The 12,000-year-old remains of a young girl found in a water-filled cavern in southern Mexico are helping shed light on how migrations over an ancient land bridge from Siberia brought the first people to the Americas.

The DNA of the remains suggests those first American were the direct ancestor of modern Native Americans, bolstering the theory that a land bridge that scientists have dubbed Beringia allowed the migrations by connecting Asia and the Americas during an ice age between 26,000 and 18,000 years ago.

The bones of the teenage girl found deep below Mexico's Yucatan Peninsula, dubbed Naia from the Greek for "water nymph," show a genetic lineage classified as Haplogroup D1, a marker present in many modern Native Americans and only found in the Americas.

Although the teenage girl did not look like present-day Native Americans, having narrower cheeks and a high forehead, the DNA maker recovered from her remains proves a genetic link between modern peoples and an early human population inhabiting the long-vanished land bridge.

"This lineage is thought to have developed in Beringia, the land that now lies beneath the Bering Sea after its Ice Age occupants became genetically isolated from the rest of Asia," lead study author Jim Chatters says.

At the time the girl was living the Yucatan region was dry and desert-like, and she may have been searching underwater caves for water before she fell into a deep chamber and died, the researchers suggest.

The chamber, also containing the remains of giant sloths, cave bears and saber-tooth cats, filled with water from melting glaciers as the Ice Age ended, sealing it off for millennia.

It was only in 2007 that divers began exploring the flooded cavern called "Hoyo Negro," Spanish for "black hole," and discovered the skull and other bones of the young girl.

The DNA analysis of Naia proves migrations over the Beringia land bridge made is as far as southern Mexico.

"We were able to identify her genetic lineage with high certainty," says University of Illinois anthropology Professor Ripan Malhi, whose lab was among several conducting the DNA analysis. "This shows that living Native Americans and these ancient remains of the girl we analyzed all came from the same source population during the initial peopling of the Americas."