Location: Texas

Saturday, November 20, 2004

Descent of Man

Scientists have long believed that modern humans, the species Homo sapiens, coexisted in parts of Europe and western Asia with another species, Homo neandertalis. For reasons unknown, the Neanderthals died out about 30,000 years ago, leaving the planet to the single species of genus Homo that now accounts for all 6.3 billion people on the planet.

That, at least, is what anthropologists thought, until Australian and Indonesian researchers gave the family tree a mighty shake last month with the reported discovery of some unusual bones in the limestone caves of Flores, a volcanic island 300 miles east of Bali.

The bones in the Liang Bua caves — seven individuals in all — were unlike anything scientists had encountered before. The bones were those of creatures that stood 3 feet high, about the size of a 21st-century toddler. Despite thick brows and "nutcracker" jaws, they clearly weren't apes. Femur and pelvis bones showed that they walked upright, but more like earlier human ancestors than modern humans.

From their worn molars, it was clear that despite their small stature, they weren't children. And from the tiny, grapefruit-size brain of one intact skull — that of an adult female that Michael Morwood and his colleagues dubbed Ebu but quickly renamed "hobbit" — it was clear they weren't pygmies either. Pygmies have brains proportionately as large as other humans.

Buried in the cave floor, the archaeologists also found ample evidence that the little people of Flores — now known as Homo floresiensis — made stone tools, hunted, fished and used fires for cooking, all activities associated with human intelligence.

The real shocker, however, was the discovery that although some of the bones were 90,000 years old, some dated to less than 18,000 years ago — a time by which modern man is thought to have inherited the earth. All of it.

"This gives us a whole new perspective on human evolution," said Rick Potts, director of the human origins program at the National Museum of Natural History.

Modern man, the Neanderthals, and now Homo floresiensis are all thought to have descended from a common ancestor, Homo erectus, which migrated out of Africa and spread throughout Europe and Asia within the last 1 million to 2 million years.

Recent and controversial discoveries elsewhere in Indonesia, however, suggest that Homo erectus may have existed there as recently as 50,000 years ago. That finding, coupled with the Flores discoveries, suggests that, for a time, at least four human species roamed the planet.

But if there were four kinds of humans, why not more? Morwood says human evolution — especially in isolated settings like Flores — may prove to be much more diverse than anyone has imagined.

"I think we're going to have a plethora of human species showing up," he said. "Some may be really weird."

Archaeologists say Flores' little people may have been small as a result of "island dwarfing," a process observed in some animals that evolve smaller forms that are better adapted to a restricted island environment.

They also suspect that the isolated island population, which may have inhabited Flores for tens of thousands of years, probably died out during a period of intense volcanic eruptions there, about 13,000 years ago.

There is no evidence that the Lilliputian people of Flores survived until modern times, although Morwood says there are tales in local folklore of a diminutive group of cave-dwelling people who disappeared about the time that the Dutch began colonizing the islands in the late 1500s.

Scientists don't have a clue about how humans managed to reach the southern end of the Indonesian archipelago. Potts and others say it's unlikely that humans could have deliberately navigated open water so early in time, but he says they might have accidentally drifted there on crude log rafts.

Nor is it clear exactly where Homo floresiensis branched off the human family tree — or how many other twists and turns in the evolution of the human species remain to be discovered.

Scientists today distinguish 12 to 19 species of early humans, but there is widespread disagreement about how they are related, which are ancestors of modern humans and which represent lineages that simply died out.

The oldest fossil of a hominid — a humanlike creature called Toumai who had the smallish brain case of an ape but sported a skull and teeth with human characteristics — was unearthed in central Africa two years ago. It is between 6 million and 7 million years old, a period when anthropologists say the human lineage was becoming distinct from that of chimpanzees.

Numerous fossil discoveries from more recent times suggest that distinctively human traits emerged gradually over the last 5 million years as nature experimented with the evolution of the human species. Most of the experiments proved to be evolutionary dead-ends.

By 4 million years ago, human ancestors were walking upright, but they had brains only about one-third the size of modern humans. As early as 2.5 million years ago, they were making and using crude stone tools.

By 2.2 million years ago, the emergence of Homo habilis in Africa showed a clear trend toward larger brains — a development followed about half a million years later by the appearance of the first fully human species, Homo erectus.

Anthropologists differ over exactly how and where Homo sapiens appeared.

Thoroughly modern man — the subspecies known as Homo sapiens sapiens, with a lighter build and a bigger brain — emerged in Africa about 130,000 years ago and spread worldwide.

Until now, however, there was nearly universal agreement that, with the disappearance of the Neanderthals 30,000 years ago, only a single species of humans remained.

A fertile field of study, no?


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