(The following is taken from an article that Meave Leakey and Alan Walker published in Scientific American, June 1997, pp74-79)
The following graph depicts the possible relationship of the hominids discussed here. It is important to realize that graphs such as this are based on what is known at the time. Relationships may become clearer as more evidence is uncovered. However, as of this time, this is one way to conceptualize the relationships of Australopithecines forms in particular.
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Evolving Story of Early Hominids
By the beginning of the 1990s, researchers knew a fair amount about the various species of Australopithecus and how each had adapted to its environmental niche. A description of any one of the species would mention that the creatures were bipedal and that they had ape-size brains and large, thickly enameled teeth in strong jaws, with nonprojecting canines. Males were typically larger than females, and individuals grew and matured rapidly. But the origins of Australopithecus were only hinted at, because the gap between the earliest well-known species in the group (A. afarensis, from about 3.6 Myr ago) and the postulated time of the last common ancestor of chimpanzees and humans (between 5 and 6 Myr ago) was still very great. Fossil hunters had unearthed only a few older fragments of bone, tooth and jaw from the intervening 1.5 million years to indicate the anatomy and course of evolution of the very earliest hominids.
Filling the Gap
Discoveries in Kenya over the past several years have filled in some of the missing interval between 3.5 and 5 Myr ago. Beginning in 1982, expeditions run by the National Museums of Kenya to the Lake Turkana basin in northern Kenya began finding hominid fossils nearly 4 Myr old. But because these fossils were mainly isolated teeth no jawbones or skulls were preserved very little could be said about them except that they resembled the remains of A. afarensis from Laetoli. But our recent excavations at an unusual site, just inland from Allia Bay on the east side of Lake Turkana, yielded more complete fossils.
The site at Allia Bay is a bone bed, where millions of fragments of weathered tooth and bone from a wide variety of animals, including hominids, spill out of the hillside. Exposed at the top of the hill lies a layer of hardened volcanic ash called the Moiti Tuff, which has been dated radiometrically to just over 3.9 Myr old. The fossil fragments lie several meters below the tuff, indicating that the remains are older than the tuff. We do not yet understand fully why so many fossils are concentrated in this spot, but we can be certain that they were deposited by the precursor of the present-day Omo River.
Today the Omo drains the Ethiopian highlands located to the north, emptying into Lake Turkana, which has no outlet. But this has not always been so. Our colleagues Frank Brown of the University of Utah and Craig Feibel of Rutgers University have shown that the ancient Omo River dominated the Turkana area for much of the Pliocene (roughly 5.3 to 1.6 Myr ago) and the early Pleistocene (1.6 to 0.7 Myr ago). Only infrequently was a lake present in the area at all. Instead, for most of the past four million years, an extensive river system flowed across the broad floodplain, proceeding to the Indian Ocean without dumping its sediments into a lake.
The Allia Bay fossils are located in one of the channels of this ancient river system. Most of the fossils collected from Allia Bay are rolled and weathered bones and teeth of aquatic animals fish, crocodiles, hippopotamuses and the likethat were damaged during transport down the river from some distance away. But some of the fossils are much better preserved; these come from the animals that lived on or near the riverbanks. Among these creatures are several different species of leaf-eating monkeys, related to modern colobus monkeys, as well as antelopes whose living relatives favor closely wooded areas. Reasonably well preserved hominid fossils can also be found here, suggesting that, at least occasionally, early hominids inhabited a riparian habitat.
Where do these Australopithecus fossils fit in the evolutionary history of hominids? The jaws and teeth from Allia Bay, as well as a nearly complete radius (the outside bone of the forearm) from the nearby sediments of Sibilot just to the north, show an interesting mixture of characteristics. Some of the traits are primitive onesthat is, they are ancestral features thought to be present before the split occurred between the chimpanzee and human lineages. Yet these bones also share characteristics seen in later hominids and are therefore said to have more advanced features. As our team continues to unearth more bones and teeth at Allia Bay, these new fossils add to our knowledge of the wide range of traits present in early hominids.
A cross Lake Turkana, some 145 kilometers (about 90 miles) south of Allia Bay, lies the site of Kanapoi, where our story began. One of us (Leakey) has mounted expeditions from the National Museums of Kenya to explore the sediments located southwest of Lake Turkana and to document the faunas present during the earliest stages of the basin's history. Kanapoi has proved to be one of the most rewarding sites in the Turkana region.
A series of deep erosion gullies, known as badlands, has exposed the sediments at Kanapoi. Fossil hunting is difficult here, though, because of a carapace of lava pebbles and gravel that makes it hard to spot small bones and teeth. Studies of the layers of sediment, also carried out by Feibel, reveal that the fossils here have been preserved by deposits from a river ancestral to the present-day Kerio River, which once flowed into the Turkana basin and emptied into an ancient lake we call Lonyumun. This lake reached its maximum size about 4.1 Myr ago and thereafter shrank as it filled with sediments.
Excavations at Kanapoi have primarily yielded the remains of carnivore meals, so the fossils are rather fragmentary. But workers at the site have also recovered two nearly complete lower jaws, one complete upper jaw and lower face, the upper and lower thirds of a tibia (the larger bone of the lower leg), bits of skull and several sets of isolated teeth. After careful study of the fossils from both Allia Bay and Kanapoiineluding Patterson's fragment of an arm bonewe felt that in details of anatomy, these specimens were different enough from previously known hominids to warrant designating a new species. So in 1995, in collaboration with both Feibel and Ian McDougall of the Australian National University, we named this new species Australopithecus anamensis, drawing on the Turkana word for lake (anam) to refer to both the present and ancient lakes.
To establish the age of these fossils, we relied on the extensive efforts of Brown, Feibel and McDougall, who have been investigating the paleogeographic history of the entire lake basin. If their study of the basin's development is correct, the anamensis fossils should be between 4.2 and 3.9 Myr old. Currently McDougall is working to determine the age of the so-called Kanapoi Tuff the layer of volcanic ash that covers most of the fossils at this site. We expect that once McDougall successfully ascertains the age of the tuff, we will be confident in both the age of the fossils and Brown's and Feibel's understanding of the history of the lake basin.
Anatomic Mosaic
A major question in paleoanthropology today is how the anatomic mosaic of the early hominids evolved. By comparing the nearly contemporaneous Allia Bay and Kanapoi collections of anamensis, we can piece together a fairly accurate picture of certain aspects of the species, even though we have not yet uncovered a complete skull.
The jaws of anamensis are primitive the sides sit close together and parallel to each other (as in modern apes), rather than widening at the back of the mouth (as in later hominids, including humans). In its lower jaw, anamensis is also chimp like in terms of the shape of the region where the left and right sides of the jaw meet (technically known as the mandibular symphysis).
Teeth from anamensis, however, appear more advanced. The enamel is relatively thick, as it is in all other species ofAustralopithecus; in contrast, the tooth enamel of African great apes is much thinner. The thickened enamel suggests anamensis had already adapted to a changed dietpossibly much harder foodeven though its jaws and some skull features were still very apelike. We also know that anamensis had only a tiny external ear canal. In this regard, it is more like chimpanzees and unlike all later hominids, including humans, which have large external ear canals. (The size of the external canal is unrelated to the size of the fleshy ear.)
The most informative bone of all the ones we have uncovered from this new hominid is the nearly complete tibia, the larger of the two bones in the lower leg. The tibia is revealing because of its important role in weight bearing: the tibia of a biped is distinctly different from the tibia of an animal that walks on all four legs. In size and practically all details of the knee and ankle joints, the tibia found at Kanapoi closely resembles the one from the fully bipedal afarensis found at Hadar, even though the latter specimen is nearly a million years younger.
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Fossils of other animals collected at Kanapoi point to a somewhat different paleoecological scenario from the setting across the lake at Allia Bay. The channels of the river that laid down the sediments at Kanapoi were probably lined with narrow stretches of forest that grew close to the riverbanks in otherwise open country. Researchers have recovered the remains of the same spiral-horned antelope found at Allia Bay that very likely lived in dense thickets. But open-country antelopes and hartebeest appear to have lived at Kanapoi as well, suggesting that more open savanna prevailed away from the rivers. These results offer equivocal evidence regarding the preferred habitat of anamensis: we know that bushland was present at both sites that have yielded fossils of the species, but there are dear signs of more diverse habitats at Kanapoi.
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An Even Older Hominid?
At about the same time that we were finding new hominids at Allia Bay and Kanapoi, a team led by our colleague Tim D. White of the University of California at Berkeley discovered fossil hominids in Ethiopia that are even older than anamensis. In 1992 and 1993 White led an expedition to the Middle Awash area of Ethiopia, where his team uncovered hominid fossils at a site known as Aramis. The group's finds include isolated teeth, a piece of a baby's mandible (the lower jaw), fragments from an adult's skull and some arm bones, all of which have been dated to around 4.4 Myr ago. In 1994, together with his colleagues Berhane Asfaw of the Paleoanthropology Laboratory in Addis Ababa and Gen Suwa of the University of Tokyo, White gave these fossils a new name: Australopithecus ramidus. In 1995 the group renamed the fossils, moving them to a new genus, Ardipithecus. Other fossils buried near the hominids, such as seeds and the bones of forest monkeys and antelopes, strongly imply that these hominids, too, lived in a closed-canopy woodland.
This new species represents the most primitive hominid knowna link between the African apes and Australopithecus. Many of the Ardipithecus ramidus fossils display similarities to the anatomy of the modern African great apes, such as thin dental enamel and strongly built arm bones. In other features, thoughsuch as the opening at the base of the skull, technically known as the foremen magnum, through which the spinal cord connects to the brain the fossils resemble later hominids.
Describing early hominids as either primitive or more advanced is a complex issue. Scientists now have almost decisive molecular evidence that humans and chimpanzees once had a common ancestor and that this lineage had previously split from gorillas. This is why we often use the two living species of chimpanzee (Pan troglodytes and P. paniscus) to illustrate ancestral traits. But we must remember that since their last common ancestor with humans, chimpanzees have had exactly the same amount of time to evolve as humans have. Determining which features were present in the last common ancestor of humans and chimpanzees is not easy.
But Ardipithecus, with its numerous chimplike features, appears to have taken the human fossil record back close to the time of the chimp-human split. More recently, White and his group have found parts of a single Ardipithecus skeleton in the Middle Awash region. As White and his team extract these exciting new fossils from the enclosing stone, reconstruct them and prepare them for study, the paleoanthropological community eagerly anticipates the publication of the group's analysis of these astonishing finds.
But even pending White's results, new Australopithecus fossil discoveries are offering other surprises, particularly about where these creatures lived. In 1995 a team lead by Michel Brunet of the University of Poitiers announced the identification in Chad of Australopithecus fossils believed to be about 3.5 Myr old. The new fossils are very fragmentaryonly the front part of a lower jaw and an isolated tooth. In 1996, however, Brunet and his colleagues designated a new species for their specimen: A. hahrelghazali. Surprisingly, these fossils were recovered far from either eastern or southern Africa, the only areas where Australopithecus had been found until now.