4 Human Evolution After you have ﬁnished reading this chapter, you should be able to: Identify important arboreal adaptations of primates and list the distinguishing features of prosimians, monkeys, and apes. Discuss the development of bipedalism in the early hominids. Compare and contrast the characteristics of fossil hominid species. Mankind stood up ﬁrst and got smarter later. Stephen Jay Gould Introduction The theory of evolution is one of the most important ideas in science. It is also a wonderful example of what science does best: tests ideas against evidence and observations in the real world to determine if the ideas are correct. The study of how the human species evolved is a good example of how science has tested the idea of evolution. II LOOKING FOR HUMAN ORIGINS It is only natural that we are extremely curious about human origins. “Where did I come from?” is a question that occurs to every person at some point in her or his life. “Where did we come from?” is the question we ask now. Two hundred million years ago, dinosaurs populated Earth. Those great reptiles had come to dominate Earth through the adaptive radiation to life on land that occurred after the evolution of the watertight egg. Living alongside the dinosaurs, but close to the ground and very small indeed— about the size of a mouse—were the ﬁrst mammals. (See Figure 4-1.) Like72
Chapter 4 / Human Evolution 73all mammals, these ancestors had hair, nursedtheir young, and likely maintained a steady,high body temperature. The early mammalslived between 220 million and 65 million yearsago. The most common parts of these animalsto be found are their teeth. Because teeth arecovered with hard enamel, they are frequentlypreserved as fossils. Studies of these teeth have Figure 4-1 The ﬁrstshown that early mammals probably ate mammals lived alongside the dinosaurs. LIVING ENVIRONMENT BIOLOGY, 2e/fig. 4-1 s/sinsects, worms, leaves, and fruits. For almost 130 million years, reptiles werethe dominant life-forms on Earth. Then suddenly, at least by geologictime, dinosaurs became extinct. Faced with fewer competitors, an enor-mous variety of mammals evolved, again by adaptive radiation. Thesenew groups of mammals included the carnivores (cats, dogs, seals, bears);hoofed mammals (pigs, deer, cattle); rodents (squirrels, porcupines, mice);whales; elephants; bats; insectivores (shrews, moles); and primates(lemurs, monkeys, apes, and humans). (See Figure 4-2.) Tamarin Coyote Bighorn Sheep Porpoise Squirrel Mole Bat ElephantFigure 4-2 Representatives of the main groups of mammals.
74 Evolution II ADAPTATIONS FOR LIFE IN THE TREES The fossils of the earliest mammals indicate that they had ﬁve separate digits on each of their four feet. As explained in Chapter 2, this was a primitive feature. Fossil remains of various types of later mammals show feet and hands that evolved into hooves for running, feet for digging, wings for flying, or flippers for swimming. Mammals whose feet and hands had fewer than ﬁve digits were said to show advanced features. However, one group of mammals—the primates—through natural selec- tion kept ﬁve digits on their feet and hands. In fact, these digits became even more fully developed. Eventually the thumb could bend over and easily touch the foreﬁnger. This is called an opposable thumb; all pri- mates have this feature. (See Figure 4-3.) What was the great advantage of an opposable thumb? How did this kind of thumb contribute to primate evolution through natural selection? An opposable thumb could hold on to tree branches. Primate evolution began with adaptations suited to an arboreal life, that is, a life in the trees. An opposable thumb was a very important adaptation for an arboreal way of life. Figure 4-3 The opposable thumb is a great advantage to tree-dwelling primates. The order of primates includes prosimians, known as the “lower pri- mates,” and monkeys, apes, and humans, known as the anthropoids or LIVING ENVIRONMENT BIOLOGY, 2e/fig. 4-3 s/s “higher primates.” Many characteristics of modern primates are related to their original arboreal way of living. For example, a baseball pitcher uses an amazing shoulder joint, which ﬁrst evolved to swing from one tree branch to another. Primate hands—with their fingernails, opposable thumbs, and strong, sensitive ﬁngers—helped these animals hang on to branches, hold food, and groom themselves. Primates’ eyes are positioned close together on the front of the face. Observe how the eyes of nonpri- mates, such as horses, are on either side of the head. (See Figure 4-4.) Because their eyes are positioned closer together, primates have stereo- scopic (3-D) vision. Knowing how near or far an object is becomes a crit-
Chapter 4 / Human Evolution 75 Figure 4-4 A horse’s eyes are far apart, while a monkey’s eyes are close together. ical piece of information when an animal needs to jump safely from oneLIVING ENVIRONMENT BIOLOGY, 2e/fig. 4-4 s/s (rev. 10/10/03) tree branch to another. You can observe the advantage of 3-D vision if you close one of your eyes. Notice how the arrangement of objects in front of you seems ﬂatter; you lose a sense of “depth” in your ﬁeld of view. A life lived in trees poses many hardships and dangers, especially for the young who need time to develop their skills. Because they could eas- ily fall to the ground if left on their own, young primates would have trouble surviving. To ensure the survival of the young, a period of parental care is vital. Primates are great parents, caring for their young for a long time. Although humans no longer live in trees, we share many traits from our arboreal origins with other primates, including prolonged care of our young until they are able to live on their own.II A CLOSER LOOK AT PRIMATES Prosimians, monkeys, apes, and humans are the main groups of primates alive today. Current research from molecular biology, combined with fos- sil evidence, indicates that the oldest common ancestor of today’s pri- mates lived between 80 and 90 million years ago, long before the dinosaurs disappeared. Weighing less than 2 pounds, this primate ances- tor, it is theorized, looked like a very small lemur, lived in tropical forests, and was nocturnal (that is, active at night). Prosimians include the lemurs of Madagascar and the lorises, bush babies, and tarsiers of tropical Africa and southern Asia. They are rela- tively small arboreal animals that feed on insects, leaves, fruits, and ﬂow- ers. Like their ancestors, the prosimians are often nocturnal. These
76 Evolution Figure 4-5 This lemur is a type of prosimian, one of the four main groups of primates alive today. animals existed in great numbers in the huge forests north and south of the equator 65 to 38 million years ago. Today, because of the destruction of their forest habitats, many prosimians are in grave danger of becoming extinct. (See Figure 4-5.) Monkeys evolved from prosimian ancestors about 50 million years ago. There are two main groups of monkeys alive today. The New World monkeys, such as capuchins, spider monkeys, squirrel monkeys, and mar- mosets, are found in Central and South America. (See Figure 4-6.) The Old World monkeys, such as baboons, vervets, langurs, and macaques, live in Africa and Asia. The higher primates, also known as hominoids, include all apes and humans. Ape fossils found in East Africa show that these primates evolved from the monkeys of Africa and Asia. The earliest known ape fossils are of an organism called Aegyptopithecus, which means “dawn ape”; they are about 35 million years old. This hominoid lived in trees and was about the size of a cat. Aegyptopithecus migrated across Asia around 25 million years ago. The apes include gibbons, orangutans, chimpanzees, bonobos, and gorillas. Apes are generally larger than monkeys, have larger brains, and lack tails. They can hang upright from tree branches and have relatively long arms and short legs. Because of the close relationship of apes to humans, we are fascinated by their behavior. Several long-term scientiﬁc studies of the apes have
Chapter 4 / Human Evolution 77 Figure 4-6 Some monkeys have grasping tails, an adaptation to living in trees.been completed. It is not only our behavior that is so close to that of theapes; most of our DNA is the same as that of the African apes, too. (SeeFigure 4-7.) Gorillas live in troops of 8 to 24 individuals with one large male asthe leader. Usually peaceful, male gorillas can appear menacing when theythreaten an enemy with screams, broken tree branches, and chest pound-ing. Female gorillas nurse their infants from two to four years. Chimpanzees are thought to be the primates most closely related to Figure 4-7 Chimpanzees— intelligent apes closely related to humans—are known to use tools in the wild.
78 Evolution humans. In fact, new research has shown that human and chimpanzee DNA is almost identical. The main difference is that greater quantities of proteins are produced by human genes, especially within brain cells, than by chimpanzee genes. Chimpanzee behavior also shows the evolution- ary closeness. They live in social groups, and the males and females form temporary bonds to mate. Friends within the group spend long hours grooming each other. Chimps love to play together and are curious, noisy, and outgoing. However, they can also be quite aggressive and do ﬁght with neighboring groups of chimpanzees. Bonobos (sometimes called pygmy chimps) are also very closely related to humans in their genetic makeup and behavior. They are considered highly intelligent and tend to be more peaceful in their social interactions than the chimpanzees. II HOMINIDS: THE EARLIEST HUMANS The rising of the Himalayan mountains and drier weather around 20 mil- lion years ago caused forest areas to diminish in size. At that time, Asian and African apes became separated from one another. Between 14 and 8 million years ago, from within the African group, the common ancestor of humans and chimpanzees evolved. This does not mean that our ances- tors were chimpanzees. We are most closely related to chimpanzees because we shared an ancestor with chimpanzees more recently than with any other animal. Very few details are known about this early stage in the evolution of humans. One of the most important discoveries in human evolution occurred in 1924 when a small fossilized skull was found in a mine in Taung, South Africa. The “Taung child” skull was sent to a skilled neurologist, Raymond Dart, who recognized that the fossil had humanlike features. (See Figure 4-8.) He concluded that this specimen was the fossilized skull of an early human, a type of hominid. The appearance of the skull, the size and shape of the brain case, and the shape of the teeth all showed that this fos- sil was from an early ancestor on the human family tree. Later fossils con- ﬁrmed that the Taung child walked on two feet. More than any other feature, walking on two feet is what makes an early human a hominid. Dart named this 3-million-year-old hominid Australopithecus africanus (southern ape of Africa). It took 25 years for scientists to accept Dart’s conclusions. During that time, many australopithecine fossils were found in different places in Africa. Raymond Dart was indeed correct about the little skull. Discoveries of more fossils provided additional evidence that A. africanus walked upright and had hands and teeth similar to ours.
Chapter 4 / Human Evolution 79 Figure 4-8 Raymond Dart and the Taung skull—the fossilized skull of an early human ancestor. In 1974, a fossil discovered in Ethiopia became famous worldwide. Sig-niﬁcant portions of a 3.18-million-year-old skeleton of a female hominidwere unearthed; she was 1 meter tall with a skull aboutthe size of a softball. This fossil also showed evidence ofupright walking. She was named Lucy after the Beatlessong the scientists were listening to at the time of theirdiscovery! Donald Johanson, of the Cleveland Museumof Natural History, led the team of scientists. Furtherwork showed that Lucy’s species, Australopithecus afaren-sis, was the ancestor of the A. africanus species identiﬁedby Raymond Dart. Recently, several other more ancientspecies have been discovered that bring us ever closer tothe dividing point (about 7 million years ago) betweenhumans and apes. (See Figure 4-9.) The most important point about australopithecinespecies is that hominids walked on two feet, not four,for at least 2 million years without much enlargementof their brain. Bipedalism (walking on two feet) mayhave helped hominids gather food and care for their Figure 4-9 Ayoung more efﬁciently by freeing their hands. Tools were drawing of annot made until much later. That is why evolutionary Australopithecus LIVING ENVIRONMENT BIOLOGY, 2e/fig. 4-9 s/sbiologist Stephen Jay Gould of Harvard University said afarensisthat “Mankind stood up ﬁrst and got smarter later.” skeleton.
80 Evolution Raymond Dart and the Skull of the Taung Child In the 1920s, large amounts of limestone were being dug from the ground in Taung, an area of South Africa. Many human fossils were also being dug up in the limestone quarry. Raymond Dart, an Australian doctor teaching at the medical school in Johannesburg, heard about the fossils. Dart was an expert on the anatomy of the human head and was anxious to examine the fossils—a natural curiosity. Dart contacted the owner of the quarry and, in time, two large boxes of fossils arrived at his home. When he examined the material in the boxes, Dart found a dome-shaped piece of stone and immediately recognized that it was shaped like a brain. In this fossil, Dart saw the folds of tissue that make up the brain and even the blood vessels on the surface. Dart realized what had happened many years before. Long ago, someone had died in the vicinity of this present quarry. Sand and water that contained minerals had entered the skull; eventually these materials hardened into rock in the exact shape of the brain. II WHY DID EARLY HUMANS STAND? Our earliest ancestors lived in the trees. One of the ﬁrst big steps on the path of human evolution occurred when early hominids walked on two feet on the ground. Where, when, how, and why did this occur? For more than 100 years, it has been widely believed that the big step from ape to human occurred the day some apes left the forest. According to this story, the apes had been spending their lives in the manner of most forest animals, enjoying the warm, humid days, with plenty of shade and abundant fruits and berries to eat. For some reason, perhaps because the forest area was decreasing, they now found themselves out on the open, where tall grasses, small shrubs, and occasional trees replaced the forest, in an environment called the savanna. (See Figure 4-10.) It was drier on the savanna, it took longer to ﬁnd food, and predators could see you more easily. Life was harder. To survive, you had to walk upright on two feet. By being upright you could see approaching danger more easily. You also had to get smarter. So here are the early hominids out on the dangerous savanna, while back in the forest the other apes are still doing what they always did, going about picking fruits and berries in an environment that was relatively safe. Interesting story, but does the evidence conﬁrm it? Science is based on proposing ideas, or hypotheses, that can be tested and then seeing if the evidence supports or contradicts the idea. The “savanna hypothesis” is now being thoroughly tested. Evidence against the hypothesis would be
Chapter 4 / Human Evolution 81On close examination, Dart felt that the fossil brain looked like it had comefrom an ape, but he recognized that the fossil also had some similarities toa human brain. The skull, he thought, might provide some clues to thebrain’s origin. Dart looked again in the box that contained the fossilizedbrain. Much to his amazement and delight, he found pieces of the lowerjaw and the skull. However, the front of the fossil skull—the face—wascovered by layers of rock. In a procedure that took several months, Dartchipped away at the rock layers. What he eventually revealed was the faceof a young creature, later dubbed the “Taung Child,” which Dart believedwas an early ancestor of the human species. His ﬁnd turned out to be oneof the most important hominid fossil discoveries ever made, adding crucialdetails to our understanding of human evolution.hominid fossils that showed upright walking and the ability to climb trees.Lucy had curved ﬁngers that might have been adapted for tree climbingeven though she could walk on two feet. To support the savanna hypoth-esis, fossils of other animals and of plants living at the same time as thehominids would have to show that the climate had become drier, thatthe forests had disappeared, and that the savannas remained as anexploitable food source. In some places in Africa, such as Tanzania, where3- to 4-million-year-old footprints of A. afarensis, Lucy’s species, werefound, it was deﬁnitely very dry, with no forests. However, in Ethiopia,Figure 4-10 The savanna has tall grasses, small shrubs, and scattered trees.
82 Evolution where Lucy lived, there were forests as well as open places. Were our bipedal hominid ancestors savanna dwellers or neighbors of other pri- mates that lived in the forests? The oldest hominid fossils found so far, Ardipithecus ramidus, have been dated at 5.8 million years old. The fossils of these individuals, who lived in Ethiopia, show that the skull was balanced at the top of the skeleton for walking erect. Meanwhile, other animal fossils found nearby indicate that A. ramidus deﬁnitely lived in the forest. If careful studies of the A. ramidus bones show that it really did walk upright, the savanna hypoth- esis will be disproved. If some apes began walking on two feet not because they left the for- est for the open savanna, what other explanations could there be? Why did some apes begin walking upright if not to gain a selective advantage out of the forest and on the open savanna? Perhaps standing up on tree branches, as chimps sometimes do, makes it easier to feed. (See Figure 4-11.) Standing erect to threaten an enemy, as gorillas are known to do, may have provided a survival advantage maintained by natural selection. Another explanation that has been offered is that apes that were born with a slightly greater ability to stand up were better able to gather food, even in the forest. Males with this advantage could bring food back to the females with whom they had mated and to their offspring. In this story, the offspring most likely to survive were those of apes that could walk erect. This would be a tremendous evolutionary advantage and could easily have led to the evolution of walking on two feet. Figure 4-11 There are many possible advantages to standing and walking upright, as this young chimpanzee is doing.
Chapter 4 / Human Evolution 83 Once again, science will put this idea to the test, if it becomes a seri- ous hypothesis. Eventually, the question of why bipedal hominids evolved will be answered. By then evolutionary biologists will have new questions to study that have not yet even been asked. Check Your Understanding Why is bipedalism such an important characteristic of hominids?II OUR OWN GENUS None of the fossils that have been discussed so far belong to the genus of modern humans, Homo. To be a hominid, as the australopithecines were, you had to walk on two feet. However, to be a hominid in the genus Homo (from the Latin word for “man”), you also need to have the enlarged brain that sets you apart from the other primates. Lucy’s species, A. afarensis, remained relatively unchanged for almost 1 million years. Then, about 3 million years ago, an adaptive radiation resulted in the Taung child species, A. africanus, and several other aus- tralopithecine species with heavier bones and much wider faces. (See Fig- ure 4-12.) Known as A. robustus and A. boisei, these robust species were ﬁrst discovered by Mary Leakey in Tanzania in 1959 and were dated at 1.8 million years ago, using the potassium-argon radioisotope dating tech- nique. Mary Leakey, her husband Louis, their son Richard, and his wife Meave, are among the most important scientists who have studied the fossil evidence of human origins. Much debate has taken place about how A. robustus and A. boisei ﬁt into the human family tree. Most researchers believe these species to be separate branches on the tree, branches that Figure 4-12 Reconstructed fossils of A. africanus and A. robustus show the differences in their jaws.
84 Evolution ended long ago. These species did not adapt successfully to changing envi- ronmental conditions. As a result, they became extinct. The adaptive radiation that led to the robust, now extinct, australo- pithecine species also led to hominids with larger brain capacities. The size of the Taung child’s brain was about 500 cubic centimeters (cc). These larger hominid skulls, sometimes found along with simple stone tools, were about 650 cc. Great arguments arose when Louis Leakey ﬁrst stated in 1962 that his 1.75-million-year-old fossils from Kenya with the larger brains belonged to the genus Homo. He named the species Homo habilis, meaning “handy man.” Most scientists now accept Leakey’s interpreta- tion. Homo habilis is placed on the human family tree. Although there is little agreement on how the Australopithecus species are related to Homo habilis, it is generally accepted that H. habilis led toward modern humans, evolving ﬁrst into Homo erectus, which later evolved into Homo sapiens (modern humans). All of the early hominid fossils discussed so far have been found only in Africa. Homo erectus was the ﬁrst hominid to migrate from Africa to Asia and into Europe. Fossils of this species were found in Java in 1896 (Java Man), in Beijing, China, in 1929 (Peking Man), and in northern Kenya in 1984, where the skeleton of a 12-year-old-boy of this species who died 1.6 million years ago was found in 1984. Homo erectus had a body skeleton much like that of modern humans. The 12-year-old boy was 1.7 meters tall and walked like modern humans. Differences are found in the size of the skull. Their brains, 700 to 1200 cc, were much larger than those of earlier species and almost as large as those of modern humans. (See Figure 4-13.) However, their jaws and teeth were much larger than those of modern humans. Homo erectus skulls had thick, low foreheads and sloping chins. 1,400 Range 1,200 Brain volume (cm3) 1,000 800 600 400 Australopithecus Homo Homo Homo sapiens Homo sapiens habilis erectus neanderthalensis sapiens Figure 4-13 A comparison of the brain volume of several important hominids. LIVING ENVIRONMENT BIOLOGY, 2e/fig. 4-13 s/s
Chapter 4 / Human Evolution 85 By this time, the larger brains of our hominid ancestors showed that they were definitely becoming more intelligent. Much more efficient tools, such as the hand ax (a stone that has a surface for grip- ping and several cutting edges), are often found with H. erectus fossils. (See Figure 4-14.) These are the ﬁrst hominids known to build fires, live in caves, and clothe themselves. With these skills they were able to migrate to colder northern climates Figure 4-14 A hand ax; such found outside Africa. H. erectus existed for tools were made and used by a long time on Earth, from 1.8 million Homo erectus. years ago to about 300,000 years ago. There is much debate about recent human evolution. In 1997, researchers in northern Spain announced the discovery of yet another ancestor of modern humans, Homo antecessor. They think that the 800,000-year-old fossils from Spain belong to the common ancestor of modern humans and other extinct hominids. Today, there are two main views of human evolution. One group of scientists sees it as a ladder, with one species at a time leading to the next species. The other group sees human evolution as a tree, with several branches. One branch leads to modern humans; the other branches lead to extinct hominid species.II THE HUMAN SPECIES The migration of humans from one place to another on Earth occurred long before travel by ship and plane. These early travelers went over land on foot. Helping them and perhaps encouraging them to move on were the Ice Ages. These were periods when Earth’s climate cooled, causing great sheets of ice to move over the land. The levels of the oceans dropped as water remained on land frozen as ice. Thus humans could walk on places once covered by oceans, traveling to the island of Java, to the island continent of Australia, and eventually walking east across the land bridge in the northern Paciﬁc Ocean to North America. The last Ice Age began about 1 million years ago and included several periods of deep cold. Extensive ice sheets covered much of North America and Europe. Hominids living after Homo erectus colonized a variety of places with varying climates, including Africa, Asia, Europe, and Australia. As Earth’s climate warmed, and the ice sheets melted and retreated, early humans extended their range. These individuals included the Neanderthals, whose
86 Evolution Figure 4-15 A drawing of how a Neanderthal might have looked. fossils have been found throughout Europe1andifthe,YGOLOIB EastMNORwho GNIVIL s/s 5 -4 .g /e2 Middle TNE and IVNE had much larger brain sizes than H. erectus—brains about the size of mod- ern human brains. Some scientists consider them to be members of our species, Homo sapiens. Others think they made up a separate species, Homo neanderthalensis. Their bodies were similar to those of modern humans. However, their faces looked different, with heavier ridges over the eyes, a long, low skull, and small cheekbones. (See Figure 4-15.) Because of these differences, these humans, living from about 400,000 to 35,000 years ago, are usually called early or “archaic” Homo sapiens. They may have been descendants of the newly discovered species Homo antecessor, representing one branch of the human tree that has ended. Neanderthals wore animal skins, made better and more varied tools than H. erectus did, and buried their dead. We know that they purposely left weapons and ﬂowers with their dead. These were individuals who thought about things, including life after death. All fossils of hominids that lived during the past 30,000 years are like modern humans both in body and skull size and shape. Modern humans have an average brain size of about 1350 cc. One of the best-known groups of these “modern” Homo sapiens is the Cro-Magnons, named for the place in France where their fossils were ﬁrst found. Other modern Homo sapiens fossils, up to 100,000 years old, have been found in Israel and throughout Africa. Cro-Magnons are well known for their advanced tools made of stone, bone, and ivory. These tools included spears, ﬁshing hooks, and needles. In addition, the magniﬁcent cave paintings of these
Chapter 4 / Human Evolution 87 Figure 4-16 Cro-Magnon drawings show the beautiful forms of many different kinds of animals, and the skill of the early human artists. humans, which show the beautiful forms of many different kinds of ani- mals, give us a sense that we are seeing humans like ourselves. Cro- Magnon fossils are the remains of people like modern humans who looked and wondered at the world around them, sometimes symbolizing their thoughts and feelings, for whatever reasons, in the form of art. (See Figure 4-16.)II MORE QUESTIONS AND SOME ANSWERS The study of the history of human evolution is full of controversies, none being debated more intensely than the question of where modern Homo sapiens ﬁrst evolved. This question is considered to be a valid scientiﬁc question because it is assumed that it can be put to the test. It is thought that evidence will eventually be found to answer the question. Then it will no longer be just a matter of opinion. This process is an important part of the scientiﬁc method. One hypothesis is that the populations of H. erectus that had migrated from Africa to a variety of places on Earth each gave rise to archaic and then modern H. sapiens independently. In this “multiregional model,” human races in each of these areas arose from different populations. Breeding between the various populations would have allowed for gene flow and prevented speciation from occurring. Today, all human races on Earth belong to one species. The other hypothesis is that modern H. sapiens evolved from H. erec- tus in just one place, Africa. According to this “Out of Africa” model, mod- ern H. sapiens, moving out from Africa, replaced the archaic H. sapiens in the various places where they met. This is a very different proposal. It would mean that the varieties or races in the world’s human population arose in just the last 100,000 years since H. sapiens left Africa, not more
88 Evolution than 1 million years ago when H. erectus began migrating. Anthropologists throughout the world strongly support one or the other of the possibili- ties. Each opposing side claims that the evidence supports its theory. This scientiﬁc question continues to be studied. Another fascinating question about human evolution concerns lan- guage. When did humans begin to speak? The answer to this question remains a mystery. Charles Darwin suggested that human speech evolved from animal cries. Critics at the time, who were opposed to Darwin’s views, called this the ”bow-wow” theory. Noam Chomsky, a famous pro- fessor from the Massachusetts Institute of Technology (MIT), has for more than 40 years claimed that the rules of human language are built-in, not learned. How these innate rules could have evolved is difﬁcult to explain. In 1994, another MIT professor, Steven Pinker, defended the idea that lan- guage evolved by natural selection, but said he could only guess that it may have begun with primate calls. Other more recent suggestions are that language evolved from primate grooming. Apes and monkeys use physical contact with each other through grooming to establish social connections. Making sounds might have become a more efﬁcient way of doing this. In 2002, a New Zealand psychologist, Michael Corbollis, pro- posed the idea that human language began with hand and face gestures. He said that the earliest hominid, some 6 million years ago, could not yet have spoken, but would have had the ability to make voluntary hand and face movements. A kind of sign language could have developed, eventu- ally switching from gestures to true speech about 50,000 years ago, after modern humans had evolved. In spite of these fascinating theories, the question of where language comes from may simply be unanswerable. If that is the case, then this mystery cannot be considered a valid scientiﬁc question. Nevertheless, despite the questions that remain unanswered, we have been richly rewarded to date in learning so much about the fascinating story of where we came from.
LABORATORY INVESTIGATION 4How Can We Determine theSequence of Hominid Evolution?INTRODUCTIONIn spite of the very incomplete fossil record, scientists who study humanevolution have been able to draw some remarkable pictures of what thedifferent hominid species might have looked like. Studying these pictureshelps us develop a deeper understanding of human evolution.One misconception that must be avoided, as the pictures are studied, isthe idea that human evolution is like a ladder with a series of steps lead-ing from the most ancient hominid species directly to our own species,Homo sapiens. This misconception often has been illustrated as a paradeof fossil hominids, with the specimens in the parade becoming moremodern as they march across the page.The more accurate understanding of human evolution is that differenthominid species often existed together at the same time and in the sameplace. Also, many of these species evolved along certain pathways thateventually led to dead ends. Rather than a ladder, a better diagram ofhominid evolution would be more like a bush having many branches,with our species being at the end of the only branch that still survives.MATERIALS“Hominid Species A–H” and “Hominid Data Sheet” handouts (from theTeacher’s Manual), scissors, glue or tape, unlined paperPROCEDURE1. Examine the drawings A–H. These are artistic impressions based on fossil evidence of different hominid species. Examine them closely. Identify three characteristics that seem to differ and three characteris- tics that seem to be similar from one ﬁgure to another. Share your list of observed characteristics with your group.2. Determine which ﬁgure you think represents the earliest hominid. Determine which ﬁgure you think represents the most recent hominid. Give reasons for your choices. Share your choices with the group, dis- cuss all opinions, and then reach a consensus. Chapter 4 / Human Evolution 89
3. Cut out the ﬁgures and, as a group, arrange them in a sequence from earliest to most recent. Glue or tape the hominid drawings to the unlined paper in the order in which you have arranged them. Make a list of the criteria that guided your choices. Compare your sequence with those of the other groups. Discuss any differences. 4. Compare your time sequence to the one determined by scientists, shown in the Chronology of Hominid Evolution table on the Hominid Data Sheet. Based on this set of data, would you change your sequence? Explain. INTERPRETIVE QUESTIONS 1. Draw a horizontal timeline that is 15 cm long. Let 3 cm equal 1 mil- lion years, going from 5 million years ago (mya) to the present. Place the letter for each hominid species listed in the Chronology of Hominid Evolution table at the correct place on your timeline. 2. Study Diagram A and Diagram B on the Hominid Data Sheet. These diagrams represent alternate ideas of the evolutionary route from ancient hominids to modern humans. Write a comparison of these two different interpretations of human origins. 3. Explain why you think the three different characteristics and three similar characteristics you observed may be important for determining the sequence of hominid evolution.90 Evolution
Chapter 4 / Human Evolution 91II CHAPTER 4 REVIEW Answer these questions on a separate sheet of paper. VOCABULARY The following list contains all of the boldfaced terms in this chapter. Deﬁne each of these terms in your own words. arboreal, bipedalism, hominid, hominoids, mammals, opposable, robust PART A—MULTIPLE CHOICE Choose the response that best completes the sentence or answers the question. 1. Humans belong to the class of animals known as a. mammals b. carnivores c. rodents d. invertebrates. 2. Which is not considered an adaptation for arboreal life? a. opposable thumb b. ﬁve digits on each foot c. stereoscopic vision d. prolonged period of parental care 3. The famous fossil known as Lucy belongs to the species a. Homo antecessor b. Homo erectus c. Australopithecus afarensis d. Australopithecus africanus. 4. Primates include a. porcupines, squirrels, and mice b. pigs, sheep, and deer c. humans, lemurs, and chimpanzees d. shrews, moles, and hedgehogs. 5. The “Taung child” skull is signiﬁcant because it a. was the ﬁrst fossil of Homo habilis to be discovered b. belongs to one of the earliest types of hominids c. showed that the earliest primates in the human line walked on all fours d. indicated that members of its species made tools. 6. The earliest mammals a. ﬁrst appeared 65 million years ago b. varied greatly in size, appearance, and lifestyle c. had ﬁve digits on their front feet and four on their back feet d. are known primarily from fossil teeth. 7. The earliest hominoid fossils are of a. Australopithecus b. Aegyptopithecus c. Ardipithecus d. Homo. 8. Chimpanzees are classiﬁed as a. prosimians b. monkeys c. hominoids d. hominids. 9. The oldest hominid fossils found so far are about a. 35 million years old b. 5.8 million years old c. 1.75 million years old d. 400,000 years old.
92 Evolution 10. Which of these is not a general characteristic of mammals? a. complex life cycle with alternation of generations b. nursing their young with milk c. maintaining a high, steady body temperature d. body covered in hair 11. An opposable thumb a. can bend and easily touch the foreﬁnger b. is an adaptation for arboreal life c. is a major characteristic of primates d. all of these. 12. Animals that live in the trees are a. nocturnal b. arboreal c. diurnal d. neanderthal. 13. Which of the following statements is true? a. Hominoids include lemurs, lorises, and tarsiers. b. Prosimians are higher primates. c. Monkeys include orangutans and gorillas. d. Prosimians, monkeys, and apes are all primates. 14. Homo erectus a. could build ﬁres b. probably did not make tools c. lived in Africa only d. is a side branch on the human family tree and not a direct ancestor of modern humans. 15. The “Out of Africa” model of human evolution a. is not supported by mitochondrial DNA evidence b. states that modern Homo sapiens evolved in Africa only c. is supported by DNA evidence from Neanderthal fossils d. states that breeding among populations of archaic Homo sapiens allowed for gene ﬂow and prevented speciation. PART B—CONSTRUCTED RESPONSE Use the information in the chapter to respond to these items. Australopithecus robustus A Australopithecus B C Homo sapiens D africanus (archaic) Australopithecus Homo Homo boisei antecessor neanderthalensis 16. The diagram shows one possible pathway of human evolution. What hominid names should appear in boxes A, B, C, and D in the diagram? 17. What do you think the discoverers of Homo antecessor might think LIVING ENVIRONMENT BIOLOGY, 2e/fig. 4-Q16 s/s about the view of human evolution expressed in the diagram? 18. What is the “savanna hypothesis”? What sort of evidence will prove or disprove it?
Chapter 4 / Human Evolution 9319. How do australopithecine fossils support the hypothesis that hominids “stood up ﬁrst and got smarter later”?20. Why are Neanderthals considered archaic Homo sapiens and Cro- Magnons considered modern Homo sapiens?PART C—READING COMPREHENSIONBase your answers to questions 21 through 23 on the information below andon your knowledge of biology. Source: Science News (May 10, 2003): vol.157, p. 302. New Fossil Weighs in on Primate Origins Excavations in Wyoming have yielded the partial skeleton of a 55- million-year-old primate that probably was a close relative of the ances- tor of modern monkeys, apes, and people. The creature was built for hanging tightly onto tree branches, not for leaping from tree to tree, as some scientists had speculated, based on earlier fragmentary ﬁnds. Also, despite expectations, the ancient primate didn’t have eyes specialized for spotting insects and other prey. Jonathan I. Bloch and Doug M. Boyer, both of the University of Michi- gan in Ann Arbor, unearthed the new specimen. It belonged to a group of small, long-tailed primates that lived just before the evolution of crea- tures with traits characteristic of modern primates—relatively large brains, grasping hands and feet with nails instead of claws, forward- facing eyes to enhance vision, and limbs capable of prodigious leaping. The new ﬁnd, in the genus Carpolestes, had long hands and feet with opposable digits, Bloch and Boyer report in the Nov. 22 Science. The animal grew nails on its opposable digits, and claws on its other ﬁngers and toes. Unlike later primates, Carpolestes had side-facing eyes and lacked hind limbs designed for leaping.21. State two characteristics of the 55-million-year-old Wyoming primate (fossil) that are different from what scientists had expected.22. Explain what characteristics are considered to be those of modern primates.23. State two characteristics of the ancient Wyoming primate that indicate it was not a member of the group of modern primates.