The thought of another human species coexisting with our own seems rather, well, alien. But this may have been the case a mere 27,000 to 53,000 years ago. A report in tomorrow's issue of Science suggests that Homo erectus, a relative of modern humans, was still alive in Java, Indonesia, at least 250,000 years after it was thought to have gone extinct in Asia. If so, this remnant population of H. erectus, a species that first appeared in the fossil record about 2 million years ago, would have been alive when modern humans and Neandertals roamed the Earth.
A team led by Carl C. Swisher of the Berkeley Geochronology Center came to this stunning conclusion after using argon-based radioactive dating techniques to redate two important fossil sites in Java where H. erectus had been found. "If the dates are right, we have three different species coexisting at the same time," says Chris Stringer, a paleoanthropologist at The Natural History Museum, London. It also means, says American Museum of Natural History paleoanthropologist Ian Tattersall, that "being alone on Earth is unusual."
The new dates also cast doubt on a theory, held by some paleoanthropologists, that the Java people were the ancestors of the first Asians, including the first Australians, who appeared in the fossil record about 40,000 years ago. It's therefore no surprise that these findings are proving controversial. "There is a real problem with the dates," claims Australian National University paleoanthropologist Alan Thorne, who points out that the site is notoriously difficult to date, and that the crucial human fossils were removed from the site decades ago. And even if the dates hold up, he and others argue, they wouldn't necessarily rule out the Java-Australian lineage.
Swisher, however, is confident of the findings, which come mainly from dating of ungulate teeth taken from a fossil site on a terrace above the Solo River in Java, in the village of Ngandong. "We have two options: Sit on our data, or say that we've done the best we can with the technology available to us and throw them out there for people to evaluate," he says. Given the startling nature of the data, there will be no shortage of evaluators.