The Dawn of Human Culture (7 page)
Bipedal Apes | 55
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 pro-gressed 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 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 56
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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 bran-dish 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 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 57
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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.
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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 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 58
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sagittal
crest
forwardly
projecting
upper jaw
0
5 cm
0
2 in
Paranthropus aethiopicus
(Kenya National Museum-West Turkana Specimen No. 17000)
FIGURE 2.9
The “Black Skull,”
Paranthropus aethiopicus,
from West Turkana, Kenya (drawn by Kathryn Cruz-Uribe from photographs) (Copyright Kathryn Cruz-Uribe).
assigned to the species
Paranthropus aethiopicus,
and it is a plausible link between
afarensis
and
boisei/robustus
. Other east African sites that date between 2.5 and 2 million years ago have provided jaws and isolated teeth that may represent either
aethiopicus
or early
boisei
.
The second lineage is sparsely represented before 2 million years ago, but many anthropologists have long assumed that it stemmed from
africanus
or a species like it. Eastern Africa has not yet, however, provided fossils resembling
africanus
. Instead, in 1999, it produced another equally old and totally unexpected species.
Just three years after he discovered the partial skeleton of
Ardipithecus ramidus
at Aramis, Yohannes Haile-Selassie spotted a skull fragment on the surface at Bouri, south of Aramis in the Middle Awash Valley. After the Middle Awash team had painstakingly turned over every 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 59
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rock and bone fragment nearby, they were able to reconstruct a remarkable skull (Figure 2.10). A lower jaw from the same deposits at another locality probably represents the same species. Potassium/argon dating demonstrates that the species existed about 2.5 million years ago, which makes it a contemporary of both
Australopithecus africanus
and
Paranthropus aethiopicus
. Yet it differed sharply from both. The part of the skull that contained the brain might have been mistaken for the same part in
afarensis
if it had been found in isolation. In contrast, based on shape and proportions, the jaws and teeth might have been mistaken for those of later humans, except that the teeth were exceptionally large. The premolars and molars equaled or exceeded those of robust australopiths in size, but in contrast to the condition in the robust australopiths, the incisors and canines were also large. “The combination of large teeth and primitive morphology was a surprise,” says Tim White. “Nobody expected that.” So White and his colleagues decided to call the species
Australopithecus garhi,
from garhi, the Afar word for “surprise.” In the April 23, 1999, issue of
Science
magazine, they suggested that “It is in the right place, at the right time, to be the ancestor of early
Homo,
however defined. Nothing about its morphology would preclude it from occupying this position.” Possible
garhi
limb bones from Bouri indicate that the forearm remained long relative to the upper arm as in apes, but the thigh was long relative to the upper arm as in humans. In other words, as humans continued to differentiate from apes, it appears that their legs lengthened before their forearms shortened.
We will see that the early
Homo
line may actually include two or even three lines, and if they split by 2.5 million years ago,
garhi
could be ancestral to only one. The east African fossil record between 3 million and 2 million years ago is actually poorer than the record for 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 60
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small
Australopithecus
afarensis
-like braincase
0
5 cm
0
2 in
Australopithecus garhi
(Bouri Vertebrate Specimen-12/130)
large premolars
large canines &
& molars
large forwardly
protruding incisors
FIGURE 2.10
Skull of
Australopithecus garhi
from Bouri, Middle Awash Valley, Ethiopia (drawn by Kathryn Cruz-Uribe from photographs) (Copyright Kathryn Cruz-Uribe).
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the preceding million years, but the difference reflects the vagaries of preservation and discovery, not the likelihood that the australopiths or their descendants had become rarer. The bottom line is that as fossil hunting continues,
garhi
will probably not be the last surprise. Meave Leakey and her team brought this point home in March 2001, when they described a remarkable new skull from 3.5-million-year-old deposits west of Lake Turkana. Prior to the new discovery, most authorities agreed that the relatively well known human fossils from between 4 and 3 million years ago represented only one evolving line—
anamensis
and its immediate descendant
afarensis
. The new skull shares thick dental enamel with both, and like all australopith skulls, it contained a small, ape-size brain. However, its molar teeth were much smaller than those of
afarensis
and
anamensis,
and its face was far flatter and less projecting. Its individual features can be matched in other australopith species, but it combines them in a unique way, and Leakey and her colleagues have assigned it to a new genus and species,
Kenyanthropus platyops,
or “the flat-faced man of Kenya.”
In its flat face and the shape of its brow,
platyops
anticipates a much larger brained 1.9-million-year-old Kenyan skull that is now often assigned to
Homo rudolfensis
. However, the facial resemblance could be simply a matter of chance, and many new fossils will be necessary to clarify the relationships of
platyops
to
Homo
and to other australopiths.
For the moment,
platyops
is important because it shows that like the monkeys, the antelopes, and other mammal groups, early humans had diversified into multiple contemporaneous forms early on. In a few short years anthropologists may be worrying less about why bipedalism was successful and more about how it could have promoted such a prolifer-ation of species.
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THE WORLD’S OLDEST
WHODUNIT
Imagine camping on an east African savanna without the benefit of tents, tools and utensils, a four-wheel-drive vehicle, or even a camp-fire. You’re small, naked, and bipedal, and your intelligence is crammed in a brain less than half the size of the one you’re using to read these words. A nearby river or a waterhole provides a reliable source of water, and when danger looms your long arms can quickly propel you into the trees. Your climbing ability is crucial, because you cannot out run the large cats, hyenas, and other predators who see you as food. But what will you eat? How will you find enough to survive?
Around 2.5 million years ago, some scrawny bipedal creature made a revolutionary discovery that greatly increased its chances for survival. It lived in woodlands or savannas where predators, accidents, disease, or starvation often killed antelopes, zebras, wild pigs, and other large mammals. Carnivores and scavengers did not claim all the available flesh or marrow, and therein lay an opportunity. What our spindly 03 Whodunit.r.qxd 1/29/02 5:04 PM Page 64
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biped found was that if it struck one stone against another in just the right way, it could knock off thin, sharp-edged flakes that could pierce the hide of a dead zebra or gazelle. It could use the same flakes to slice through the tendons that bind muscle to bone. In effect, it had found a way to substitute stone flakes for the long slicing teeth that cats and other carnivores employ to strip meat from a carcass. Our primitive inventor also discovered that it could use heavy stones to crack bones for their nutritious, fatty marrow, and in this, it unwittingly imitated hyenas who employ hammer-like premolars for the same purpose. Its use of stone tools conferred a reproductive advantage over individuals who could not do likewise, and those who could soon increased in number. In extending their anatomy with tools so that they could behave more like carnivores, they set in train a co-evolutionary interaction between brain and behavior that culminated in the modern human ability to adapt to a remarkable range of conditions with culture alone.
