THE earliest inhabitants of North America differed subtly but significantly from modern native Americans. But anthropologists cannot agree why.
The difference is clearly seen in the skull shapes of the first people to colonise the continent, who had longer, narrower skulls than modern people. One theory says it is because two distinct groups of people migrated to North America at different times. But another says that just one population reached the continent and then evolved different physical attributes except for a few isolated groups.
Anthropologists once assumed the earliest Americans resembled modern native Americans. That changed with the discovery of a 10,500-year-old skeleton called Luzia in Brazil, and the 9000-year-old skeleton of Kennewick man in Washington state 鈥� both had long, narrow skulls. Some researchers argued that they were simply unusual individuals, but scientists have now identified the same features in other remains more than 8000 years old from South America, and in recent remains from an isolated population that lived at the southern tip of Baja California in Mexico.
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Six skulls dating from 8200 to 9500 years ago were found in a rock shelter at Santana do Riacho, Brazil. They have long, narrow brain cases and low faces, reports a team led by Walter Neves of the University of Sao Paulo in Brazil (Journal of Human Evolution, vol 45, p 19). Neves says the skulls resemble those of modern Australians and Africans, but they do not look like those of modern native Americans, or people living in northern Asia, who are thought to be their closest relatives.
Neves proposes that humans colonised the Americas in two distinct waves. The first came from south-east Asia about 14,000 years ago, made up of people related to modern Australians and Polynesians. Neves thinks they migrated rapidly south along the Pacific coast while central North America remained icebound. The second comprised short-skulled people who came from north-east Asia about 11,000 years ago, and spread into central North America as the ice retreated (see Map). The second wave may have been larger, and eventually came to dominate the Americas, Neves says.
The first-wave people survived until just a few hundred years ago as the Peric煤 hunter-gatherers of Baja California, says Rolando Gonz谩lez-Jos茅 of the University of Barcelona (Nature, vol 425, p 62). He measured 33 Peric煤 skulls and found their features were similar to those of the ancient Brazilian skulls. Gonz谩lez-Jos茅 thinks the formation of the Sonora desert isolated the Peric煤 for thousands of years, but vanished when Europeans disrupted their culture.
鈥淭he discovery is exactly what I have been predicting since the late 1980s,鈥� Neves told New Scientist. Joseph Powell, an anthropologist at the University of New Mexico in Albuquerque, is not convinced. He thinks the earliest Americans did come from south-east Asia, but believes they evolved into modern native Americans. 鈥淓ven with two waves, each would have changed over the past 10,000 to 12,000 years through adaptation and microevolution,鈥� he says. Neves argues that the change in skull shape after 8000 years ago is too sudden for evolution.