Reading the Fossil Record

The clues to man's past are chiefly fossils, a farrago of frequently undecipherable--and occasionally contradictory--bits of evidence that often raise more questions than they answer. Fossils, the souvenirs of ages gone by, have survived through a still incompletely understood process whereby minerals from the soil infiltrate and gradually replace the very molecules of bone or other hard tissues of an organism, leaving its form and many features preserved.

Fossilization takes place only under special conditions. An animal or plant that dies and is soon after buried in mud or covered by volcanic ash stands a decent chance of being preserved; one that perishes in a jungle or rain forest will probably break down into its chemical components and simply disappear. Few fossils that are formed survive; most are destroyed by the continuing erosion of wind and water. Fewer still are discovered. Though hominid fossils may exist elsewhere, they are found most abundantly--and frequently--in eastern Africa. There, geological processes have exposed layer upon layer of sediments that skilled anthropologists can mine for fossils.

Finding fossils requires both sharp eyes and a sort of anthropological sixth sense. An experienced fossil hunter, Ka-moya Kimeu, who heads one of Richard Leakey's search teams, has found scores of fragments that attest to early man's presence in East Africa. Most people would walk past these small brownish objects without seeing them.

Interpreting these finds requires even more skill. The age of a fossil can often be determined by analyzing the layer of rock or soil in which it was found and determining, often by the so-called potassium-argon method, just how old the layer is. Interpreting the messages of the fossil is usually more difficult. ; The first step in studying a fossil, ; which is often fragmented, is to separate -it from its rock or soil matrix. Next the ! fragments are assembled, a task considerably harder than putting together a jigsaw puzzle with pieces all the same color.

The most difficult task, however, is reconstructing an image of the creature who left these fossils behind. "It's a matter of comparative anatomy," explains Simons. "You study other animals --apes, humans and other primates. Then when you find a piece of bone, you note similarities and differences." The shape of the pelvis tells clearly whether its erstwhile owner walked on all fours or stood erect. Teeth, which are frequently preserved because of their tough, protective enamel, tell even more. Animals that eat meat need teeth shaped to cut and slice; vegetarians need broad molars to chew their fibrous foods. Fossilized bones can indicate a creature's size and weight, just as the length of a thigh bone of a modern human can be used to accurately estimate his height. But often anthropologists must interpolate. Anatomists studying jawless skulls of Australopithecus robustus could not help noticing the creature's well-developed zygomatic arch, the structure to which the jaw muscles are anchored; they deduced that the man-ape had an enormous jaw. The discovery of large A. robustus jawbones proved that they were right.

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