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incisors
canine
incisors
canine
premolars
0
0
premolars
molars
molars
5 cm
2 in
modern human
chimpanzee
canine socket
incisors
canine
(small)
incisor
incisor
sockets
sockets
very large
premolars
relatively large
large
premolars
premolars
large
very large
large
molars
molars
molars
Australopithecus
Australopithecus
Paranthropus
afarensis
africanus
robustus
(AL 200 from Hadar)
(STS 17 from Sterkfontein)
(SK 46 from Swartkrans)
FIGURE 2.4
Upper jaws of a chimpanzee, a modern human, and various australopiths (top redrawn after D. C. Johanson & M. E. Edey 1981,
Lucy: The Beginnings of Humankind
. 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 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 39
Bipedal Apes | 39
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 pro-gressively, 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 australopiths and other mammals into the South African caves. If this were true, it might follow that they possessed a typically human interest in 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 40
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sagittal crest
sagittal crest
forwardly placed,
widely flaring
cheekbone
“dished”
face
0
5 cm
0
2 in
Paranthropus robustus
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).
02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 41
Bipedal Apes | 41
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
robustus
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 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 42
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whoever was. To address this important issue and to continue the story of the bipedal apes, we must now turn to eastern Africa.
* * *
“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 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 43
Bipedal Apes | 43
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 strad-dling 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 trow-els, 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 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 44
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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 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 45
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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.
* * *
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 tugenensis,
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 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 46
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be an early australopith as opposed perhaps to an ancestral chimpanzee.