(Kenya National Museum-West Turkana Specimen No. 17000)
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 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
Skull of Australopithecus garhi from Bouri, Middle Awash Valley, Ethiopia (drawn by Kathryn Cruz-Uribe from photographs) (Copyright Kathryn Cruz-Uribe).
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 proliferation of species.
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 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.
It should come as no surprise that the world's oldest known stone tools come from the Awash Valley of north-central Ethiopia, famous for its early australopith fossils. In one locality or another, the Awash Valley contains ancient river or lake deposits that span the entire range of human evolution, from before 6 million years ago until recent times. Fossil and artifact hunters look for places where fossils or artifacts have eroded from ancient deposits. When they find what they are seeking, they first attempt to establish the layer of origin, and if the layer remains intact nearby, they often excavate to recover objects that are "in situ," that is, still sealed in their original resting places.
The most ancient artifacts come from the drainage of the Gona River, a tributary of the Awash, between Hadar on the north and Bouri and Aramis on the south (Figure 3.1). Rutgers University archeologist Jack Harris made the first discovery in 1976, but it was only between 1992 and 1994 that a team including Harris and his Ethiopian colleague, Seleshi Semaw, excavated a large number of pieces in situ and firmly established their geologic age. The excavated sample numbers more than 1000 pieces from two separate sites, and it is supplemented by about 2000 pieces that had eroded onto the surface near to the excavations.
For raw material, the Gona artifact makers selected volcanic pebbles or cobbles from ancient streambeds, and they left behind sharp-edged flakes, the faceted "cores" from which the flakes were struck, and the battered "hammerstones" that were used to strike the cores. The Gona people clearly understood that to obtain flakes routinely, they had strike the edge of a core forcefully at an oblique angle. When a flake is removed this way, it usually exhibits a distinct swelling or "bulb of percussion" on the inner surface immediately adjacent to the point of impact or "striking platform." Archeologists rely heavily on bulbs to distinguish human flaking from natural fracturing, since collisions between rocks in a stream or under a waterfall tend to be more glancing, and the fracture products rarely show distinct bulbs. The Gona flakes regularly do (Figure 3.2), and they come from silty, low-energy floodplain deposits where natural collisions were unlikely to occur. Their origin as artifacts is thus assured.
The geologic antiquity of the Gona artifacts has been equally well fixed by a combination of potassium/argon and paleomagnetic dating. The potassium/argon method shows that a volcanic ash above
jTChemerc ffl ^ d Olduvai Gorge %
jTChemerc ffl ^ d Olduvai Gorge %
Was this article helpful?