(5 of 10)
The 1470 skull and a similar find, labeled 1590, proved that Homo habilis—from whom man could have descended—coexisted with Australopithecus, thus weakening arguments that the latter was man's direct ancestor. Then, in 1975, the Turkana site yielded a Homo erectus skull resembling that of Peking man and with a brain size of 900 cc. The age of the fossil, about 1.5 million years, showed that Homo erectus had emerged even earlier and was hunting in the African plains while Australopithecus still roamed the earth. Because the more advanced Homo erectus was almost certainly a direct ancestor of modern man, the new skull, in the view of some anthropologists, effectively eliminated Australopithecus from man's ancestry.
The result of these findings is a radical revision of long-held views of evolution. As recently as a decade ago, scientists talked about a direct, unbranching line of descent —Australopithecus, Homo erectus, modern man—one following the other in logical order. Now all that has changed. "We can no longer talk of a great chain of being in the 19th century sense, from which there is a missing link," says Phillip Tobias, 51, Dart's successor as professor of anatomy at the University of the Witwatersrand medical school in Johannesburg. "We should think rather of multiple strands forming a network of evolving populations, diverging and converging, some strands disappearing, others giving rise to further evolutionary development."
Anthropologists now believe that man's family tree (see chart) goes back to a primate called Dryopithecus, a true ape that appeared some 20 million years ago. Much later—by 14 million years ago—the Dryopithecus line had split into three branches.
One branch evolved into the ancestors of today's great apes—the gorillas, chimpanzees and orangutans, which are man's closest living cousins. Another produced a creature called Gigantopithecus, a huge ground ape that roamed the valleys of Asia for a few million years before it became extinct. A third branch gave rise to Ramapithecus, which most anthropologists believe was a distant ancestor of man.
The cause of the changes can be traced, at least in part, to plate tectonics, the movement of the great crustal plates that ride on the earth's semimolten mantle and provide its solid outer shell. Some 45 million to 50 million years ago, the plate that carries the Indian subcontinent was pushing up into the underbelly of Asia, slowly thrusting up the massive mountain range now called the Himalayas. This new barrier to global wind circulation helped change weather patterns, altering average temperatures around the world. By about 14 million years ago, climates that had been tropical had turned largely temperate, jungles had thinned out, and fruits and nuts normally available year round began to appear only seasonally.
The changing food supply offered new opportunities for feeding outside the forest. Some of the forest-dwelling apes began venturing into the savanna, or grasslands, in search of food such as roots, seeds and finally the meat of other animals.
The creature most likely to have resulted from this transition, anthropologists believe, was Ramapithecus. Anthropologists theorize that once out of the forest, Ramapithecus began to evolve rapidly. The process of