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These ages might never have been seriously questioned were it not for a scientific maverick: the IHO's Curtis, one of the authors of the Science article. In 1970 he applied a radioactive-dating technique to bits of volcanic pumice from the fossil-bearing sediments at Mojokerto. Curtis' conclusion: the Mojokerto child was not a million years old but closer to 2 million. Nobody took much notice, however, because the technique is prone to errors in the kind of pumice found in Java. Curtis' dates would remain uncertain for more than two decades, until he and Swisher could re-evaluate the pumice with a new, far more accurate method.
The new dates ended up validating Curtis' previous work. The Mojokerto child and the Sangiran fossils were about 1.8 million and 1.7 million years old, respectively, comparable in age to the oldest Homo erectus from Africa. Here, then, was a likely solution to one of the great mysteries of human evolution. Says Swisher: "We've always wondered why it would take so long for hominids to get out of Africa." The evident answer: it didn't take them much time at all, at least by prehistoric standards -- probably no more than 100,000 years, instead of nearly a million.
If that's true, the notion that H. erectus needed specialized tools to venture from Africa is completely superseded. But Swisher doesn't find the conclusion all that surprising. "Elephants left Africa several times during their history," he points out. "Lots of animals expand their ranges. The main factor may have been an environmental change that made the expansion easier. No other animal needed stone tools to get out of Africa."
Scientists already have evidence that even the earliest hominids, the australopithecines, could survive in a variety of habitats and climates. Yale paleontologist Elisabeth Vrba believes that their evolutionary success -- and the subsequent thriving of the genus Homo as well -- was tied to climate changes taking place. About 2.5 million to 2.7 million years ago, an ice age sent global temperatures plummeting as much as 20F, prompting the conversion of moist African woodland into much drier, open savanna.
By studying fossils, Vrba found that the populations of large mammals in these environments underwent a huge change. Many forest antelopes were replaced by giant buffalo and other grazers. Vrba believes that early hominid evolution can be interpreted the same way. As grasslands continued to expand and tree cover to shrink, forest-dwelling chimpanzees yielded to bipedal creatures better adapted to living in the open. H. erectus, finally, was equipped to spread throughout the Old World.
If early humans' adaptability let them move into new environments, Walker of Johns Hopkins believes, it was an increasingly carnivorous diet that drove them to do so. "Once you become a carnivore," he says, "the world is different. Carnivores need immense home ranges." H. erectus probably ate both meat and plants, as humans do today. But, says Walker, "there was a qualitative difference between these creatures and other primates. I think they actively hunted. I've always said that they should have gotten out of Africa as soon as possible." Could H. erectus have traveled all the way to Asia in just tens of thousands of years? Observes Walker: "If you spread 20 miles every 20 years, it wouldn't take long to go that far."