Deep within a flooded cave in Mexico's Yucatan Peninsula, along with skeletons of saber-toothed tigers and giant sloths, scientists found the oldest complete human skeleton yet discovered in the Americas. The skeleton of a teenage girl, nicknamed Naia, was first found by a team of divers exploring the Hoyo Negro cave in 2007. The deep pit, part of a complex cave system, reaches more than 100 feet below sea level.

Besides being a completely awesome exercise in underwater archeology, the team of scientists studying the skeleton think that it provides critical information that will help us understand the human colonization of the Americas.

The first humans arrived in North America from Northeast Asia via a land bridge. After entering what is now Alaska about 20,000 years ago, they slowly spread into the continent. But archeologists studying the remains of paleoamericans from several thousand years ago have found that, morphologically, the skulls and teeth of early Americans were more similar to modern people in Africa or Australia than to modern Native Americans or Siberian people. These differences suggest that either the Americans were colonized more than once or that evolutionary changes after colonization were responsible for the mismatched traits.

This question has remained under debate because very few skeletons of ancient Americans have been discovered to fill in the gap. Fewer than 30 remains have been found that are older than 10,000 years and only five from more than 12,000 years ago. Of these few remains scientists have, it's rare to have intact skulls and teeth to work with.

Which is why finding Naia submerged in a cave was such a big deal—she's by far the most complete set of remains found that is at least 12,000 years old.

How did researchers find out her age? Traditional radiocarbon dating on the collagen in the bones didn't work since the material was not preserved well under water. But there are some other indications of age. For starters, geologic history suggests that the caves began flooding about 10,000 years ago, as the last ice age ended. Before the cave permanently flooded, calcite crystals formed on the human and animal bones in the damp cave, similar to how stalagmites form from water droplets.

These formations, referred to as florets by the researchers, came in very handy. They were also found on other bones in the cave, those from saber-toothed tigers and ground sloths, which were largely extinct by 13,000 years ago.

The florets also allowed the researchers to try a radiometric dating method using thorium and uranium, which is commonly used on calcium carbonate materials such as the bone florets. It works because uranium in the water gets deposited in anything that precipitates from it, like the florets. Then, once the cave is inundated and the florets stop growing, the uranium decays to thorium. Measuring the ratio of the two isotopes allows the scientists to calculate an age.

Using this technique, the researchers determined that the minimum age of the bones is 12,000 years old. A separate analysis on the tooth enamel suggested a maximum age of 13,000 years, leading them to conclude that the 15- or 16-year-old girl fell into the cave between 12,000 and 13,000 years ago. Her broken pubic bones suggest that she fell, but otherwise, the skull and bones are largely intact.

Morphologically, the skull shape is similar to that of other American remains that are greater than 10,000 years old. The scientists report that her face is short and broad, with a broad nose and wide-set eyes.

Unlike many older skeletons, the scientists were able to extract DNA from one of her molars. Using techniques developed for DNA fragments from ancient specimens, they sequenced it and found that on a key characteristic, she fits in a lineage of Asian descent found only in the Americas. The signature in question in commonly found today in indigenous people from Chile and Argentina.

The DNA evidence linking her to modern indigenous Americans and the morphological evidence consistent with other skeletons studied from the same era strongly supports the idea that the Americas were only colonized once and that, subsequently, the new American populations evolved in their new landscapes.

Science, 2014. DOI: 10.1126/science.1252619 (About DOIs).