The original Americans came from Siberia in a single wave no more than 23,000 years ago, at the height of the last Ice Age, a new study has found.  They may have stayed in the north for thousands of years before spreading in two distinct populations throughout North and South America, according to a new genomic analysis.

This Ice Age migration over a land bridge between Siberia and Alaska is distinct from the arrival of the Inuit and Eskimo, who were latecomers, spreading throughout the Arctic beginning about 5,500 years ago, the researchers said.

The analysis, using the most comprehensive genetic data set from Native Americans to date, was conducted using three different statistical models.

The data consisted of the sequenced genomes of 31 living Native Americans, Siberians and people from around the Pacific Ocean, and the genomes of 23 ancient individuals from North and South America, spanning between 200 and 6,000 years ago.

"There is some uncertainty in the dates of the migration and the divergence between the norther and southern Amerindian populations, but as we get more ancient genomes sequenced, we will be able to put more precise dates on the times of migration," said co-author Yun Song, associate professor at the University of California, Berkeley.

The international team concluded that the northern and southern Native American populations diverged between 11,500 and 14,500 years ago, with the northern branch leading to the present day Athabascans and Amerindians broadly distributed throughout North America.

The southern branch peopled Central and South America, as well as part of northern North America.

"The diversification of modern Native Americans appears to have started around 13,000 years ago when the first unique Native American culture appears in the archaeological record - the Clovis culture," said Rasmus Nielsen, a UC Berkeley professor.

"We can date this split so precisely in part because we previously have analysed the 12,600-year-old remains of a boy associated with the Clovis culture," Nielsen said.

Researchers found that both populations of Native Americans have a small admixture of genes from East Asians and Australo-Melanesians, including Papuans, Solomon Islanders and Southeast Asian hunter gatherers.

"It's a surprising finding and it implies that New World populations were not completely isolated from the Old World after their initial migration," said Eske Willerslev from the Natural History Museum, University of Copenhagen, who headed the study.

"We cannot say exactly how and when this gene flow happened, but one possibility is that it came through the Aleutian Islanders living off the coast of Alaska," Willerslev added.

The study appears in the journal Science.