Researchers at the University of Utah have studied the DNA of two babies buried 11,500 years ago. After assessing them, the researchers said that Native Americans have come from people who crossed the Bering land bridge before they settled in North and South America.

The babies were unearthed together from the Upward Sun River site, which is now central Alaska. The researchers said that babies were having different mothers. One of the babies was between 6 to 12 weeks old and the other one was a pre-term fetus.

Both the babies are considered to be earliest human remains in northern North America. Study's senior author University of Utah anthropology professor Dennis O'Rourke said, "We see diversity that is not present in modern Native American populations of the north and we see it at a fairly early date. This is evidence there was substantial genetic variation in the Beringian population before any of them moved south".

By showing that both genetic lineages lived so far north so long ago, the study supports the "Beringian standstill model." It says that Native Americans descended from people who migrated from Asia to Beringia - the vast Bering land bridge that once linked Siberia and Alaska - and then spent up to 10,000 years in Beringia before moving rapidly into the Americas beginning at least 15,000 years ago.

Another theory was that the two Native Americans lineages evolved as the people moved south and dispersed, not while they still were in Beringia, says Justin Tackney, the new study's first author and a University of Utah anthropology doctoral student. But finding those lineages in the infants only a few thousand years after the migration south began indicates those lineages already were present before the migration started.

"It supports the Beringian standstill theory in that if they [the infants] represent a population that descended from the earlier Beringian population, it helps confirm the extent of genetic diversity in that source population," O'Rourke says.

In 2013, Potter's team found the remains of two more buried infants beneath the first. They had't been cremated. Potter says it's hard to tell how the infants died. Potter, who co-authored the new study, asked O'Rourke to analyze their mitochondrial DNA.

O'Rourke and Tackney worked with University of Utah geneticists to sequence the mitochondrial DNA of the two infants - known as USR1 and USR2 for Upward Sun River. Mitochondrial DNA is located in mitochondria, or the power plants of cells.

The researchers identified infant USR1 as belonging to Native American lineage C1b, while infant USR2 is part of a more common native lineage known as B2. (Native American lineages begin with the letters A, B, C, D or X.)

"It's not common to find infants buried together that are not related maternally," O'Rourke says. "It raises questions about the social structure and mortuary practices of these early people," including whether the babies had a common father.

Lineage C1 (most remains aren't identified to the subgroup C1b level) is found most often among the Pima and Hualapai Indians of Arizona, the Delta Yuman of California, and six other tribes, including the Ignaciano in Bolivia, the extinct Tainos in Puerto Rico and a group represented by 700-year-old bones at Norris Farms in Illinois.

Before this discovery, there was not enough evidence available to support the 'Beringian standstill model'. As per this theory, ancestors of modern Native Americans must have spent a long time isolated from different populations, so that they can come up with uniquely American lineages.

It also says that people migrated from Siberia 2,000 to 30,000 years ago and remained there for around 10,000 years and then moved to North and South America 15,000 years ago. But there are some who are not convinced with it.