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Gradually, anthropologists realized that all these fossils were from creatures so similar that they could be assigned to a single species: Homo erectus. Although the African bones were the last to be discovered, some were believed to be much more ancient than those found anywhere else. The most primitive Asian fossils were considered to be a million years old at most, but the African ones went back at least 1.8 million years. The relative ages, plus the fact that H. erectus' ancestors were found exclusively in Africa, led scientists to conclude that H. erectus first emerged on that continent and then left sometime later.
When and why did this footloose species take off from Africa? Undoubtedly, reasoned anthropologists, H. erectus made a breakthrough that let it thrive in a much broader range of conditions than it was accustomed to. And there was direct evidence of a major technological advance that could plausibly have done the trick. Excavations of sites dating back 1.4 million years B.P., 4,000 centuries after H. erectus first appeared, uncovered multifaceted hand axes and cleavers much more finely fashioned than the simple stone tools used before. These high-tech implements are called Acheulean tools, after the town of St. Acheul, in France, where they were first discovered. With better tools, goes the theory, H. erectus would have had an easier time gathering food. And within a few hundred thousand years, the species moved beyond Africa's borders, spreading first into the Middle East and then into Europe and all the way to the Pacific.
The theory was neat and tidy -- as long as everyone overlooked the holes. One problem: if advanced tools were H. erectus' ticket out of Africa, why are they not found everywhere the travelers went? Alan Thorne, of the Australian National University in Canberra, suggests that the Asian H. erectus built advanced tools from something less durable than stone. "Tools made from bamboo," he observes, "are in many ways superior to stone tools, and more versatile." And bamboo, unlike stone, leaves no trace after a million years.
The most direct evidence of the time H. erectus arrived in Asia is obviously the ages of the fossils found there. But accurate dates are elusive, especially in Java. In contrast to East Africa's Rift Valley, where the underground record of geological history has been lifted up and laid bare by faulting and erosion, most Javan deposits are buried under rice paddies. Since the subterranean layers of rock are not so easy to study, scientists have traditionally dated Javan hominids by determining the age of fossilized extinct mammals that crop up nearby. The two fossils cited in the new Science paper were originally dated that way. The "Mojokerto child," a juvenile skullcap found in 1936, was estimated to be about 1 million years old. And a crushed face and partial cranium from Sangiran were judged a bit younger.