File 473

After you have finished reading this chapter, you should be able to:

Identify important arboreal adaptations of primates and list thedistinguishing 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

IntroductionThe theory of evolution is one of the most important ideas in science. Itis also a wonderful example of what science does best: tests ideas againstevidence and observations in the real world to determine if the ideas arecorrect. The study of how the human species evolved is a good exampleof how science has tested the idea of evolution.

nn 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 atsome point in her or his life. “Where did we come from?” is the questionwe ask now.

Two hundred million years ago, dinosaurs populated Earth. Those greatreptiles had come to dominate Earth through the adaptive radiation tolife on land that occurred after the evolution of the watertight egg. Livingalongside 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.) Like

72

Human Evolution4

all 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 haveshown that early mammals probably ateinsects, 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.)

Chapter 4 / Human Evolution 73

LIVING ENVIRONMENT BIOLOGY, 2e/fig. 4-1 s/s

Figure 4-1 The firstmammals lived alongsidethe dinosaurs.

Tamarin

Elephant

Squirrel

Bat

Porpoise

CoyoteBighorn Sheep

Mole

Figure 4-2 Representatives of the main groups of mammals.

nn ADAPTATIONS FOR LIFE IN THE TREES

The fossils of the earliest mammals indicate that they had five separatedigits on each of their four feet. As explained in Chapter 2, this was aprimitive feature. Fossil remains of various types of later mammals showfeet and hands that evolved into hooves for running, feet for digging,wings for flying, or flippers for swimming. Mammals whose feet andhands 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 becameeven more fully developed. Eventually the thumb could bend over andeasily touch the forefinger. This is called an opposable thumb; all pri-mates have this feature. (See Figure 4-3.) What was the great advantage ofan opposable thumb? How did this kind of thumb contribute to primateevolution through natural selection? An opposable thumb could hold onto tree branches. Primate evolution began with adaptations suited to anarboreal life, that is, a life in the trees. An opposable thumb was a veryimportant adaptation for an arboreal way of life.

The order of primates includes prosimians, known as the “lower pri-mates,” and monkeys, apes, and humans, known as the anthropoids or“higher primates.” Many characteristics of modern primates are related totheir original arboreal way of living. For example, a baseball pitcher usesan amazing shoulder joint, which first evolved to swing from one treebranch to another. Primate hands—with their fingernails, opposablethumbs, and strong, sensitive fingers—helped these animals hang on tobranches, hold food, and groom themselves. Primates’ eyes are positionedclose 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-

74 Evolution


Figure 4-3 The opposable thumb isa great advantage to tree-dwellingprimates.

ical piece of information when an animal needs to jump safely from onetree branch to another. You can observe the advantage of 3-D vision ifyou close one of your eyes. Notice how the arrangement of objects infront of you seems flatter; you lose a sense of “depth” in your field ofview.

A life lived in trees poses many hardships and dangers, especially forthe 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 havetrouble surviving. To ensure the survival of the young, a period of parentalcare is vital. Primates are great parents, caring for their young for a longtime. Although humans no longer live in trees, we share many traits fromour arboreal origins with other primates, including prolonged care of ouryoung until they are able to live on their own.

nn A CLOSER LOOK AT PRIMATES

Prosimians, monkeys, apes, and humans are the main groups of primatesalive 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 thedinosaurs 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, bushbabies, 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


LIVING ENVIRONMENT BIOLOGY, 2e/fig. 4-4 s/s (rev. 10/10/03)

Figure 4-4 A horse’s eyes are far apart, while amonkey’s eyes are closetogether.

animals existed in great numbers in the huge forests north and south ofthe equator 65 to 38 million years ago. Today, because of the destructionof their forest habitats, many prosimians are in grave danger of becomingextinct. (See Figure 4-5.)

Monkeys evolved from prosimian ancestors about 50 million yearsago. There are two main groups of monkeys alive today. The New Worldmonkeys, such as capuchins, spider monkeys, squirrel monkeys, and mar-mosets, are found in Central and South America. (See Figure 4-6.) TheOld World monkeys, such as baboons, vervets, langurs, and macaques,live in Africa and Asia.

The higher primates, also known as hominoids, include all apes andhumans. Ape fossils found in East Africa show that these primates evolvedfrom the monkeys of Africa and Asia. The earliest known ape fossils areof an organism called Aegyptopithecus, which means “dawn ape”; they areabout 35 million years old. This hominoid lived in trees and was about thesize of a cat. Aegyptopithecus migrated across Asia around 25 million yearsago.

The apes include gibbons, orangutans, chimpanzees, bonobos, andgorillas. Apes are generally larger than monkeys, have larger brains, andlack tails. They can hang upright from tree branches and have relativelylong arms and short legs.

Because of the close relationship of apes to humans, we are fascinatedby their behavior. Several long-term scientific studies of the apes have

76 Evolution

Figure 4-5 This lemur is a typeof prosimian, one of the four maingroups of primates alive today.

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 4-6 Some monkeyshave grasping tails, anadaptation to living in trees.

Figure 4-7 Chimpanzees—intelligent apes closely related tohumans—are known to use toolsin the wild.

humans. In fact, new research has shown that human and chimpanzeeDNA is almost identical. The main difference is that greater quantities ofproteins are produced by human genes, especially within brain cells, thanby chimpanzee genes. Chimpanzee behavior also shows the evolution-ary closeness. They live in social groups, and the males and females formtemporary bonds to mate. Friends within the group spend long hoursgrooming each other. Chimps love to play together and are curious, noisy,and outgoing. However, they can also be quite aggressive and do fightwith neighboring groups of chimpanzees. Bonobos (sometimes calledpygmy chimps) are also very closely related to humans in their geneticmakeup and behavior. They are considered highly intelligent and tend tobe more peaceful in their social interactions than the chimpanzees.

nn 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, Asianand African apes became separated from one another. Between 14 and 8million years ago, from within the African group, the common ancestorof humans and chimpanzees evolved. This does not mean that our ances-tors were chimpanzees. We are most closely related to chimpanzeesbecause we shared an ancestor with chimpanzees more recently than withany other animal. Very few details are known about this early stage in theevolution of humans.

One of the most important discoveries in human evolution occurredin 1924 when a small fossilized skull was found in a mine in Taung, SouthAfrica. The “Taung child” skull was sent to a skilled neurologist, RaymondDart, who recognized that the fossil had humanlike features. (See Figure4-8.) He concluded that this specimen was the fossilized skull of an earlyhuman, a type of hominid. The appearance of the skull, the size andshape 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 otherfeature, 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’sconclusions. During that time, many australopithecine fossils were foundin different places in Africa. Raymond Dart was indeed correct about thelittle skull. Discoveries of more fossils provided additional evidence thatA. africanus walked upright and had hands and teeth similar to ours.

78 Evolution

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 theiryoung more efficiently by freeing their hands. Tools werenot made until much later. That is why evolutionarybiologist Stephen Jay Gould of Harvard University saidthat “Mankind stood up first and got smarter later.”


Figure 4-8 Raymond Dart and theTaung skull—the fossilized skull ofan early human ancestor.


Figure 4-9 Adrawing of anAustralopithecusafarensisskeleton.

nn WHY DID EARLY HUMANS STAND?

Our earliest ancestors lived in the trees. One of the first big steps on thepath of human evolution occurred when early hominids walked on twofeet on the ground. Where, when, how, and why did this occur?

For more than 100 years, it has been widely believed that the big stepfrom ape to human occurred the day some apes left the forest. Accordingto this story, the apes had been spending their lives in the manner ofmost forest animals, enjoying the warm, humid days, with plenty ofshade and abundant fruits and berries to eat. For some reason, perhapsbecause the forest area was decreasing, they now found themselves out onthe open, where tall grasses, small shrubs, and occasional trees replacedthe forest, in an environment called the savanna. (See Figure 4-10.)

It was drier on the savanna, it took longer to find food, and predatorscould see you more easily. Life was harder. To survive, you had to walkupright on two feet. By being upright you could see approaching dangermore easily. You also had to get smarter. So here are the early hominidsout on the dangerous savanna, while back in the forest the other apes arestill doing what they always did, going about picking fruits and berries inan environment that was relatively safe.

Interesting story, but does the evidence confirm it? Science is basedon proposing ideas, or hypotheses, that can be tested and then seeing ifthe evidence supports or contradicts the idea. The “savanna hypothesis”is now being thoroughly tested. Evidence against the hypothesis would be

In the 1920s, large amounts of limestone were being dug from the groundin Taung, an area of South Africa. Many human fossils were also being dugup in the limestone quarry. Raymond Dart, an Australian doctor teaching atthe medical school in Johannesburg, heard about the fossils. Dart was anexpert on the anatomy of the human head and was anxious to examine thefossils—a natural curiosity. Dart contacted the owner of the quarry and, intime, two large boxes of fossils arrived at his home.

When he examined the material in the boxes, Dart found a dome-shapedpiece of stone and immediately recognized that it was shaped like a brain. Inthis fossil, Dart saw the folds of tissue that make up the brain and even theblood vessels on the surface. Dart realized what had happened many yearsbefore. Long ago, someone had died in the vicinity of this present quarry.Sand and water that contained minerals had entered the skull; eventuallythese materials hardened into rock in the exact shape of the brain.

O

80 Evolution

Raymond Dart and the Skull of the Taung Child

On 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.

where Lucy lived, there were forests as well as open places. Were ourbipedal 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 beendated at 5.8 million years old. The fossils of these individuals, who livedin Ethiopia, show that the skull was balanced at the top of the skeletonfor walking erect. Meanwhile, other animal fossils found nearby indicatethat 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? Whydid some apes begin walking upright if not to gain a selective advantageout of the forest and on the open savanna? Perhaps standing up on treebranches, as chimps sometimes do, makes it easier to feed. (See Figure4-11.) 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 bornwith 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 tothe females with whom they had mated and to their offspring. In thisstory, the offspring most likely to survive were those of apes that couldwalk erect. This would be a tremendous evolutionary advantage and couldeasily have led to the evolution of walking on two feet.

82 Evolution

Figure 4-11 There are manypossible advantages to standingand walking upright, as thisyoung chimpanzee is doing.

Once again, science will put this idea to the test, if it becomes a seri-ous hypothesis. Eventually, the question of why bipedal hominidsevolved will be answered. By then evolutionary biologists will have newquestions to study that have not yet even been asked.

nn OUR OWN GENUS

None of the fossils that have been discussed so far belong to the genus ofmodern 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 brainthat sets you apart from the other primates.

Lucy’s species, A. afarensis, remained relatively unchanged for almost1 million years. Then, about 3 million years ago, an adaptive radiationresulted 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 werefirst discovered by Mary Leakey in Tanzania in 1959 and were dated at1.8 million years ago, using the potassium-argon radioisotope dating tech-nique. Mary Leakey, her husband Louis, their son Richard, and his wifeMeave, are among the most important scientists who have studied thefossil evidence of human origins. Much debate has taken place about howA. robustus and A. boisei fit into the human family tree. Most researchersbelieve these species to be separate branches on the tree, branches that

Check Your Understanding

Why is bipedalism such an important characteristic of hominids?


Figure 4-12 Reconstructed fossils of A. africanus and A. robustusshow the differences in their jaws.

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. Thesize of the Taung child’s brain was about 500 cubic centimeters (cc). Theselarger hominid skulls, sometimes found along with simple stone tools,were about 650 cc. Great arguments arose when Louis Leakey first statedin 1962 that his 1.75-million-year-old fossils from Kenya with the largerbrains 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 islittle agreement on how the Australopithecus species are related to Homohabilis, 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 onlyin Africa. Homo erectus was the first hominid to migrate from Africa toAsia and into Europe. Fossils of this species were found in Java in 1896(Java Man), in Beijing, China, in 1929 (Peking Man), and in northernKenya in 1984, where the skeleton of a 12-year-old-boy of this specieswho 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 thoseof modern humans. (See Figure 4-13.) However, their jaws and teeth weremuch larger than those of modern humans. Homo erectus skulls had thick,low foreheads and sloping chins.

84 Evolution


Australopithecus Homo erectus

Homo habilis

Homo sapiens neanderthalensis

Homo sapiens sapiens

Range

Bra

invo

lum

e(c

m3 )

1,400

1,200

1,000

800

600

400

Figure 4-13 A comparison of the brain volume of several important hominids.

By this time, the larger brains of ourhominid ancestors showed that they weredefinitely becoming more intelligent.Much more efficient tools, such as thehand ax (a stone that has a surface for grip-ping and several cutting edges), are oftenfound with H. erectus fossils. (See Figure4-14.) These are the first hominids knownto build fires, live in caves, and clothethemselves. With these skills they wereable to migrate to colder northern climatesfound outside Africa. H. erectus existed fora long time on Earth, from 1.8 millionyears 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 anotherancestor of modern humans, Homo antecessor. They think that the800,000-year-old fossils from Spain belong to the common ancestor ofmodern humans and other extinct hominids. Today, there are two mainviews of human evolution. One group of scientists sees it as a ladder, withone species at a time leading to the next species. The other group seeshuman evolution as a tree, with several branches. One branch leads tomodern humans; the other branches lead to extinct hominid species.

nn THE HUMAN SPECIES

The migration of humans from one place to another on Earth occurredlong before travel by ship and plane. These early travelers went over landon foot. Helping them and perhaps encouraging them to move on werethe Ice Ages. These were periods when Earth’s climate cooled, causinggreat sheets of ice to move over the land. The levels of the oceans droppedas water remained on land frozen as ice. Thus humans could walk onplaces once covered by oceans, traveling to the island of Java, to the islandcontinent of Australia, and eventually walking east across the land bridgein the northern Pacific Ocean to North America. The last Ice Age beganabout 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 withvarying climates, including Africa, Asia, Europe, and Australia. As Earth’sclimate warmed, and the ice sheets melted and retreated, early humansextended their range. These individuals included the Neanderthals, whose


Figure 4-14 A hand ax; suchtools were made and used byHomo erectus.

fossils have been found throughout Europe and the Middle East and whohad much larger brain sizes than H. erectus—brains about the size of mod-ern human brains. Some scientists consider them to be members of ourspecies, Homo sapiens. Others think they made up a separate species, Homoneanderthalensis. Their bodies were similar to those of modern humans.However, their faces looked different, with heavier ridges over the eyes, along, low skull, and small cheekbones. (See Figure 4-15.) Because of thesedifferences, these humans, living from about 400,000 to 35,000 years ago,are usually called early or “archaic” Homo sapiens. They may have beendescendants of the newly discovered species Homo antecessor, representingone branch of the human tree that has ended.

Neanderthals wore animal skins, made better and more varied toolsthan H. erectus did, and buried their dead. We know that they purposelyleft weapons and flowers with their dead. These were individuals whothought about things, including life after death.

All fossils of hominids that lived during the past 30,000 years are likemodern humans both in body and skull size and shape. Modern humanshave an average brain size of about 1350 cc. One of the best-knowngroups of these “modern” Homo sapiens is the Cro-Magnons, named forthe place in France where their fossils were first found. Other modernHomo sapiens fossils, up to 100,000 years old, have been found in Israeland throughout Africa. Cro-Magnons are well known for their advancedtools made of stone, bone, and ivory. These tools included spears, fishinghooks, and needles. In addition, the magnificent cave paintings of these

86 Evolution


Figure 4-15 A drawing of how aNeanderthal might have looked.

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 wholooked and wondered at the world around them, sometimes symbolizingtheir thoughts and feelings, for whatever reasons, in the form of art. (SeeFigure 4-16.)

nn MORE QUESTIONS AND SOME ANSWERS

The study of the history of human evolution is full of controversies, nonebeing debated more intensely than the question of where modern Homosapiens first evolved. This question is considered to be a valid scientificquestion because it is assumed that it can be put to the test. It is thoughtthat evidence will eventually be found to answer the question. Then itwill no longer be just a matter of opinion. This process is an importantpart of the scientific method. One hypothesis is that the populations ofH. erectus that had migrated from Africa to a variety of places on Eartheach gave rise to archaic and then modern H. sapiens independently. Inthis “multiregional model,” human races in each of these areas arose fromdifferent populations. Breeding between the various populations wouldhave 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 inthe various places where they met. This is a very different proposal. Itwould mean that the varieties or races in the world’s human populationarose in just the last 100,000 years since H. sapiens left Africa, not more


Figure 4-16 Cro-Magnondrawings show the beautifulforms of many differentkinds of animals, and theskill of the early humanartists.

than 1 million years ago when H. erectus began migrating. Anthropologiststhroughout the world strongly support one or the other of the possibili-ties. Each opposing side claims that the evidence supports its theory. Thisscientific question continues to be studied.

Another fascinating question about human evolution concerns lan-guage. When did humans begin to speak? The answer to this questionremains a mystery. Charles Darwin suggested that human speech evolvedfrom animal cries. Critics at the time, who were opposed to Darwin’sviews, called this the ”bow-wow” theory. Noam Chomsky, a famous pro-fessor from the Massachusetts Institute of Technology (MIT), has for morethan 40 years claimed that the rules of human language are built-in, notlearned. 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 itmay have begun with primate calls. Other more recent suggestions arethat language evolved from primate grooming. Apes and monkeys usephysical contact with each other through grooming to establish socialconnections. Making sounds might have become a more efficient way ofdoing 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 yethave spoken, but would have had the ability to make voluntary hand andface movements. A kind of sign language could have developed, eventu-ally switching from gestures to true speech about 50,000 years ago, aftermodern humans had evolved.

In spite of these fascinating theories, the question of where languagecomes from may simply be unanswerable. If that is the case, then thismystery cannot be considered a valid scientific question. Nevertheless,despite the questions that remain unanswered, we have been richlyrewarded to date in learning so much about the fascinating story of wherewe came from.

88 Evolution


LABORATORY INVESTIGATION 4How Can We Determine the Sequence of Hominid Evolution?

INTRODUCTION

In 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 paper

PROCEDURE

1. Examine the drawings A–H. These are artistic impressions based onfossil 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 listof 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.

90 Evolution

3. Cut out the figures and, as a group, arrange them in a sequence fromearliest to most recent. Glue or tape the hominid drawings to theunlined paper in the order in which you have arranged them. Make alist of the criteria that guided your choices. Compare your sequencewith 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 HominidData 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. Placethe letter for each hominid species listed in the Chronology ofHominid Evolution table at the correct place on your timeline.

2. Study Diagram A and Diagram B on the Hominid Data Sheet. Thesediagrams represent alternate ideas of the evolutionary route fromancient hominids to modern humans. Write a comparison of thesetwo different interpretations of human origins.

3. Explain why you think the three different characteristics and threesimilar characteristics you observed may be important for determiningthe sequence of hominid evolution.

nn 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. Defineeach 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. mammalsb. 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. stereoscopicvision d. prolonged period of parental care

3. The famous fossil known as Lucy belongs to the speciesa. Homo antecessor b. Homo erectus c. Australopithecus afarensisd. Australopithecus africanus.

4. Primates include a. porcupines, squirrels, and miceb. pigs, sheep, and deer c. humans, lemurs, and chimpanzeesd. shrews, moles, and hedgehogs.

5. The “Taung child” skull is significant because it a. was the firstfossil of Homo habilis to be discovered b. belongs to one of theearliest types of hominids c. showed that the earliest primates inthe human line walked on all fours d. indicated that members ofits species made tools.

6. The earliest mammals a. first appeared 65 million years agob. varied greatly in size, appearance, and lifestyle c. had fivedigits on their front feet and four on their back feet d. are knownprimarily from fossil teeth.

7. The earliest hominoid fossils are of a. Australopithecusb. Aegyptopithecus c. Ardipithecus d. Homo.

8. Chimpanzees are classified as a. prosimians b. monkeysc. hominoids d. hominids.

9. The oldest hominid fossils found so far are about a. 35 millionyears old b. 5.8 million years old c. 1.75 million years oldd. 400,000 years old.


10. Which of these is not a general characteristic of mammals?a. complex life cycle with alternation of generations b. nursingtheir young with milk c. maintaining a high, steady bodytemperature d. body covered in hair

11. An opposable thumb a. can bend and easily touch the forefingerb. is an adaptation for arboreal life c. is a major characteristic ofprimates d. all of these.

12. Animals that live in the trees are a. nocturnal b. arborealc. diurnal d. neanderthal.

13. Which of the following statements is true? a. Hominoids includelemurs, 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 toolsc. lived in Africa only d. is a side branch on the human familytree and not a direct ancestor of modern humans.

15. The “Out of Africa” model of human evolution a. is notsupported by mitochondrial DNA evidence b. states that modernHomo sapiens evolved in Africa only c. is supported by DNAevidence from Neanderthal fossils d. states that breeding amongpopulations of archaic Homo sapiens allowed for gene flow andprevented speciation.

PART B—CONSTRUCTED RESPONSE

Use the information in the chapter to respond to these items.

16. The diagram shows one possible pathway of human evolution.What hominid names should appear in boxes A, B, C, and D in thediagram?

17. What do you think the discoverers of Homo antecessor might thinkabout the view of human evolution expressed in the diagram?

18. What is the “savanna hypothesis”? What sort of evidence willprove or disprove it?

92 Evolution

LIVING ENVIRONMENT BIOLOGY, 2e/fig. 4-Q16 s/s

A B C D

Australopithecus robustus

Australopithecus africanus

Homo sapiens (archaic)

Homo neanderthalensis

Homo antecessor

Australopithecus boisei

19. How do australopithecine fossils support the hypothesis thathominids “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 COMPREHENSION

Base 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.

21. State two characteristics of the 55-million-year-old Wyomingprimate (fossil) that are different from what scientists had expected.

22. Explain what characteristics are considered to be those of modernprimates.

23. State two characteristics of the ancient Wyoming primate thatindicate it was not a member of the group of modern primates.

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 forhanging tightly onto tree branches, not for leaping from tree to tree, assome scientists had speculated, based on earlier fragmentary finds. Also,despite expectations, the ancient primate didn’t have eyes specialized forspotting 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 groupof small, long-tailed primates that lived just before the evolution of crea-tures with traits characteristic of modern primates—relatively largebrains, 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 withopposable digits, Bloch and Boyer report in the Nov. 22 Science. Theanimal grew nails on its opposable digits, and claws on its other fingersand toes. Unlike later primates, Carpolestes had side-facing eyes andlacked hind limbs designed for leaping.
