The earliest human ancestors lived in Africa, and the same was true of the even earlier common ancestor we share with other great apes--or so most researchers believe. But new results reported in the 30 July/15 August issue of Current Biology suggest instead that the ancestor was an unknown ape from Europe or Asia that dispersed into Africa 10 million years ago.
This controversial finding emerged from a phylogenetic tree showing the evolutionary relationships of extinct and living apes and Old World monkeys. To construct the tree, the researchers--molecular evolutionist Caro-Beth Stewart of the State University of New York, Albany, and molecular anthropologist Todd Disotell of New York University--used differences in DNA sequences from dozens of studies. To determine branching points, the researchers applied dates from a molecular clock, which converts DNA differences into a time since two lineages split apart. To see where extinct primates fit in this molecular tree, Stewart and Disotell consulted syntheses of data from the fossil record. They then labeled the tree with the geographic ranges of the living and extinct apes.
Finally, with the help of a computer, they set about testing migration scenarios that could produce such a family tree. The scenario that worked best shows that at least one African ape left the continent about 18 million to 20 million years ago, "when Africa was chockful of apes," says Disotell. Its descendants eventually gave rise to an array of early apes in Asia and Europe, including the ancestors of gibbons and orangutans in Asia. By about 10 million years ago, one of those Eurasian apes moved back to Africa, where it evolved into the living African apes and humans.
But other paleoanthropologists say the fossil record is simply too fragmentary to support this interpretation. Harvard University paleoanthropologist David Pilbeam says there is a gaping hole in the fossil record in Africa during the time when the living African apes evolved. He also doubts whether the fossilized bones of extinct apes will ever offer enough clues to fill in the branches of the ape family tree reliably.