by 9080 ± 60 B.P. and 8090 ± 60B.P., respectively, each date representing a direct AMSphytolith determination on the phytolith assemblage (Piperno and Pearsall, 1998b, pp. 188-197, 203-205; Piperno et al., 1998). Phytolith dates for the first two early squash contexts are forthcoming, but phytolith assemblages with dates of 5560 ± 80 B.P. and 6910 ± 60 B.P. occur stratigraphically above the earliest Panamanian Cucurbita remains (Piperno and Pearsall, 1998b, pp. 213-217). The Ecuadorian sequence is most clear in indicating the local presence and exploitation of a wild gourd during the terminal Pleistocene and early Holocene periods and its subsequent domestication by ca. 9000 B.P., whereas the Amazonian Colombian and possibly, Panamanian se- quence indicate an introduction of a domesticated form of Cucurbita by ca. 8000 B.P. The Upper Cauca Valley context for Cucurbita possibly does not date much later than 9500 B.P., and there, like in the terminal Pleistocene/early Holocene Ecuadorean context, phytoliths have sizes typi- cal of modern wild gourds (D.R. Piperno, unpublished data). It is almost certainly not coincidental that the earliest domesticated C. pepo squash remains from Guila Naquitz cave in Mexico (Smith, 1997) date to about the same time as the domesticated Cucurbita phytoliths at Vegas. Cucurbita species appear to have been among the earliest plants brought under cultivationand domesticated in both Mesoamerica and South America. Archaeological deposits that date to after ca. 7000 B.P. generally contain few to no Cucurbita phytoliths, an expected result ifearly Holocene domestication occurred because many modern domesticated populations do not produce them. Third, two now-minor root crops, leren and arrowroot, are consistently present in the archaeological phytolithassemblages that were directly dated to between 9000 B.P. and 7000 B.P. from southwest Ecuador, Panama, and Amazonian Colombia discussed previously (Piperno and Pearsall, 1998b, pp. 182-227). These data are in accord with the notion that pre-7000 B.P. food production systems were characterized by a simple kind ofhorticulture practiced largely in house gardens (no significant clearing of the forest for larger-scale agricultural plots yet). Paleoecological data from Panama and elsewhere support this notion (Piperno et al, 1991 and later this chapter). Fourth, manioc pollen and starch grains that compare favorably with manioc are present in sites in Belize, Panama, and the Colombian Amazon dating to between ca.7000 B.P. and 4750 B.P. The starch grains were found on grinding stone tools from Central Panama that occurred in secure late preceramic (ca.7000 B.P.-5000 B.P.) contexts, and where associated phytol- ith assemblages discussed previously yielded direct AMS dates of 6910 B.P. and 5560 B.P. (Piperno and Pearsall, 1998b, pp. 209-227; Piperno and Hoist, 1998). Manioc pollen occurred in sediment cores from Belize and 440 Piperno
  • Amazonia Colombia that likely penetrated former fields. The Colombian manioc occurred 10 cm below a 2-cm core level with dates of 4330 ± 45 B.P. and 4695 ± 40 B.P. (Mora et al, 1991). The Belizean manioc was present in two different sites located 50km apart and was directly associated with dates of 4591 B.P. from one site and 4750 B.P. from the other (Pohl et al, 1996). If, as believed, manioc domestication occurred in or near the Orinoco Basin, then even earlier remains should be found in northern South America. Fifth, maize phytoliths, pollen, and/or starch grains are present in archaeological and paleoecological sediments and/or on stone tools from many of the same sites in lowland southern Central America and northern South America just discussed, in deposits that date to between ca. 7100 B.P. and 5000 B.P. (Piperno and Hoist, 1998; Piperno and Pearsall, 1998b). Conversely, pre-7100 B.P. contexts from these sites do not contain microfos- sil remains of maize. This, as several investigatorshave noted, is earlierthan the current maize evidence from Mesoamerica (Smith, 1995a,b). However, archaeological data from maize's hearth in the Pacific lowlands of Mesoam- erica are extremely sparse, making comparisons with the chronology of events to the south difficult. Sixth paleoecologic sequences from the middle Cauca Valley, Colom- bia, the Colombian Amazon, the Ecuadorian Amazon, central Panama, Belize, and the MexicanGulf Coast are consonant in indicating the develop- ment of slash and burn agriculture with maize between 7000 B.P. and 4200 B.P. (Monsalve, 1985; Bush et al, 1989; Mora et al, 1991; Piperno et al, 1991; Rue, 1987; Pohl et al, 1996; Pope and Pohl, 1998). At present, this evidence is time transgressive.That is,data from southern Central America and northern South America adduce this form of agriculture by between 7000 B.P. and 5000 B.P., whereas it is first detectable in sites from Mesoam- erica between ca. 5000 B.P. and 4200 B.P. Again, fewer sites have been examined in Mesoamerica, so current trends may be more apparent than real. In summary,the evidence from several different regions pointsstrongly to the development of food production in the lowland tropical forestduring the early Holocene, and the subsequent emergence of truly productive slash-and-burn systems 2000 to 3000 years later. Paleoethnobotany and Explanations of Culture Change Producers and consumers of paleothnobotanical data alike have com- mented that the subdiscipline of paleoethnobotany has only weaklycontrib- uted to general theoretical explanations of culture change (Ford, 1988; Marquardt, 1988; Gardner, 1995). However, with the increasingquality and 441Tropical Paleoethnobotany Through Microfossil Analysis
  • quantity of information that is becoming available to them, paleoethnobota- nists are perfectly positioned to actively formulate and structure theoretical debates in archaeology. For example, paleoethnobotanical data have sub- stantial potential for testing evolutionary ecological and more strictly selec- tionist evolutionary explanations for agriculturalorigins (Piperno and Pear- sail, 1998b). Other theoretical constructs for major changes that occurred in prehistoric subsistence and settlement systems, such as risk theory (Win- terhalder and Goland, 1997), can be tested through paleoethnobotany. The increasing refinement of the data also means that paleoethnobotanists can be more innovative in their uses of traditional data sets. For example, by combining information on the efficiency of food procurement in different modern habitats with reconstructions of vegetational history, broad esti- mates can be made of foraging return rates (rates of energy capture) in prehistoric ecosystems through time (Piperno and Pearsall, 1998b). Other aspects of past environments that must have significantly affected human subsistence decisions (e.g., changes in the seasonality of precipitation and temperature) are open to study with microfossils. The close links between paleoethnobotanical data, subsistence, and paleoenvironment does not mean that paleoethnobotanists should distance themselves from explanations of the origins of social complexity. Many investigators of this issue find features such as the presence of highly produc- tive crop plants, food surpluses, and decreasing access to good agricultural land to be closely connected to social competition and conflict, the hallmarks of cultural complexity, and all these features leave tangible paleoethnobo- tanical evidence (e.g., Cooke and Ranere, 1992; Pohl et al, 1996). The next decade should bring us to a clearer understanding about when, why, and how peoples of the lowland tropical forest came to form close relationships with their complex flora, brought some species under cultivation and domesticated them, and arguably developed effective sys- tems of agriculture earlier than they are presently demonstrable in the cooler and drier regions of the Neotropics. Microfossil studies will likely form the underpinning for much of this research. ACKNOWLEDGMENTS The author's phytolith and starch research was made possible by sup- port from the Smithsonian Tropical Research Institute (STRI) and a grant to the STRI from the Andrew W. Mellon Foundation. The grass silica body studies were also supported by a grant to the author and Deborah M. Pearsall from the National Science Foundation (BNS-89-2365). Part of the grass phytolith research was carried out at the Museum Applied Science Piperno442
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