During the last ice age, an enormous stretch of fertile grassland connected Eurasia and the Americas. Known today as Beringia, this lost landscape was home to a small but important population of humans. Now, the discovery of one skull in an underwater cave tells an intriguing story about what happened to the Beringians when they came to America.

Humans colonized most of the world, including Australia, tens of thousands of years before they settled in the Americas. It wasn't until the Earth's most recent ice age, 2o,000 years ago, that humans began their journey to the last remaining continent untouched by our species. Some may have come along the coast, in reed boats similar to the ones that took their ancestors to Australia. But thousands of them came across a land bridge that grew between the two continents as the ocean receded, it waters locked into glacier ice.

The term "land bridge" is misleading because it sounds as if there was a slim finger of land connecting two vast continents. Beringia was an enormous tract of land, some 1000 miles across at its widest extent, and its grassy ecosystems overlapped with those in northern Asia and America. Rather than a bridge between continents, it might make more sense to think of Beringia as simply one region in a vast mega-continent that ringed the polar region of the Earth's northern hemisphere.

Beringia, the Lost Territory Where Americans Evolved

Image via Smithsonian

Many anthropologists believe that this place was home to a small population of humans for nearly 5,000 years. When glaciers blocked off their access to both Asia and the Americas, this group thrived on the animals and plants in the region — they also changed both biologically and culturally, cut off as they were from other human groups for so long. Today their homes are buried under freezing water, and we may never know if they left any paintings or artifacts behind the way their ancestors did in Eurasia.

Who Came to America?

One thing we do know for certain is that humans began hiking into the Americas as soon as the glaciers started to retreat roughly 15,000 years ago. They came in waves, with small groups migrating out of Beringia over many centuries. Archaeologists have found signs of humans everywhere in the Americas after that point, especially along the coasts. At various fossil sites, they've discovered a wide range of weapons, the remains of ancient campsites, and gorgeous rock art.

Of course, humans were not the only animals to migrate through Beringia. Horses and camels, which evolved in the Americas, traveled across the grassy plains and arrived in Asia just as Homo sapiens was coming to North America.

A question that's bothered anthropologists for decades is whether Native Americas are direct descendants of the Beringians. The common sense answer would seem to be yes, but many pieces of evidence have suggested otherwise. First of all, the skull shapes of the earliest Americans, called Paleoamericans, were very distinctive, with wide-set eyes and narrow, projecting faces. They look quite different from the skulls of the Mesoamericans who built great civilizations in the Americas and were the direct ancestors of today's Native Americans. In addition, these Paleoamericans have distinctive toolkits that differ strongly from those of Mesoamericans. This suggests different cultural traditions for Paleoamericans and Native Americans.

Making this picture even more complicated is the recent discovery that all Native Americans share a genetic lineage that stretches back to the emergence of the Beringians into the Americas. If Native Americans came from Beringia, who are those Paleoamericans with their distinctive facial features and tookits? Some scientists have suggested that they might be arrivals from outside Beringia, perhaps from Europe or Australia, who were eventually overrun by the Mesoamericans who colonized the continents.

Wild vs. Domestic Humans

Now we're closer than ever to an answer. Just yesterday, scientists announced that they'd analyzed a beautifully-preserved skull, roughly 12 or 13 thousand years old, discovered off the coast of Mexico in 2007. That skull, which belonged to a young woman, offers an intriguing story.

Beringia, the Lost Territory Where Americans Evolved

It was found in an underwater cavern that was once part of a warm, dry tunnel system in the Yucatan. But, like the land that brought people into the Americas, it was long ago submerged by rising seas.

Scientists Deborah Bolnick and James Chatters were part of an international team that analyzed the skull and published about it in Science magazine yesterday. The team immediately recognized the distinctive look of a Paleoamerican in the skull's shape — there were the characteristic wide-set eyes and narrow, prominent facial features. But when they sequenced DNA from a tooth in the skull, they discovered something incredible. This woman was a Beringian, just like today's Native Americans. It appears that the Paleoamericans were Beringians, and they are the same people whose distant progeny eventually built the incredible civilizations of the Inka, the Maya, and the Aztecs.

Said Bolnick in a press conference yesterday:

Our results suggest that paleoamericans and contemporary Native Americans both have Beringia ancestry so the physical differences between them are more likely due to changes that occurred in Beringia or the Americas over the last 9,000 years rather than separate origins. And so what this study is presenting for the first time is the evidence that Paleoamericans with those distinctive features can also be directly tied to the same Beringian source population as contemporary Native Americans.

Chatters added that the changes we see between the Paleoamerican group and contemporary Native Americans is consistent with what happens in a lot of animals as they are domesticated. The Beringian woman discovered in Mexico would have been what biologists call a "wild type" human — her genome had not yet been altered by thousands of years of sedentary life made possible by agriculture and cities. Typically, Chatters said, domesticated humans develop more childlike faces, with their eyes set more closely together and their facial features softening.

This early Native American woman's ancestors migrated out of Beringia over thousands of years. She came from a population whose distinctive look and culture died out when humans traded a nomadic life for an agricultural one. Her progeny eventually made permanent homes in Central and South America, domesticating beans, corn, and squash as they slowly domesticated themselves.

While this skeleton offers only one data point in a vast and complicated story, it has settled one debate definitively. The ancient Paleoamerican peoples shared DNA with today's Native Americans. The differences between them came from when and how they lived, not where they came from.

Read the full scientific paper in Science