KNOW-IT-ALLS ARE fond of pointing out that the only true Americans are the descendants of the diverse tribes found by Columbus and Cortes when they first arrived in the New World. That's wrong, of course. Even North and South American Indians had immigrants for ancestors: northeastern Asians who crossed from Siberia to Alaska in prehistoric times across the bridge of land that then spanned the Bering Strait.
But when did these adventurous souls reach Alaska? What kind of people were they? How fast did they spread down through the Americas? For decades, archaeologists felt sure they knew the answers: the first Americans were skilled hunter-gatherers and toolmakers who arrived about 11,500 years ago and moved rapidly southward, reaching deep into South America within about five centuries as well as helping drive to extinction such prehistoric mammals as mastodons and woolly rhinos.
Now, a competing theory about the original Americans, once touted by only a small band of renegade archaeologists, has become too compelling to ignore. Its thesis is that the first migration took place not 11,500 years ago but 20,000 or 30,000 or even 50,000 years ago. Although the evidence is still sketchy, archaeological digs in Chile, Brazil, Venezuela, the U.S. and Canada have yielded tantalizing clues that this radical notion might be correct. "This is a hot area of research," says Dennis Stanford of the Smithsonian Institution. "Man's origin in the New World is one of the major unanswered questions of archaeology."
The old assumption that humans arrived in the Americas about 11,500 years ago is known as the Clovis hypothesis. The name comes from the 1933 discovery of a fluted stone spearpoint dated to that era in a pile of mammoth bones near Clovis, New Mexico. Over the years, similar spearpoints were unearthed all over North America, all apparently about the same age. Because the weapons, known as "Clovis points," were so widespread, and because essentially no artifacts at all were found in older sediments, archaeologists and anthropologists concluded that the Clovis people were the first and that they came over from Asia in the last years before melting glaciers and rising sea levels submerged the Bering land bridge.
The theory has flaws, though. One is the idea that prehistoric people could have populated an entire continent in a mere 500 years, the span between the time of the presumed land migration and the time by which Clovis spearpoints had been deposited throughout North America. Even more problematic are signs of very early culture in South America. "Humans don't sprint through their environment," says Mercyhurst College archaeologist James Adovasio. "But that's what the Clovis guys would have us believe. There's no analogue for that in archaeological history."
More doubts arise from studies of linguistic and genetic diversity. Modern North and South American Indian languages presumably evolved from a single ancestral tongue, but they differ so greatly that it is hard to imagine how this could have happened in just a few score centuries. Similarly, the mitochondrial DNA in the cells of Native Americans differs so much from tribe to tribe that a single, relatively recent ancestral group seems unlikely.