NEW YORK – Thousands of years ago, a teenage girl toppled into a deep hole in a Mexican cave and died. Now, her skeleton and her DNA are bolstering the long-held theory that humans arrived in the Americas by way of a land bridge from Asia, scientists say.
The girl's nearly complete skeleton was discovered by chance in 2007 by expert divers who were mapping water-filled caves north of the city of Tulum, in the eastern part of the Yucatan Peninsula. One day, they came across a huge chamber deep underground.
"The moment we entered inside, we knew it was an incredible place," one of the divers, Alberto Nava, told reporters. "The floor disappeared under us and we could not see across to the other side."
They named it Hoyo Negro, or black hole.
Months later, they returned and reached the floor of the 100-foot tall chamber, which was littered with animal bones. They came across the girl's skull on a ledge, lying upside down "with a perfect set of teeth and dark eye sockets looking back at us," Nava said.
The divers named the skeleton Naia, after a water nymph of Greek mythology, and joined up with a team of scientists to research the find.
The girl was 15 or 16 when she met her fate in a cave, which at that time was dry, researchers said. She may have been looking for water when she tumbled into the chamber some 12,000 or 13,000 years ago, said lead study author James Chatters of Applied Paleoscience, a consulting firm in Bothell, Washington. Her pelvis was broken, suggesting she had fallen a long distance, he said.
The analysis of her remains, reported Thursday in the journal Science by researchers from the United States, Canada, Mexico and Denmark, addresses a puzzle about the settling of the Americas.
Most scientists say the first Americans came from Siberian ancestors who lived on an ancient land bridge, now submerged, that connected Asia to Alaska across the Bering Strait. They are thought to have entered the Americas sometime after 17,000 years ago from that land mass, called Beringia. And genetic evidence indicates that today's native peoples of the Americas are related to these pioneers.
But the oldest skeletons from the Americas -- including Naia's -- have skulls that look much different from those of today's native peoples. To some researchers, that suggests the first Americans came from a different place.
Naia provides a crucial link. DNA recovered from a molar contains a distinctive marker found in today's native peoples, especially those in Chile and Argentina. The genetic signature is thought to have arisen among people living in Beringia, researchers said.
That suggests that the early Americans and contemporary native populations both came from the same ancestral roots in Beringia -- not different places, the researchers concluded. The anatomical differences apparently reflect evolution over time in Beringia or the Americas, they said.
The finding does not rule out the idea that some ancient settlers came from another place, noted Deborah Bolnick, a study author from the University of Texas at Austin.
Dennis O'Rourke, an expert in ancient DNA at the University of Utah who didn't participate in the work, said the finding is the first to show a genetic link to Beringia in an individual who clearly had the anatomical signs of a very early American. He said he considered the notion of multiple migrations from different places to be "quite unlikely."
Last February, other researchers reported that DNA from a baby buried in Montana more than 12,000 years ago showed a close genetic relationship to modern-day native peoples, especially those in Central and South America. An author of that study, Mike Waters of Texas A&M University, said the Mexican finding fits with the one in Montana.
There are so few such early skeletons from the Americas, he said, that "every single one of them is important."
However, Richard Jantz, a retired professor of forensic anthropology at the University of Tennessee in Knoxville, said he still believes early settlers arrived by boat from east Asia before any migration occurred via Beringia. That's based on anatomical evidence, he said. The argument in the new paper "leaves a lot of unanswered questions," he said in an email.