Recent African Origins or Regional Evolution?
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Recent African Origins or Regional Evolution?

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Compares the two major models of human migration and the means of testing them.

Compares the two major models of human migration and the means of testing them.

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Recent African Origins or Regional Evolution? Recent African Origins or Regional Evolution? Presentation Transcript

  • Recent African Origins or Regional Evolution? Where did Modern Humans Come From, and When?
  • Human Origins: An Acrimonious Debate
    • There is a longstanding debate as to how we got to be we, namely become modern Homo sapiens?
    • Why do the experts, whom we are supposed to respect for their careful research, differ so intensely that sparks fly?
  • Why the Debate: Several Reasons
    • The evidence itself is fragmentary, as you’ve seen in the past lessons; we just don’t have complete skulls or postcranial skeletons handed to us on a platter
    • Therefore we have to make inferences from our bone fragments, and different experts make different inferences from those bone fragments
    • Every expert comes to the field and the lab with her or his own assumptions
  • Points of Agreement
    • There is one agreement: almost all paleoanthropologists agree that we evolved from Homo erectus
    • They also agree that Homo erectus migrated from Africa into Asia and Europe
    • This is where the experts split into two camps—When and where did we evolve from Homo erectus ?
  • The Recent African and Multiregional Evolution Models
    • Did our ancestors evolve at different places around the Old World from Homo erectus?
    • That is known as the Multiregional Evolution Model
    • Or did they all evolve from Homo erectus in Africa?
    • That is known as the Recent African Origin model, because we originated from Africa only in the past one or two hundred thousand years BP (before the present)
    • This is also known as the Out of Africa model.
  • The Two Models Compared
    • Left: Multiregional Hypothesis: Homo erectus migrates to Asia and Europe and evolves in each region into Homs sapiens .
    • The little arrows indicate interbreeding across the regions, keeping our species intact..
    • Right: Recent African Origin (Out of Africa) Hypothesis: Homo erectus evolves into Homo sapiens, then migrates to the rest of the Old World—and the new one
  • Multiregional Evolution Model
    • This map depicts humans evolving from regional points and migrating from there
    • What prevents speciation, of humans from becoming new species? Milford Wolpoff (lower left) has an answer:
    • Whatever else hominins may do, they always interbreed.
    • Thus they maintain control over, or prevent, speciation
    • Alan Thorne (lower right) traces Homo erectus ‘s transition to Homo Sapiens in Southeast Asia
  • Recent African Origin Model
    • This map places modern human origins, according to one conception, somewhere in the pink shaded area of S. and E. Africa
    • Then humans migrate in waves: the darker the red, the more recent the waves.
    • Ian Tattersall (with bony colleague) argues the following;
    • Modern Homo sapiens evolved in Africa around 200,000 BP
    • They migrated to Europe and Asia
    • They displaced archaic H. sapiens including H. neanderthalensis
    • Modern and archaic forms did not interbreed, extinguishing the latter
  • Partial Replacement Model: A Middle Ground?
    • Gunter Br äuer, Univ. of Hamburg, Germany
    • Modern H. sapiens arose in Africa around 100,000 BP
    • They both replaced and interbred with archaic sapient forms
    • Through interbreeding, modern populations gradually replaced the premodern hominins
    • Fred Smith: replacement occurred as much through gene flow as through migration: they let the genes do the walking
  • Testing the Models
    • It is very well to speculate on and interpret the two models
    • How do we test them?
    • We offer one set of tests to compare the hypotheses generated by the two models
    • We then show how that one test alone is fraught with ambiguities of the data—the fragmented hominin remains.
  • Out-of-Africa Model: Test Expectations I
    • Oldest modern sapient fossils should be found only in Africa (see model to the right)
    • Transitional forms (e.g. H. Heidelbergensis) should be found only in Africa
    • Elsewhere, emigrant modern humans should coexist with archaic humans until the latters’ extinction
  • Out-of-Africa Model: Test Expectations II
    • There should be a break between premodern (H. heidelbergensis and earlier forms) and modern fossil humans outside Africa
    • Modern human material cultures (e.g. tools) should make a sudden appearance outside Africa, with no transitional forms
    • Modern humans should be genetically distinct from premodern humans outside Africa
  • Multiregional Evolutionary Model: Test Expectations I
    • Early modern human fossils should be found across all or many regions, none much older than the others (left model)
    • Intermediate humans should be found across the regions because evolution occurred everywhere.
    • Premodern features should grade into modern forms everywhere as modern genes replace premodern ones (e.g. reduction of prognathism)
  • Multiregional Evolutonary Model: Test Expectation II
    • Local skeletal traits should show continuity between modern and premodern forms everywhere
    • There should be a continuous development between premodern and modern material cultural remains
    • There should be genetic continuity between modern and premodern forms in every region
  • Shifting Evidence
    • Klases River Mouth, South Africa yielded the following finds:
    • Fragments of modern skulls and a jaw with a modern chin (upper left)
    • The jaw and fragments were dated 90,000 years BP, the oldest up to that time (1970s)
    • The find would confirm the RAO hypothesis
    • Then a modern skull was found in Liujiang, China (lower left) in 1958, dated 20 to 30,000 BP
    • An analysis in 2002, however, dated the skull 100,000 years BP, favoring the Multiregional Evolution model
    • Later finds place a modern find in Omo, Ethiopia, at 195,000 years BP, favoring the RAO Model—for now.
  • Current Status of the Models: Modern Homo sapiens
    • At the moment, multiple sites indicate that African sapient sites are older
    • Homo sapiens skulls are oldest in Africa
    • They range between 100,000 and 200, 000 years BP, including the Omo find
    • Israel has remains ranging between 92,000 and 120,000 BP
    • European skulls range between 10,000 and 27,000 BP
    • China’s range is 10,000 to 100,000 BP
    • Australian skulls range from 40,000 upward
    • So far these data support the Recent African Origins Model
  • Current Status of the Models: “Archaic Homo sapiens ”
    • The same pattern applies to Homo heidelbergensis , or Archaic Homo sapiens
    • African remains vary between 400,000 and 700,000 BP
    • European remains vary between 160,000 and 475,000 BP (780,000 in Spain)
    • China and India: 130,000-200,000 BP
    • These fit the pattern of the Recent African Origins model.
  • Typological Ambiguities
    • Also open to controversy is what constitutes modern Homo sapiens
    • Harold Dibble argues that typologies often lead us down blind alleys
    • We often find differences that aren’t really there , creating even more squabbles—like this one or the one about the human status of Neanderthals
    • Artist’s conception of Homo heidelbergensis (lower left)—or is it Homo sapiens ?
  • Taxonomic Questions
    • This comparison of a Neanderthal (left) and human skeleton reflects a major controversy about human typology
    • Wolpoff goes so far as to suggest that Homo erectus (lower left) and Homo sapiens (lower right( could be one species
    • As the clichés have it, you be the judge:
    • From the two sets of pictures, does Wolpoff make a prima facie case for his one-species argument?
  • Testing the Model
    • Are the oldest modern forms found in Africa or are they also found in Europe and Asia?
    • Are transitional forms found only in Africa or are they also found in Europe and Asia?
    • What’s the evidence from genetic mutation?
    • Mitochondrial DNA in the female lineage>
    • Y Chromosomes?
    • Is the transformation from archaic to modern forms sudden or gradual outside Africa?
    • What do the archaeological finds say?
  • Case Example: Mitochondrial DNA
    • Principles of mitorchorndral DNA (mDNA) tests
    • mDNA samples only from living persons
    • mDNA is used to retrodict past mutations
    • mDNA is passed only by women
    • Sperm leaves behind all its mDNA
    • Fetus inherits mDNA only from ovum
    • Number of mDNA mutations indicates antiquity of species
  • Out of Africa: Procedures
    • Rebecca Cann and colleagues:
      • Sampled 147 women
        • mDNA shows little diversity
        • Sample was to be worldwide
          • Africans should show the most mutations
          • New Guinea (NG)/Australian, dated 80K BP, included
          • Africans showed 3 times mutation of NG./Australians
          • Other populations similarly sampled
      • Results tended to support claim
        • Mutations traceable to a single African female
        • Hence, “Mitochondral Eve
  • Out of Africa: mDNA a flawed methodology
    • Nature of the flaws
      • Sample was too small: 147 out of 2 billion
      • All “African” women were American
        • Potential admixture with Europeans, Native Americans, and Asians
      • Order of data input influenced results
    • Alan Templeton reran the tests
      • Found mutation rates equal for Africans, Europeans, and Asian
      • Supports mulitregional theory
  • Out of Africa Theory: Retests of mDNA
    • Laurence Excoffier & Andre Langanay
      • Tested larger sample
        • Africans from Africa exhibited less diversity
        • Than European and Asians.
    • Cann and colleagues ran a retest
      • Sample much larger--5,000
      • Africans from African included
      • Support more modest
        • Africans showed more diversity
        • But the variations were not statistically significant
  • Mitochondrial DNA: General Results, If Any
    • There might have been several migrations out of Africa, not just one.
    • The migrations could have taken place various times from 2 billions years BP—to the present
    • Thus it is not surprising that Templeton found varied and even contradictory results
    • The genetics of the world population isn’t all that diverse in the first place.
    • For full argumentation, see pp. 331-344.
  • Conclusion
    • Fragmentary data makes the controversy less than conclusive.
    • It depends on our typology, from Tattersall’s extreme splitter taxonomy to Wolpoff’s lumper’s imagination.
    • And how we interpret our finds in relation to the typology we have accepted.
    • The artists’ conceptions involve knowledge of human/hominin anatomy with a great deal of subjective interpretation.
    • Finally, DNA results are too varied to be conclusive—mtDNA or Y chromosomes