JANUARY 17, 2000 VOL. 155 NO. 2

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MODERN HUMANS

Just as australopithecus afarensis eventually gave rise to the genus Homo, so one species came to stand out among the Homo line and eventually led to modern humans. The fossil record is far too spotty to say how Homo habilis (handy man) and other members of its genus--H. rudolfensis, H. ergaster and H. erectus--were related, to what extent they overlapped or even whether they all represent distinct species. Many scientists believe, though, that it was H. erectus that was the ultimate victor, the direct ancestor of our own species.

ALSO IN TIME

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H. erectus was also the first hominid to emigrate from Africa, at least 1.8 million years ago, spreading all the way to China and Indonesia. Then, at some point--for reasons still mysterious--the lineage diverged, with one branch leading to Neanderthals and another to modern humans.

Exactly when and how it happened is unclear. The oldest Neanderthal fossils in hand date only to 200,000 B.P., and the oldest Homo sapiens to about 100,000. But some recent discoveries may help answer those questions. A 1 million-year-old cranium from Buia, Eritrea, for example, has characteristics of both H. erectus and H. sapiens. And what Asfaw and his colleagues call a "spectacular" partial cranium of the same age from Ethiopia should help as well when it's formally unveiled.

An unusually rich trove of fossils has been found at two sites in northern Spain's Atapuerca mountains. One, known as Gran Dolina, has yielded 800,000-year-old hominids that Spanish researchers believe are a new species, perhaps the most recent common ancestor of modern humans and Neanderthals. Named Homo antecessor (Latin for explorer or pioneer), they had a primitive jaw and prominent brow ridges but a projecting face, sunken cheekbones and tooth development similar to that of modern humans.

Less than a kilometer away, antecessor's co-discoverer, Juan Luis Arsuaga of the Universidad Complutense de Madrid, is excavating at Sima de los Huesos (Pit of Bones), deep inside a natural cave. So far, his team has found thousands of fossils from at least 33 hominids of all ages. About 300,000 years old, they appear to represent an early stage of Neanderthal evolution. Explains Eric Delson, a professor of anthropology at Lehman College in New York City: "For the first time, we have a good population from a single place and enough variation to show Neanderthal features being distilled and standardized."

What occurred some 200,000 years later, when Homo sapiens first met their Neanderthal cousins--the only other hominid species that hadn't dwindled into extinction--is a matter of much speculation, scientific and otherwise. Our species would end up the only one left standing, but whatever happened to the Neanderthals didn't happen quickly. Plentiful archaeological evidence proves that Homo sapiens and Homo neanderthalensis inhabited the same general turf in many parts of Europe and the Middle East for thousands of years. That doesn't prove, however, that they lived as peaceable neighbors. Populations were so sparse that run-ins probably would have been rare.

A romantic notion of how the Neanderthals disappeared has been around for decades: perhaps they were eliminated by interbreeding with us. Maybe we all carry a bit of Neanderthal in our DNA. Two years ago, molecular biologists tested that hypothesis by extracting some DNA from a Neanderthal fossil and comparing it with that of modern humans. Their conclusion: the differences are great enough to rule out significant interbreeding, even though such mating would have been biologically possible.

But a skeleton discovered in Portugal in December 1998 gives new life to the old idea. Co-discoverer João Zilhão, director of the Portuguese Institute of Archaeology, and consultant Erik Trinkaus of Washington University in St. Louis, Mo., claim that the 24,500-year-old remains of a four-year-old child show a mix of human and Neanderthal features. The boy could simply be the love child from a single prehistoric one-night stand--except that the last true Neanderthals had disappeared from the area at least 3,000 years earlier. Plenty of experts are unwilling to be swayed by romance, however--especially the American Museum's Ian Tattersall, who says flatly, "It's just a chunky modern kid. There's nothing special about it."

Besides, one isolated case can't explain the demise of an entire population spread across thousands of kilometers. The mystery is all the greater as paleoanthropologists learn how similar to our own ancestors the Neanderthals were. They hunted cooperatively, they buried their dead, and their brains were as big as ours. The species' relative equality, says Trinkaus, "makes perfect sense, given that the two groups coexisted for several thousand years without one or the other being dominant."

What may have happened, suggests Tattersall, is that some 50,000 years after modern humans arose, we began using our brains in a fundamentally different way. Despite their burials, for example, the Neanderthals left no clear evidence of any ritual or any belief in an afterlife. Nor is there any hint of Neanderthal language. Most telling of all, Homo sapiens began, some 40,000 years ago, to create art in an astonishing variety of forms, including cave paintings and female statuettes. All this, Tattersall and others believe, represents a single, profound change: the development of symbolic thought. "Art, symbols, music, notation, language, feelings of mystery, mastery of diverse materials and sheer cleverness: all these attributes, and more, were foreign to the Neanderthals and are native to us," he writes in his 1998 book, Becoming Human. For the first time, innovation was a routine part of human life that could easily be shared with others--not just something that occurred every million years or so. Against that kind of competition, no other human species could hold out.

THE END OF EVOLUTION?

The development of symbolic thought and complex communication did nothing less than alter human evolution. For one thing, high-tech transportation means that the world, though ethnically diverse, now really consists of a single, huge population. "Everything we know about evolution suggests that to get true innovation, you need small, isolated populations," says Tattersall, "which is now unthinkable."

Not only is a new human species next to impossible, but technology has essentially eliminated natural selection as well. During prehistory, only the fittest individuals and species survived to reproduce. Now strong and weak alike have access to medicine, food and shelter of unprecedented quality and abundance. "Poor peasants in the Third World," says University of Michigan anthropologist Milford Wolpoff, "are better off than the Emperor of China was 1,000 years ago."

And technology shows no signs of slowing down, which means that even dramatic changes in the natural world won't necessarily have evolutionary consequences. Argues Wolpoff: "We're not going to [adapt to] the next ice age by changing our physical form. We'll set off an atom bomb or set up a space mirror or whatever [to control climate]." Manipulation of the human genome, meanwhile, will eventually let us change the basic characteristics of our species to order. Evolution by natural selection could be replaced, perhaps chillingly, with evolution by human intervention.

That's not to say humanity can't become extinct. An 80-km-wide asteroid crashing down from space would do it. So could a sudden and thorough collapse of earth's ecosystem through pollution, deforestation and the like--unless we establish some colonies in space beforehand. But whatever happens, the long history of multiple hominid species struggling for supremacy on earth is over. After millions of years, evolution by natural selection, operating blindly and randomly, has produced a creature capable of overturning evolution itself. Where we go from here is now up to us.

With reporting by Martha de la Cal/Lisbon, Peter Hawthorne/Cape Town, Edward Owen/Madrid and Simon Robinson/Addis Ababa

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