Modern man reached to North America from Eurasia over an Ice Age-land bridge, slogging over as a single group about 15,000 years ago and spreading southwards, the thinking had been. That theory has now been trashed by the finding that a few Amazon Indian tribes are genetically closer to Australian and Melanesian aboriginals than to the Siberians from whom they were assumed to have arisen.
One implication from the paper published in Nature is that these Amazonian groups arose from an earlier arrival in the Americas. A second is that indigenous American population had more than one founding group.
Until now, genetic analysis had supported the theory of a single group of origin for indigenous American populations. But some morphological studies had been hinting otherwise: Early skeletons found in North America were discovered to be more similar to contemporary Australasians (natives of Australia, Melanesia, Southeast Asian islands) than to their postulated Siberian forebears. For instance, some skulls were longer and narrower in the Melanesian style, rather than rounder and broader in the native American style.
Now a genetics group at Harvard headed by David Reich tested more populations – and demonstrated that the Surui, a small clan that today grows coffee, the Xavante and the Karitiana Indians of central Brazil are genetically closer to native Australians, New Guineans and Andaman Islander than to the Eurasians from which they were assumed to have arisen. "This [genetic] signature is not present to the same extent, or at all, in present-day Northern and Central Americans," writes the team – and concludes that the set of founding populations of the Americas was more diverse than previously thought.
It bears saying that the peopling of Australia – how, when – remains a controversial topic, but present theory has the founding population of today's indigenous Australians reaching there about 50,000 years ago.
So what we have is three groups in very different parts of the planet sharing a genetic signature. The team therefore postulates that they had a common ancestor in Asia. Over the generations this family split and migrated in multiple directions – one spreading to Australia, one heading for the Andamans, and one walking north to Siberia and crossing over to the Americas before the Eurasian surge.
There's no telling why no trace of this earlier American founding group has been discovered in North America or other parts of the southern continent. They might not have survived for any number of reasons, including possibly being wiped out by that later surge of migrants from Siberia. Or more testing could still find surviving groups.
Meanwhile, 23,000 years ago
Meanwhile, a separate and rather enormous group of scientists from around the world published a paper in Science saying pretty much the opposite – that based on their genetic studies, all native Americans, from north and south, arose from a single group of founders that crossed over from Siberia "no later" than 23,000 years ago. (Which is coincidentally the age of recent findings in Israel of the earliest proto-agriculture: previously agriculture had been thought to have begun around 12,000 years ago.)
Anyway, they say the group reached North America and split around 13,000 years ago, with one group spreading to South America. "Subsequent gene flow resulted in some Native Americans sharing ancestry with present-day East Asians (including Siberians) and, more distantly, Australo-Melanesians," they write.
But their finding need not negate the conclusions of the Harvard team. Indeed most of the native populations of the Americas could have arisen from the Eurasian crossing over the Beringia bridge; but some could still have arisen from an earlier crossing. None of the teams purport to have tested every human in the world.