Thousands of years ago, a teenage girl toppled into a deep hole in a Mexican cave and died. After being hidden there for more than 12,000 years — along with the bones of dozens of extinct ice age beasts — 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 said.

The girl’s 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 Yucatán 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, said. “The floor disappeared under us and we could not see across to the other side.”

Months later, they returned and reached the floor of the 100-foot tall chamber — which was littered with the bones of saber-toothed tigers, giant tapirs and bears — and divers quickly spotted the skull as they swept the chamber with flashlights. “It was a small cranium laying upside-down with a perfect set of teeth and dark eye sockets looking back at us,” recalled Nava of Bay Area Underwater Explorers, a nonprofit conservation organization based in Berkeley, Calif. The divers named the skeleton Naia, after a water nymph of Greek mythology.

Now, researchers have published an analysis of Naia’s skeletal remains in the journal Science, calling it the oldest, most complete specimen ever discovered in the Americas.

The study authors say that the bucktoothed 15- or 16-year-old girl’s mitochondrial DNA reveals she is related to 11 percent of living American Indians, and links them genetically to a population of early humans who inhabited a land now submerged beneath the Bering Sea.

Carbon-dating of her teeth and isotope data from crystals that formed on her bones helped study authors determine that the girl lived 12,000 to 13,000 years ago in what would have been a parched environment. They believe she was probably searching for water when she entered a dark, underground cave and then plummeted 100 feet into the massive chamber. now called Hoyo Negro, or black hole.

Unable to escape — her hip bone shattered from the fall — she died amid a menagerie of similarly doomed megafauna. As the ice age ended and glaciers melted, sea levels rose and filled the chamber with water, sealing it off from humanity.

Or at least it did until 2007, when scuba divers discovered “a time capsule” of Central American life at the end of the ice age, said study leader James Chatters, a paleoarchaeologist at Applied Paleoscience, a private research company in Bothell, Wash.

The analysis — reported by researchers from the United States, Canada, Mexico and Denmark — is among a number of genetic findings that have radically altered the debate over how humans came to inhabit the New World.

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 archaeologists, that suggests that the Americas were originally populated by people from Europe, as well as Asia.

Within the past 10 years, however, DNA research has suggested that America’s early dwellers emerged from a single ancestral population that initially came from Asia.

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. “Paleoamericans and Native Americans descended from the same homeland,” Chatters said.

John Hoffecker, a paleoarchaeologist at the University of Colorado, Boulder, said it would help reshape debate on the first Americans. “For years archaeologists have been debating this transatlantic thing, and really it’s been an enormous distraction. This helps us focus on Beringia, which is what we should have been doing all along.”

The Los Angeles Times contributed to this report.