Experts in evolution determined decades ago that the human species arose in Africa. Fossil bones found there are far more ancient than human remains found anywhere else. But when and why our ancestors first ventured away from the mother continent to take up residence in other parts of the world have been matters of debate. The conventional wisdom had long held that Homo erectus, the immediate ancestor of Homo sapiens, made the exodus about 1 million years ago, after developing relatively sophisticated stone tools that enabled him to gather food more efficiently during his wanderings.
But that wisdom was upset in 1994, when a Homo erectus skull from Indonesia was dated to 1.8 million years B.P. (before the present). And while that discovery was disputed by some scholars, a find reported in last week's Science should end all doubt. Cranial bones from two H. erectus individuals, discovered in Dmanisi, in the Republic of Georgia, have been dated to at least 1.7 million years B.P. What's more, researchers unearthed at the site primitive stone implements resembling those found at H. erectus digs in Africa, proving that fancy tools weren't the trigger for the departure.
Instead, suggests Carl Swisher, a dating expert from the Berkeley (Calif.) Geochronology Center and co-author of the Science paper, wanderlust may simply have been part of H. erectus' personality. The species evolved some 2 million years ago, and armed with a larger brain and body than its predecessor, H. habilis, "it was probably changing its range and its living habitat almost immediately," says Swisher. H. erectus also developed a more carnivorous appetite and probably moved to follow game. "As soon as they lost this dependency on vegetation," says Alan Walker, a Pennsylvania State University paleoanthropologist, "they changed their lifestyle. Then they got out."
Of all the H. erectus bones found outside Africa, these new ones are closest in form to African H. erectus--and may belong to a distinct species, Homo ergaster, which some experts have until now assigned to Africa. Since modern Homo sapiens is believed to have descended directly from H. ergaster, the discovery of closely related bones in Eurasia suggests that our own species may have evolved outside the ancestral continent or arisen in several places simultaneously. Says Swisher: "If you have the ancestral form outside Africa, then you have to entertain those thoughts."
But without complete fossil records, it's difficult to know definitively one way or the other. It's also unclear what route H. erectus took. The study's authors think the species went east to Asia and gave rise to the Asian branch of H. erectus, perhaps then turned north and finally west back to Europe. The fossil record in Europe is especially spotty, with about a million years separating the Dmanisi finds and any other hominid remains.
Researchers expect to continue uncovering a wealth of bones in Europe and are encouraged by the treasures at Dmanisi. "Certainly if they [H. erectus] were out that early, they've got to be other places as well," says Susan Anton, a University of Florida anthropologist and co-author of the Science paper. "It's just a matter of finding them."
--Reported by Sora Song/New York