ORIGINS early hominid fossils from AFRICA The year was 1965. Bryan Patterson, a paleoanthropologist ern humans than the one other Australo- Yet Patterson’s fossil would eventu- from Harvard University, unearthed a pithecus humerus known at the time. ally help establish the existence of a new fragment of a fossil arm bone at a site And yet the age of the Kanapoi fossil species of Australopithecus—the oldest called Kanapoi in northern Kenya. He proved somewhat surprising. Although yet to be identified—and push back the and his colleagues knew it would be hard the techniques for dating the rocks where origins of upright walking to more than to make a great deal of anatomical or the fossil was uncovered were still fairly four million years ago. But to see how evolutionary sense out of a small piece of rudimentary, the group working in Ken- this happened, we need to trace the steps elbow joint. Nevertheless, they did rec- ya was able to show that the bone was that paleoanthropologists have taken in ognize some features reminiscent of a probably older than the various Austra- constructing an outline for the story of ) species of early hominid (a hominid is lopithecus specimens that had previous- hominid evolution. right any upright-walking primate) known as ly been found. Despite this unusual result, and Australopithecus, first discovered 40 however, the significance of Patterson’s An Evolving Story years earlier in South Africa by Raymond discovery was not to be confirmed for an- SCIENTISTS CLASSIFY the immediate center Dart of the University of the Witwater- other 30 years. In the interim, researchers ancestors of the genus Homo (which in- srand. In most details, however, Patterson identified the remains of so many impor- cludes our own species, Homo sapiens) and his team considered the fragment of tant early hominids that the humerus in the genus Australopithecus. For sev- arm bone to be more like those of mod- from Kanapoi was rather forgotten. eral decades it was believed that these ancient hominids first inhabited the AUSTRALOPITHECUS earth at least three and a half million ANAMENSIS (right) lived roughly four million years ago. The specimens found in South years ago. Only a few Africa by Dart and others indicated that anamensis fossils have there were at least two types of Austra- been found—the ones lopithecus—A. africanus and A. robus- shown at the left tus. The leg bones of both species sug- include a jawbone and ); ALAN WALKER; © NATIONAL MUSEUMS OF KENYA ( part of the front of the gested that they had the striding, bipedal left face (left), parts of an locomotion that is a hallmark of humans arm bone (center) and among living mammals. (The upright fragments of a lower leg posture of these creatures was vividly bone (right)—and thus confirmed in 1978 at the Laetoli site in researchers cannot determine much about Tanzania, where a team led by archae- ); ROBERT CAMPBELL ( the species’ physical ologist Mary Leakey discovered a spec- appearance. But tacular series of footprints made 3.6 mil- scientists have lion years ago by three Australopithecus illustration established that individuals as they walked across wet anamensis walked upright, making it the volcanic ash.) Both A. africanus and A. earliest bipedal creature robustus were relatively small-brained MATT MAHURIN ( yet to be discovered. and had canine teeth that differed from 14 SCIENTIFIC AMERICAN UpdatedNEW LOOK from AT the HUMAN June 1997EVOLUTION issue COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. A new species of Australopithecus, the ancestor of Homo, pushes back the origins of bipedalism to some four million years ago By Meave Leakey and Alan Walker those of modern apes in that they hard- of hominid bones and teeth discovered ly projected past the rest of the tooth at Laetoli, as well as a large and very im- row. The younger of the two species, A. portant collection of specimens from the robustus, had bizarre adaptations for Hadar region of Ethiopia (including the chewing—huge molar and premolar famous “Lucy” skeleton). The group teeth combined with bony crests on the named the new species afarensis. Radio- skull where powerful chewing muscles metric dating revealed that the species would have been attached. had lived between 3.6 and 2.9 million Paleoanthropologists identified more years ago, making it the oldest Aus- species of Australopithecus over the next tralopithecus known at the time. several decades. In 1959 Mary Leakey This early species is probably the best unearthed a skull from yet another East studied of all the Australopithecus rec- African species closely related to robus- ognized so far, and it is certainly the one tus. Skulls of these species uncovered that has generated the most controversy during the past 45 years in the north- over the past 30 years. The debates have eastern part of Africa, in Ethiopia and ranged over many issues: whether the Kenya, differed considerably from those afarensis fossils were truly distinct from found in South Africa; as a result, re- the africanus fossils from South Africa; searchers think that two separate robus- whether there was one or several species tus-like species—a northern one and a at Hadar; whether the Tanzanian and southern one—existed. Ethiopian fossils were of the same spe- In 1978 Donald C. Johanson, now at cies; and whether the fossils had been the Institute of Human Origins at Ari- dated correctly. zona State University, along with his col- But the most divisive debate con- leagues, identified still another species of cerns the issue of how extensively the Australopithecus. Johanson and his bipedal afarensis climbed in trees. Fossils team had been studying a small number of afarensis include various bone and www.sciam.com SCIENTIFIC AMERICAN 15 COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. joint structures typical of tree climbers. BONOBO Some scientists argue that such charac- teristics indicate that these hominids CHIMPANZEE must have spent at least some time in the HOMO trees. But others view these features as Ardipithecus ? simply evolutionary baggage, left over Orrorin ramidus Au. afarensis from arboreal ancestors. Underlying this Au. robustus Au. africanus discussion is the question of where Aus- tralopithecus lived—in forests or on the Sahelanthropus open savanna. Au. anamensis Au. aethiopicus By the beginning of the 1990s, re- searchers knew a fair amount about the Au. boisei various species of Australopithecus and how each had adapted to its environ- 6 MYR 5 MYR 4 MYR 3 MYR 2 MYR 1 MYR mental niche. A description of any one of AGO AGO AGO AGO AGO AGO the species would mention that the crea- tures were bipedal and that they had ape- FAMILY TREE of the hominid Australopithecus (red) includes a number of species that lived between roughly 4 million and 1.25 million years (Myr) ago. Just over 2 Myr ago a new genus, Homo (which size brains and large, thickly enameled includes our own species, H. sapiens), evolved from one of the species of Australopithecus. teeth in strong jaws, with nonprojecting canines. Males were typically larger than 1982, expeditions run by the National River dominated the Turkana area for females, and individuals grew and ma- Museums of Kenya to the Lake Turkana much of the Pliocene (roughly 5.3 to 1.8 tured rapidly. But the origins of Aus- basin in northern Kenya began finding million years ago) and the early Pleis- tralopithecus were only hinted at, because hominid fossils nearly four million years tocene (1.8 to 0.7 million years ago). Only the gap between the earliest well-known old. But because these fossils were main- infrequently was a lake present in the species in the group (afarensis, from ly isolated teeth—no jawbones or skulls area at all. Instead, for most of the past about 3.6 million years ago) and the pos- were preserved—very little could be said four million years, an extensive river sys- tulated time of the last common ancestor about them except that they resembled tem flowed across the broad floodplain, of chimpanzees and humans (about six the remains of afarensis from Laetoli. proceeding to the Indian Ocean without million years ago, according to molecular But our excavations at an unusual site, dumping its sediments into a lake. evidence) was still very great. Fossil just inland from Allia Bay on the east The Allia Bay fossils are located in hunters had unearthed only a few older side of Lake Turkana [see maps on page one of the channels of this ancient river fragments of bone, tooth and jaw from 18], yielded more complete fossils. system. Most of the fossils collected the intervening 1.5 million years to indi- The site at Allia Bay is a bone bed, from Allia Bay are rolled and weathered cate the anatomy and course of evolution where millions of fragments of weath- bones and teeth of aquatic animals— of the earliest hominids. ered tooth and bone from a wide variety fish, crocodiles, hippopotamuses and the of animals, including hominids, spill out like—that were damaged during trans- Filling the Gap of the hillside. Exposed at the top of the port down the river from some distance DISCOVERIES IN KENYA over the hill lies a layer of hardened volcanic ash away. But some of the fossils are much past several years have filled in some of called the Moiti Tuff, which has been better preserved; these come from the the missing interval between 3.5 million dated radiometrically to just over 3.9 animals that lived on or near the river- and 5 million years ago. Beginning in million years old. The fossil fragments banks. Among these creatures are sever- lie several meters below the tuff, indi- al different species of leaf-eating mon- MEAVE LEAKEY and ALAN WALKER, to- cating that the remains are older than keys, related to modern colobus mon- gether with Leakey’s husband, Richard, the tuff.