When scientists dig through the ruins of a medieval castle and stumble across goblets, maces or other relics, they consider themselves lucky. The anthropologists working at the castle in the nation of Georgia were very, very lucky: after heavy rains last May washed away some sediments, an archeology student giving a tour of the site to visiting schoolchildren noticed a bone sticking out of the mud. It turned out to be a skull from a young man; a second skull, unearthed a few months later, was an adolescent girl's. But the pair was not a knight and his lady love, as one might expect to find entombed at a medieval site. They were, instead, 1.7 million-year-old fossils of early human ancestors and, anthropologists suspect, members of the species that first journeyed out of the continent where the human species arose. Announcing the discovery last week in the journal Science, the scientists conclude, "These hominids may represent the species that initially dispersed from Africa."
Who took that momentous step, and when, has long fueled debate. The standard story held that a species called Homo erectus got smart, learned to make relatively sophisticated tools and then bravely migrated to Asia and Europe 1 million years ago. But the Dmanisi skulls are dead ringers for a primitive African species called Homo ergaster, who lived in Kenya some 1.9 million years ago and was no genius: the brain case of the Dmanisi male measures 780cc, compared with 1,500cc for people today. And the 1,000 stone artifacts found with the skulls were rudimentary chipped stone tools, not the hand axes that supposedly gave the first emigrants the techno-wherewithal to conquer new worlds. "These are really small-brained ancestors," says anthropologist Milford Wolpoff of the University of Michigan, who was not involved in the find. "The tools were very primitive: they basically smashed rocks and picked out the pieces they liked." Yet even with a small brain and primitive tools, says Christopher Stringer of London's Natural History Museum, "humans were capable of surviving. It's extremely important material."
If Homo ergaster was indeed the first human species out of Africa, more than 1.7 million years ago, it's puzzling that he does not resemble the ancient humans who later occupied Europe, like Neanderthals. It could be that our African ancestors walked through the Levant and into the Caucasus, but turned right toward Asia. Either later out-of-Africa migrants became the Homo sapiens of Europe, or the children of the first Asians found their way to Europe about 900,000 years ago. "It's likely there was not one mass exodus" giving rise to Asian Homo erectus as well as Neanderthals, says Susan Anton of the University of Florida. "Groups probably came out at different times and went in different directions." But why did they leave their natal continent? By 1.7 million years ago our ancestors had bulked up. Bigger bodies need more and better food, which means meat, says Anton. If the new fossils indeed represent the first humans out of Africa, they will have solved the mystery not only of who first left the continent where humans were born, but why: they followed big game because they were hungry.