The largest collection of early American skulls ever studied is lending credence to a controversial theory that two distinct populations of humans--rather than one--colonized the New World. If true, the findings indicate that people who shared an ancestry with modern day Australians and Melanesians may have settled on the continents somewhat earlier than immigrants from northeast Asia.
Not so long ago, the origins of the first Americans seemed fairly certain: Beginning about 12,000 years ago, people from northeast Asia entered North America via the Bering landbridge in several waves of immigration. These ancestors of present-day Native Americans spread out to populate the entire New World. But in recent years, some archaeologists have argued that the first immigrants to the Americas were people from southeast Asia who share ancestors with native Australians and Melanesians. Chief amongst them has been Walter Neves of the University of Sao Paolo in Brazil.
This week, Neves claims new support for this hypothesis from an analysis of 81 prehistoric skulls found in the Lagoa Santa region of southeast Brazil. Unearthed over a 150 year period, most of the skulls range from 7500 to 8500 years old, although two skulls were dated to around 11,500 years ago. Detailed measurements of the skulls, combined with statistical analyses of their morphology, shows that they most closely resemble those of present-day people from Australia and Melanesia, whose skulls tend to be long and narrow with projecting faces, Neves and his Sao Paolo colleague Mark Hubbe report online this week in The Proceedings of the National Academy of Sciences. People from northeast Asia, on the other hand, tend to have skulls that are short and wide with relatively flat faces.
Because skulls similar to those at Lagoa Santa have been found in North and South America, Neves and Hubbe conclude that their data support a hypothesis in which two distinct populations colonized the New World: one group from southeast Asia that is morphologically similar to Australians and Melanesians, which arrived around 12,000 years ago (also via the Bering landbridge), and a second group from northeast Asia, which followed soon after and eventually gave rise to today's Native Americans. As for the immigrants fromwith southeast Asia, they may have been replaced after the second group from Asia arrived or may even have held on until Europeans arrived on the continent, Neves says.
Archaeologist Tom Dillehay of Vanderbilt University in Nashville, Tennessee, says he had some doubts about Neves' hypothesis when it was first proposed, but "Neves is building a more solid case with these skeletons." But physical anthropologist Clark Larsen of Ohio State University in Columbus notes that the differences between the Lagoa Santa skulls and those of Native Americans do not necessarily indicate that they represent two biologically distinct groups. Changes in diet over time can modify the jaw muscles in ways that also alter skull shape, without major genetic changes, Larsen argues. Concludes Dillehay: "Neves' hypothesis may now be the most plausible explanation, but it is not yet fully acceptable."