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Mtugenensis

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Iramidus anamensis afarensisj

Iplatyops africanusj^ ■III aethiopicus

I boisei/

I robustus

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I rudolfensis habilis

I I I I I I I I I | I I I I I I I I I | I I I I I I I I I | I I I I I I I I I | I I I I I I I I I | millions of years ago

brain expansion reduction of chewing teeth stone artifact manufacture ■ archeological sites ■ consumption of large mammals ■

key anatomical

behavioral traits

Figure 2.3

Top: Time spans of the most commonly recognized human species that existed before 1 million years ago. Bottom: Time spans of some key anatomical and behavioral traits. Broken lines imply less secure or more speculative dating.

Among other ape-like characteristics that africanus and robustus shared, the most conspicuous was their small brain size. In both species, adult brain volume averaged less than 500 cubic centimeters (cc). This compares to roughly 400 cc in chimpanzees and to 1400 cc in living people. Even when the averages for africanus and robustus are adjusted for small body size, their brains were less than half the size of ours. Both species also possessed very ape-like upper bodies with long, powerful arms that would have made them agile tree climbers. They differed from apes primarily in their lower bodies, which were shaped for habitual bipedal locomotion on the ground, and in their teeth.

The dental differences are important for two reasons. First, teeth and jaws strongly outnumber other fossil bones, because they are much more durable. They tell us we have australopiths even at sites where limb bones are not preserved. Second, teeth are a window on diet and other aspects of behavior. Chimpanzees and gorillas concentrate on soft foods like ripe fruit and fresh leaves that do not require heavy chewing. Their molar teeth are thus relatively small, and they are encased in relatively thin enamel that soft foods are unlikely to wear away. In their chewing, they do not have to move the jaws from side to side with the mouth nearly closed, and they can therefore have large canines. These are particularly large in males who use them in threat displays and sometimes in violent conflict.

In contrast, africanus and robustus had greatly expanded molar teeth that were encased in thick enamel (Figure 2.4). The implication is that they often consumed tough, hard, gritty, or fibrous foods that required heavy chewing. Such foods probably included seeds that they found on the ground or bulbs and tubers for which they had to dig. Members of both species also had small canines that would not have

Figure 2.4

Upper jaws of a chimpanzee, a modern human, and various australopiths (top redrawn after D. C. Johanson Ö M. E. Edey 1981, Lucy: The Beginnings ofHumankind. New York: Simon and Schuster, p. 367; bottom after T. D. White et al. 1981, South African Journal of Science 77, fig. 9).

Figure 2.4

Upper jaws of a chimpanzee, a modern human, and various australopiths (top redrawn after D. C. Johanson Ö M. E. Edey 1981, Lucy: The Beginnings ofHumankind. New York: Simon and Schuster, p. 367; bottom after T. D. White et al. 1981, South African Journal of Science 77, fig. 9).

impeded sideways movement of the jaws and that males could not have used in threat displays. This may indicate that dietary change was accompanied by a reduction in male-on-male aggression, or, more generally, by greater social tolerance.

The main differences between africanus and robustus were in the size of the chewing teeth—the premolars and molars that line the cheeks—and in the power of the chewing muscles. In robustus, the molars were huge, the premolars had become almost like molars, and the chewing muscles were extraordinarily well developed. The muscles themselves of course are not preserved, but their bony attachments are, and these include large, forwardly placed, widely flaring cheekbones, and in many individuals, a bony (sagittal) crest along the top of the skull (Figure 2.5). For their huge chewing teeth and rugged skulls, robustus and a closely related east African species, Paranthropus boisei, have been called the "robust" australopiths. However, they were small-bodied, even petite like africanus, and in every essential anatomical respect, including small brain size and ape-like upper body form, they exemplify bipedal apes equally well.

Apes use only the most rudimentary technology, and there is little to suggest that the australopiths were different. Flaked stone tools that show a technological advance beyond the ape level appear for the first time around 2.5 million years ago, and the robust australopiths might have produced some. Several findings, however, implicate an early member of the genus Homo as the more likely maker. Perhaps like some chimpanzees, africanus and robustus modified twigs to probe termite nests or they employed naturally occurring rocks or pieces of wood to crack nuts, but such tools would be archeologically invisible. And if the tools were as simple as their chimpanzee counterparts, their use could have been lost and reinvented many times, with minimal impact on the species. In strong contrast, human technology accumulates progressively, it could not be easily reinvented from scratch, and its loss would imperil the species. Even the earliest stone tool makers would probably have quickly vanished if they had somehow forgotten how to flake.

Dart proposed that the australopiths carried the bones of aus-tralopiths and other mammals into the South African caves. If this were true, it might follow that they possessed a typically human interest in

Paranthropus Robustus Skeleton

Figure 2.5

A reconstructed skull of Paranthropus robustus (redrawn after F. C. Howell 1978, in Evolution of African Mammals, Harvard University Press: Cambridge, MA, fig. 10.7).

Figure 2.5

A reconstructed skull of Paranthropus robustus (redrawn after F. C. Howell 1978, in Evolution of African Mammals, Harvard University Press: Cambridge, MA, fig. 10.7).

meat and marrow. C. K. Brain, who excavated at Swartkrans Cave for twenty years and carefully studied the bones, has argued alternatively that large cats or other large carnivores probably introduced the bones of australopiths and other creatures. His most gripping clue is a robus-tus skull fragment with puncture marks that are the right form and distance apart to have been made by a leopard's canines. Like baboons, the australopiths probably sometimes sheltered in caves at night, where they would have provided tempting targets for leopards or extinct saber-tooth cats. If a successful predator consumed its victim on the spot, many of the bones would have fallen to the floor, to become part of the cave deposit. Perhaps like chimpanzees, africanus and robustus sometimes hunted monkeys or other small mammals, but the South African caves indicate that they were more often the hunted than the hunter.

Since africanus lived in South Africa before robustus, it could have been its ancestor, and teeth and skulls of africanus anticipate those of robustus in some respects. The history of the robust australopiths, however, extends to 2.5 million years ago in eastern Africa, where africanus is unknown, and the ancestry of robustus probably lies there. Robust australopiths are unlikely ancestors for true humans, because their teeth and skulls were so specialized and because they coexisted with more plausible ancestors after 2.5 million years ago. The robust australopiths became extinct by 1 million years ago, perhaps because they could no longer compete with evolving true humans or because they could not adjust to a decline in rainfall that occurred about the same time. Africanus is a different matter. In both its anatomy and its presence before true humans, it remains a possible human ancestor, and some anthropologists believe that if it was not, it closely resembled whoever was. To address this important issue and to continue the story of the bipedal apes, we must now turn to eastern Africa.

Both laypeople and anthropologists know that eastern Africa is vital to our understanding of early human evolution. It is no exaggeration to say that this is due largely to the extraordinary dedication and talent of Louis and Mary Leakey. Beginning in 1935, from their base in Nairobi, Kenya, the Leakeys repeatedly traveled to northern Tanzania (known as Tanganyika before independence), where they scoured Olduvai Gorge for traces of early people. They always found artifacts and fossil animal bones, but it was only in 1959 that they recovered their first significant human fossil. This was the well-preserved skull of an adolescent "robust" australopith. We now assign it to the species Paranthropus (or Australopithecus) boisei, although boisei may have been simply an east African variant of South African Paranthropus robustus. Bones of boisei have been found at eight other east African sites, from Ethiopia on the north to Malawi on the south.

The Leakeys' success in 1959 brought them richly deserved financial support, and they excavated much more deposit over the next fourteen years at Olduvai than they had in the previous thirty. They recovered many additional human fossils, and they showed that boisei had coexisted with early true humans after 2 million years ago, just as robustus had in South Africa. They also illuminated the course of human evolution after robustus and boisei became extinct and only true humans survived.

The Leakeys' research revolutionized paleoanthropology not only because it provided key fossils and artifacts at Olduvai, but because it encouraged others to tap the great paleoanthropological potential of eastern Africa. The expedition leaders came from many countries, and the long list includes the Leakeys' son Richard from Kenya and his wife, Meave, Clark Howell, Donald Johanson, William Kimbel, and Tim White from the United States, Berhane Asfaw from Ethiopia, Yves Coppens and Maurice Taieb from France, and Gen Suwa from Japan. The expeditions have met their greatest success at sites near Lake Turkana straddling northern Kenya and southern Ethiopia and along the margins of the Awash River in north-central Ethiopia. In their quest, the fossil hunters have followed an important precedent that Mary Leakey established at Olduvai and at the older site of Laetoli nearby. She knew that ancient fossils and artifacts have little value if their stratigraphic position is not carefully recorded, and she therefore collaborated closely with the geologist Richard Hay, whose careful geologic mapping ensured the correct stratigraphic ordering. It also allowed him to reconstruct the landscape in which early people lived. Other fossil expeditions have routinely engaged field geologists for the same purpose, and like the Leakeys, they have also relied on geochemists to date the deposits in years and on paleontologists to identify the animal remains for both dating and environmental reconstruction. In short, research into early human evolution in eastern Africa has succeeded because it has been truly multidisciplinary, and it was the Leakeys who provided the model.

Eastern Africa has two distinct advantages over South Africa for the study of human evolution. First, the east African fossils often occur in relatively soft river or lake deposits that can be excavated with trowels, brushes, and other standard archeological tools. In contrast, the rock-hard South African cave breccias commonly require dynamite and pneumatic drills. Second, the east African sites often contain layers of lava or volcanic ash (tiny particles of lava that were erupted into the atmosphere and later settled to Earth). Lava and ash cool in a geologic eye-blink, and the time when they cooled can be estimated by the potassium/argon technique. This depends on the observation that rocks commonly contain small amounts of naturally occurring radioactive potassium-40 and of its daughter (decay) product argon-40. Argon-40 is a gas that disappears from molten rocks and that reaccumulates in cooled rocks in direct proportion to the known decay rate ("half-life") of potassium-40. The ratio of potassium-40 to argon-40 thus tracks the time of cooling in years, and the time when lava or ash cooled can be used in turn to date fossils and artifacts that are stratified within the same deposits. The South African cave breccias contain neither lava nor ash, and the South African australopiths must thus be dated mostly by associated animal species whose time ranges have been established at east African sites.

The twin advantages of the east African sites reflect their proximity to the eastern branch of the Great Rift Valley (Figure 2.1). This is essentially a gigantic geologic fault that marks the boundary between two massive continental plates. Tension and compression along the fault have forced its bottom down and its sides up, creating a trough more than 2000 kilometers (1200 miles) long and 40 to 80 kilometers (25 to 50 miles) wide. Repeated crustal movements in and around the Rift have often blocked streams to create lake basins that trapped and preserved fossil bones and artifacts. When later earth movements caused the lakes to drain, sparse vegetation and episodically violent rainfall encouraged erosion that exposed fossils for discovery. Rifting also promoted the volcanic activity that supplied lava and ash for dating. In contrast, the landscape of southern Africa was stable over the entire course of human evolution. It provided few internal basins to trap fossils and no active volcanoes. The result is that we have only the cave breccias and the challenge they present to excavation and to dating.

East African discoveries have not only extended the geographic range of the australopiths, they have also pushed the australopith record back beyond 4 million years ago (Figure 2.3). Ultimately, they will push it back to between 7 and 5 million years ago, the time when geneticists estimate that people and chimpanzees last shared a common ancestor. Two teams—one French and the other Ethiopian/American—already claim to have done this. Early in the winter of 2001, the French team announced the discovery of thirteen tantalizing, fragmentary fossils from deposits dated to 6 million years ago in the Tugen Hills of northern Kenya. They assigned the fossils to the new species Orrorin tuge-nensis, from the location of the site and the word "orrorin," meaning "original man" in the local Tugen language. Then in the summer of 2001, the Ethiopian/American team reported eleven fossils dated to between 5.8 and 5.2 million years ago from the arid margin of the middle stretch of the Awash river, about 300 kilometers (180 miles) northeast of Addis Ababa, Ethiopia. The Ethiopian/American team tentatively assigned their specimens to an older variant of the previously known species, Ardipithecus ramidus, whose discovery at the site of Aramis we recount below.

Neither the Kenyan fossils nor the Ethiopian ones include bones that unequivocally demonstrate bipedalism, and team members and other specialists are currently debating which species is more likely to be an early australopith as opposed perhaps to an ancestral chimpanzee. Conceivably, one or the other might even represent the last shared ancestor of australopiths and chimpanzees. Their status cannot be resolved without additional, more complete fossils, and in the meanwhile, the oldest widely accepted australopith comes from the site of Aramis, also in the Middle Awash Valley. We expand on Aramis here, for it nicely illustrates both the difficulties and rewards that fossil hunters can encounter in eastern Africa.

Aramis today is an inhospitable patch of sparse vegetation and ultrahigh temperatures. Ticks, vipers, and scorpions call it home, and at first glance, it looks like an unlikely place to seek fossils. Yet an international team of scientists who began working at the site in 1992 showed that when they look hard, sometimes crawling on their hands and knees, shoulder to shoulder, for days on end, they can recover fascinating traces of ancient life: seeds, fossilized wood, insect remains, and bones of birds, reptiles, and mammals. Potassium/argon analysis of volcanic ash shows that the fossils accumulated at Aramis about 4.4 million years ago.

The hard-won finds from Aramis reveal a far less forbidding ancient landscape. A dense forest lined the river. Acrobatic colobus monkeys clambered through the trees, and spiral-horned kudu antelopes browsed on leaves near ground level. Monkeys and kudus seem to have been the most common animals, but many other species were also present. These ranged in size from tiny rodents and bats to hippos, giraffes, rhinos, and elephants. The carnivores included large cats, hyenas, and other species that we would expect in Africa, and a bear that seems oddly out of place. The same bear occurs at other ancient African sites as far south as the Cape of Good Hope, and its presence underscores how much Aramis—and Africa—have changed over the past four million years.

The carnivores that hunted and scavenged near the river often chewed and crushed bones, and few specimens have survived intact. Partial skeletons are particularly rare, with one prominent exception. This represents a creature who, to the great fortune of paleontologists, died as floodwaters rose and covered its body with a layer of silt—a crucial step on the path to bone preservation.

In November 1994, University of California graduate student Yohannes Haile-Selassie was crawling across the surface at Aramis when he spotted some broken hand bones eroding from below. When he and his coworkers scraped the subsurface, more of the skeleton appeared: a tibia or shin bone, a heel bone, part of the pelvis, forearm bones, hand and wrist bones, and part of the skull. The bones were very fragile, and a careless touch could have turned them to powder, so the excavators softened the deposit with water, and they worked with surgical precision. Their painstaking efforts eventually retrieved more than one hundred pieces of the skeleton, including a nearly complete set of wrist bones and most of the finger bones from one hand. They recovered a lower jaw nearby.

The new skeleton proved to come from the same 4.4-million-year-old australopith species that Tim White, Gen Suwa, and Berhane Asfaw had described from other find spots at Aramis just two months before. Their description, in the journal Nature, was based on seventeen fossils, including a lower jaw, isolated teeth, pieces of skull, and three left arm bones. The species was roughly one-half million years older than any previously known australopith, and it was significantly more ape-like. To signal its position near the bottom of the human family tree, White and his colleagues named it Australopithecus ramidus, from "ramid," meaning "root" in the language of the local Afar people. Later, they concluded that it was so distinct that it deserved its own genus, and they renamed it Ardipithecus ramidus. Ardi means "ground or floor" in the Afar language, and the new name reinforced both the basal position of the species in human ancestry and the likelihood that it spent much of its time on the ground.

In the parts that have been described, ramidus was remarkably ape-like even for a bipedal ape. Its canines, for example, were exceptionally large relative to its molars, and its teeth were covered by thin enamel. It was also decidedly ape-like in the power of its arms, and it probably even possessed the ability to lock the elbow joint for added stability during climbing. If the teeth and arm bones were all we had, we might conclude that ramidus was only an ape, but a fragment from the base of the skull suggests that it carried its head in the human (bipedal) position. We will know just how bipedal when White and his colleagues describe the leg and foot bones of the partial skeleton.

Bipedalism is amply documented for the next youngest australo-pith, which Meave Leakey and her paleoanthropologist colleague Alan Walker described in 1995 from the sites of Kanapoi, southwest of Lake Turkana, and Allia Bay on the lake's eastern margin. They named the species Australopithecus anamensis, from "anam," meaning "lake" in the language of the local Turkana people. Potassium/argon dating shows that anamensis lived near Lake Turkana between 4.2 and 3.8 million years ago (Figure 2.3). Accompanying animal fossils show that the environs were wooded, but trees were probably sparser than they were at Aramis.

The bone sample of anamensis includes thirteen partial jaws, fifty isolated teeth, a piece of skull from around the ear region, two bones of the arm, a hand bone, a wrist bone, and the pièce de résistance, a tibia or shin bone. The jaws and teeth show that anamensis retained relatively large canines, but it also had the broadened molars and thickened enamel that mark virtually all later australopiths. The arm bones suggest that it preserved an ape-like ability to climb, but the tibia shows even more clearly that it was habitually bipedal on the ground. In people, in contrast to chimpanzees, the flat, articular surface at the knee end of the tibia is almost perpendicular to the shaft, and the shaft itself is heavily buttressed near both ends (Figure 2.6). These and other features allow people to shift their weight from one leg to the other during bipedal movement, and they are all present in the anamensis tibia. Together, then, the teeth, the arm bones, and the tibia unequivocally finger anamensis as a bipedal ape.

On known parts, anamensis closely resembled Australopithecus afarensis, which occurred in the immediately succeeding time period, and when anamensis becomes better known, it may turn out to be simply an early version of afarensis. Since afarensis was recognized first, its name would be applied to both species.

Afarensis illustrates the bipedal ape character of the australopiths more clearly than any other species, because it is known from virtually every bone of the skeleton, often in multiple copies. That we know afarensis so well is due almost entirely to the efforts of Donald Johanson and his coworkers beginning in 1973 at Hadar, immediately north of Aramis in Ethiopia, and to the work of Mary Leakey between 1974 and 1979 at Laetoli, 45 kilometers (27 miles) south of Olduvai Gorge in northern Tanzania. At one small site, Johanson's team recovered forty percent of the skeleton of a single individual (Figure 2.7), whom they immortalized as "Lucy," from the lyrics of a Beatles tune

Australopithecus Africanus Jaws

Figure 2.6

Front views of tibias (shin bones) of a chimpanzee, Australopithecus anamensis, and of a living human (redrawn after M. G. Leakey 1995, National Geographic 190 (9), p. 45).

Australopithecus anamensis (Kanapoi)

Figure 2.6

Front views of tibias (shin bones) of a chimpanzee, Australopithecus anamensis, and of a living human (redrawn after M. G. Leakey 1995, National Geographic 190 (9), p. 45).

that was popular at the time. A partial skeleton is worth far more than the sum of its parts, because unlike isolated bones, it permits anthropologists to reconstruct bodily proportions, including, for example, the length of the arms relative to the length of the legs. At another small site, Johanson's team found more than two hundred bones from at least nine adults and four juveniles who have been dubbed the "First Family."

Lucy Australopithecus Mary Leakey

Figure 2.7

Left: The forty-percent-complete skeleton of "Lucy" (Australopithecus afarensis) from Hadar, Ethiopia (drawn from a photograph in M. H. Day 1986, Guide to Fossil Man. Chicago: University of Chicago Press, p. 250). Right: a reconstruction of the entire skeleton based on mirror-imaging and on other specimens of the same species (drawn after K. F. Weaver, 1985, National Geographic 168, p. 564).

Figure 2.7

Left: The forty-percent-complete skeleton of "Lucy" (Australopithecus afarensis) from Hadar, Ethiopia (drawn from a photograph in M. H. Day 1986, Guide to Fossil Man. Chicago: University of Chicago Press, p. 250). Right: a reconstruction of the entire skeleton based on mirror-imaging and on other specimens of the same species (drawn after K. F. Weaver, 1985, National Geographic 168, p. 564).

Together with fossils from other sites, they allow highly reliable estimates of variability within afarensis, including the degree of sexual dimorphism.

Based on the Hadar and Laetoli samples, Johanson, Tim White, and Yves Coppens defined afarensis in 1978, and they took the name from the Afar region of Ethiopia that includes Hadar, Aramis, and other key fossil sites. Potassium/argon analysis shows that Hadar fossils of afarensis accumulated between about 3.4 and 2.9 million years ago and that the Laetoli fossils are somewhat older. They take the species back to roughly 3.8 million years ago (Figure 2.3). Thus, even if anamensis is kept separate, afarensis spanned an interval of about a million years, and it changed little over this long span. At Laetoli, it occupied a dry environment, with few trees, but at Hadar it enjoyed generally moister, more wooded conditions. It was thus flexible in its environmental requirements.

Afarensis had a small ape-sized brain that may have been even smaller on average than the brains of africanus or robustus. It shared their relatively small body size, but it was much more dimorphic. Males not only averaged perhaps fifty percent taller and heavier than females, they also had significantly larger canines. In both males and females, the jaws protruded farther forwards below the nose than in any other known member of the human family, and body proportions were intermediate between those of apes and later humans. Thus, the arms were very long relative to the legs, and the forearm was particularly long and powerful. Combined with ape-like curvature of the finger and toe bones (phalanges), the arms imply an ape-like agility in the trees.

At the same time, in all key respects, the pelvis, the leg, and the foot demonstrate bipedalism (Figure 2.8). The pelvis was shortened from

Human Bipedalism

Figure 2.8

Lower limbs of a modern human, of Australopithecus afarensis, and of a chimpanzee (redrawn after D. C. Johanson & M. E. Edey 1981, Lucy: The Beginnings ofHumankind. New York: Simon and Schuster, p. 157).

Figure 2.8

Lower limbs of a modern human, of Australopithecus afarensis, and of a chimpanzee (redrawn after D. C. Johanson & M. E. Edey 1981, Lucy: The Beginnings ofHumankind. New York: Simon and Schuster, p. 157).

top to bottom and broadened from fore to rear to center the trunk over the hip joints and thereby reduce fatigue during upright, bipedal locomotion. The femur slanted inwards towards the knee and formed a distinct (valgus) angle with the tibia so that the body could balance on one leg while the other was off the ground. And the foot had the expanded heel, upward arch, and non-divergent (non-opposable) big toe that are essential for human walking. In humans, each step involves a heel strike, followed by the planting of the foot over the arch, and finally by a pushing off from the big toe. If there were any doubt about this sequence in afarensis, Mary Leakey laid it to rest in what for anyone else would have been the discovery of a lifetime. In her excavations at Laetoli, her team uncovered a 27-meter (89-foot) long trail of footprints left by two afarensis individuals strolling together on a mushy surface that hardened about 3.6 million years ago. In heel strike, arch, and non-divergent big toe, the prints match ones that living humans make when they walk barefoot on a soft substrate.

If paleontologists wanted to construct a bipedal ape from scratch, they could probably not produce a more persuasive species than Australopithecus afarensis, and nothing could provide more compelling evidence that humans descend from apes. For opponents of this idea, afarensis is an even more formidable foil than the flamboyant Clarence Darrow, who found himself defending evolution in a Tennessee court half a century before afarensis was discovered.

A reader who has reached this point may be thinking "OK, bipedal apes, but why bipedal?" What natural selective force could have prompted an ape to become bipedal and what advantage would bipedalism have conferred? These questions are not trivial, but they are also not easy to answer. With regard to what stimulated the shift to bipedalism, the most likely cause is environmental change. Between 10 million and 5 million years ago, global climate became cooler and drier, and grasslands expanded while forests shrank or thinned out. The change spelled doom for many forest-adapted species, including a variety of apes that lived in Africa and Eurasia before 10 million years ago. In equatorial Africa, however, one ape species adapted to the changing conditions by spending an increasing amount of time on the ground. Life on the ground presented new challenges and opportunities that favored those individuals whose anatomy and behavior gave them a reproductive edge, however slight, over their peers. In retrospect, it appears that the most important anatomical advantage was an enhanced ability to walk and run bipedally.

The shift to a lifestyle grounded in bipedalism may have progressed gradually over a long interval, or it may have occurred abruptly, as African environments changed in response to a particularly dramatic decline in global temperature and humidity between 6.5 and 5 million years ago. During this interval, periodic growth in the Antarctic ice cap sucked so much water from the world ocean that the Mediterranean Sea was drained. The loss of moisture from the Mediterranean accelerated forest contraction on the adjacent continents, and animal communities responded. In Africa, the antelopes burgeoned into the wide variety we know historically, and the human line may have emerged at the same time. If so, its origin would constitute a punctuational event. For the moment this idea must remain conjectural, but ongoing research in eastern Africa will one day provide the fossils to test it.

As to the advantages that bipedalism would have offered a ground-dwelling ape, the first and perhaps most obvious is that the arms and hands could now be used to carry food to widely scattered trees or to other group members. In addition, as Darwin noted more than a century ago, the hands would now be freer for tool manufacture and use. Today, this idea is less compelling, because archeology shows that tool use beyond the level of living apes occurred only about 2.5 million years ago, long after bipedalism. Among other less obvious natural selective advantages, bipedalism may have reduced the energy that ground-dwelling apes needed to travel between widely scattered trees or tree clumps, and it could have lessened their danger of heat stroke, if they were often forced to forage in the open at midday. This is because the sun's most intense rays would have fallen only obliquely on upright backs.

Modern experiments have failed to confirm that bipedalism increases energy efficiency, while animal and plant fossils show that the bipedal apes, particularly the earliest ones, lived in environments where shade trees were plentiful. It was only about 1.7 million years ago that people invaded savannas where shade may have been sparse, and they evolved a different body form to meet the challenge. Novel explanations of bipedalism are thus still welcome, and Nina Jablonski and George Chaplin of the California Academy of Sciences have offered a particularly intriguing one. It draws on the observation that free-ranging chimpanzees and gorillas stand upright mainly to threaten each other over food or mates. In the process, they wave their arms, beat their chests, and sometimes even brandish branches to enhance their displays. When male gorillas feel threatened, they often stand erect before charging, while chimpanzees swagger and raise their hair so that they seem even more imposing. When an opponent fails to back down, violent, deadly struggles may ensue. Humans of course also signal their status or intentions with posture, and Jablonski and Chaplin propose that an increase in bipedal displays for dominance and appeasement—standing up or backing down—may have been important to reduce violent aggression among early bipedal apes. The potential for aggression may actually have increased, if forest fragmentation had concentrated the most desirable food in small, dense patches. Individuals who learned to defuse tense situations with bipedal displays could have reduced their risk of injury or death and thus, by definition, improved their reproductive chances. In this scenario, bipedalism may have been important for promoting social tolerance even before it facilitated carrying or tool use.

The initial advantages of bipedalism may always remain a matter for speculation, but they must have been significant, for the bipedal apes not only survived, they eventually proliferated. Anthropologists disagree on whether ramidus is a likely ancestor for anamensis and afarensis, but most agree that between 3.5 and 2.5 million years ago, multiple bipedal species appeared (Figure 2.3). By 2.5 million years ago, there were at least two highly distinct bipedal lines—one that produced the later robust australopiths and another that led to true people of the genus Homo and ultimately to ourselves.

The robust line is better documented, mainly thanks to a spectacular skull that Alan Walker and his colleagues described in 1986 from a site to the west of Lake Turkana in northern Kenya. As it lay in the ground, the skull had been permeated with manganese which turned it blue-black, and it is thus been dubbed the "Black Skull" (Figure 2.9). It had a face like that of afarensis, in which the jaws projected far forward, but it also had very large chewing teeth and a powerfully developed sagittal crest like those of robustus and boisei. It is now commonly

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