National Geographic : 1995 Sep
Early steps on the road to humankind Sifting through the fossil rich sediments at Turkana's Allia Bay site, a research team under Alan Walker (right, at rear right) of Pennsyl vania State University leaves no sand unturned in the search for fossils exposed by erosion and fragmented by the ele ments. Though tedious, only interminable sieving can yield the tiny pieces of bone and teeth that make up a vital part of the fossil record. Since bipedalism isthe primary factor separat ing apes from hominids, the Turkana researchers were particularly inter ested in leg bones. From the Kanapoi site, two sections of a tibia- the larger bone of the lower leg-show clearly that its owner walked upright. The socket-like condyles at the knee (top, at right) are both concave-a hominid trait; inan ape one would be convex. And a buttress of flared bone at the bottom of the tibia, at left, where it joins the ankle, is also hominid-like. A built-in shock absorber, the muscle large enough for bone enlargement indi cates an animal that bore more upright weight than an ape could. An other indication of the animal's bipedalism isthe relative delicacy of the fibula implied by its small junction at the knee. Too frail to support a a toe-grasping ape, the fibula might have been large enough for a homi nid with toes more dex terous than a modern human's. To what extent these hominids were tree dwelling, or even tree climbing, remains a question.