Scientists have long been somewhat confounded by a mystery of how the Americas were first settled. The first people who walked across the Bering Strait from Asia around 20,000 years ago are the ancestors of Native Americans, yet modern people look rather different from those first immigrants. A theory that explains this difference is that other migrations took place afterward, and the different waves of immigrants interbred. DNA evidence from a skeleton found in an underwater cave now sheds some light on the question.
Between 12,000 and 13,000 years ago, a 15-year-old girl wandered into a cave now called Hoyo Negro (“black hole”) on the Yucatan Peninsula in Mexico and died, apparently from a fall in the darkness of the cave into a 150 foot hole. Then the Ice Age glaciers melted and the cave was filled with water. In 2007, divers discovered her skeleton on a ledge, her skull at rest on an arm bone. Ribs and a broken pelvis lay nearby. Her upside-down skull with intact teeth caught the eye of a man in scuba gear.
The divers contacted archeologist Pilar Luna of Mexico’s National Institute of Anthropology and History, and with support from the National Geographic Society they continued to explore the pit and document the fossils at the bottom, including two saber-toothed cats, six bears, three cougars and two ground sloths.
A series of delicate measurements followed. Scientists examined material scraped from the surface of the bones, and used multiple techniques to probe one of Naia’s molars. They estimated the age of the skeleton at 12,000 to 13,000 years old.
Tests on mitochondrial DNA taken from Naia show that she had a genetic marker common today across the Americas, one that scientists say evolved in a prehistoric population that had been isolated for thousands of years in Beringia, the land mass between Alaska and Siberia that formed a bridge between the continents during the Ice Ages.
Thus, according to the report published in the journal, Science, the Native Americans and the Paleoamericans are the same people, descended from the same Beringia population. They just look different because of recent, relatively rapid, human evolution, and not the result of subsequent migrations of peoples into the Americas.
“This is truly an extraordinary discovery,” said Yemane Asmerom, a geochemist at the University of New Mexico who co-wrote the report. He compared Hoyo Negro to the Awash Valley of Ethiopia, the site of the 1974 discovery of “Lucy,” an early human ancestor.
Most scientists have assumed that the first humans to come to the Americas traveled from Eurasia across the Bering land bridge that existed before the oceans rose after the Ice Ages. But there is great debate about whether this represented a single migratory event or multiple pulses of people from different parts of Eurasia and via different routes, including a coastal migration.
Adding to the mystery is that the Paleoamericans, such as Naia, didn’t look like later Native Americans. Naia had a small, projecting face, with narrow cheekbones, wide-set eyes and a prominent forehead. Native Americans of later millennia tended to have broader, longer, flatter faces, and rounder skulls, said James Chatters, an independent researcher and the lead author of the paper.
The distinct morphology of the Paleoamericans is most famously found in the “Kennewick Man,” a 9,000-year-old skeleton discovered two decades ago along the Columbia River in Washington state. Facial reconstruction resulted in someone who looked a bit like the actor Patrick Stewart (“Star Trek: The Next Generation,” “X-Men”). Scientists theorized that he could have been related to populations in East Asia that spread along the coast and eventually colonized Polynesia. Under that scenario, more recent Native Americans could be descended from a separate migratory population.
Chatters said in an interview, “For 20 years I’ve been trying to understand why the early people looked different. The morphology of the later people is so different from the early ones that they don’t appear to be part of the same population.”
He went on: “Do they come from different parts of the world? This comes back with the answer, probably not.”
One of the co-authors of the paper, Deborah Bolnick, an anthropologist at the University of Texas at Austin, said the new genetic tests support the hypothesis of a single ancestral population for Native Americans.
“It’s a lineage that we see across the Americas,” she said, “and a variety of different studies, different lines of evidence over several decades — archaeological studies, genetic studies, morphological studies — all suggest that Native Americans can be traced to a Beringian source population.”
Douglas Owsley, a forensic anthropologist at the Smithsonian’s National Museum of Natural History, and a leading expert on the Kennewick Man, cautioned that the new study is based on “a sample of one.” He said he hadn’t read the paper — titled “Late Pleistocene Human Skeleton and mtDNA Link Paleoamericans and Modern Native Americans” — and would like to see more genetic evidence to bolster the report’s central hypothesis.
When there is a rapid change in the appearance of a population, he said, “I have to think you’re talking about migrations and people coming in.”
But, he added, “I think it’s a great discovery.”