Some of the first Americans may have been Australians. A new study of Brazilian skulls ranging from 11,000 to 7,500 years old has revealed that they have more in common with Aboriginal Australians and Melanesians than modern Native Americans.

"The earliest Americans are very different from nowadays Indians or later archaeological material," says Walter Neves of the University of Sao Paulo in Brazil, co-author of the study that is being published online this week by the Proceedings of the National Academy of Sciences. "We are proposing that the Americas were populated by waves of humans."

Neves and his colleague Mark Hubbe studied skulls found in the Lagoa Santa region of central Brazil, an area riddled with caves. Since 1842, at least 250 ancient human skeletons ranging from ancient to nearly modern have turned up there. The scientists dated 22 of the oldest specimens by measuring their isotopes and found them to be between 8,500 and 7,500 years old.

The researchers then enlarged this sample by including all remains buried in a similar fashion--in shallow graves covered with small blocks of limestone or quartz--which they presume to hail from the same time period as the dated material. They ended up with 55 well preserved skulls that fit the criteria and compared them with standardized skull measurements of more than 2,500 modern humans. By comparing the size and shape of the skulls as well as their noses and eye sockets, Neves found that the oldest Brazilian specimens most closely resembled the skulls of the Tolai people of New Britain in Papua New Guinea, followed by other Australo-Melanesians. The closest match among modern Native Americans are Eskimos. "Eskimos get close because they are one of the few American Indians to have a long skull," Neves explains.

This broader skull data seems to support Neves' theory that two distinct waves of people populated the Americas, perhaps both crossing the Bering Strait during different periods. But recent genetic studies have been interpreted to indicate only one such migration. "DNA lineages are often lost during the course of evolution, even in short periods of time," Neves counters. "Today, no South American native group presents the X [mitochondrial DNA] lineage, which is universal among North American native groups. However, DNA extracted a few years ago from human skeletons from the Brazilian Amazon, dated to only a couple thousand years ago, showed clearly that the X lineage was present in South America."

The lineage picture is further obscured by the decimation of the native population upon the arrival of Europeans beginning in the 15th century. But the discrepancies between the genetic and fossil data may yet be resolved. "I bet that if [molecular biologists] come to use the genes responsible for cranial morphology, our results will certainly agree," Neves says. "When the earliest Native Americans are taken into account, it becomes clear that the two most different and opposite architectural plans in terms of human cranial morphology existing today on the planet were represented in the New World."