for National Geographic News
December 27, 2005
Two archaeologists are challenging what many experts consider to be the basic assumption of human migration—that humankind arose in Africa and spread over the globe from there.
The pair proposes an alternative explanation for human origins: arising in and spreading out of Asia.
Robin Dennell, of the University of Sheffield in England, and Wil Roebroeks, of Leiden University in the Netherlands, describe their ideas in the December 22 issue of Nature.
They believe that early-human fossil discoveries over the past ten years suggest very different conclusions about where humans, or humanlike beings, first walked the Earth.
New Asian finds are significant, they say, especially the 1.75 million-year-old small-brained early-human fossils found in Dmanisi, Georgia, and the 18,000-year-old "hobbit" fossils (Homo floresiensis) discovered on the island of Flores in Indonesia.
Such finds suggest that Asia's earliest human ancestors may be older by hundreds of thousands of years than previously believed, the scientists say.
"What seems reasonably clear now," Dennell said, "is that the earliest hominins in Asia did not need large brains or bodies." These attributes are usually thought to be prerequisites for migration.
The authors maintain that, although there is no absolute proof, putting all the evidence together requires an open mind about other geographical origins of the first humans.
The authors point out that there is very little solid information about the first early humans in Asia, and paleontologists are left with assumptions that are too often treated as historical facts.
There is no archaeological or fossil evidence to prove that early humans moved from southern Africa to the Nile Valley in the early Pleistocene (1.8 million years ago to 11,500 years ago), they say.
The earliest evidence of a human ancestor in Asia appears to be the 1.8-million-year-old cranium found in Mojokerto, Indonesia. But, the authors note, the fact that no older specimens have been found in Asia does not prove that they didn't exist.
Dennell and Roebroeks get support for their proposal from other experts.
"I think this is an interesting and constructively provocative paper," said Chris Stringer, a researcher in the department of palaeontology at London's Natural History Museum.
"Evidence of humans in the Caucasus [region of Asia], China, and Java more than 1.6 million years ago implies either a very rapid spread from Africa after about 1.8 millions years ago, or that such populations were established outside Africa earlier than present evidence suggests," he said.
"I certainly think we should keep an open mind about the big picture."
The earliest tools found in Asia are routinely attributed to Homo erectus, a species known to have come from Africa.
H. ergaster—an African species that many experts believe gave rise to H. erectus—is assumed to have been the only primate capable of migrating out of Africa.
Experts cite its body form—long limbs, humanlike proportions, and a brain capable of figuring out how to hunt for meat—as evidence that it was the only species suited to life in prehistoric Asian terrain.
This might be a persuasive argument—except for the fact that australopithecines, an older form of humanlike primates, had colonized the African savannah by 3.5 million years ago.
Similar grasslands extended across Asia at the time, suggesting that australopithecines could have survived quite well in the region, the authors say.
What's more, fossil evidence for H. ergaster in Asia in the early Pleistocene is weak.
No one yet knows where H. floresiensis first came from, but it may turn out that the diminutive species has its origins in Asia.
Stringer, of the Natural History Museum, sees this as a possibility.
"The unresolved status of the intriguing Flores finds attributed to H. floresiensis leaves open the possibility that this species is the end result and last survivor of an ancient migration of very primitive humans, or even prehumans, that formerly existed more widely across Asia."
So when did early humans first leave Africa? Could they have left as early as 2.6 million years ago, as soon as they started making stone tools?
"Hominins could easily have left Africa two million years ago," Dennell said. "After all, they certainly didn't need big brains or bodies to do so."
Maybe, he concluded, "the Dmanisi [Georgia] hominins are an extremely primitive version of H. erectus that is the ancestor of the H. erectus populations in both Java and those in East Africa.
"In other words, we might be looking at [human migration] 'out of Asia,' and not 'out of Africa.'"