The discovery of a near-complete human skeleton in a watery cave in Mexico is helping scientists answer the question, "Who were the first Americans?"
The skull from the approximately 12,000-year-old skeleton looks markedly different from that of modern Native Americans, but DNA extracted from one of its teeth shares a distinctive genetic signature with them.
The find, reported in the 16 May issue of the journal Science, sheds new light on a decades-long debate among archaeologists and anthropologists. It suggests that the earliest Americans and Native Americans share a single source population, and that the Paleoamericans represent an early population expansion out of Beringia, the landmass that once connected Asia and North America.
On the basis of genetics, modern Native Americans are thought to descend from Siberians who moved into eastern Beringia between 26,000 and 18,000 years ago and then spread southward.
Despite widespread support for this idea, the ancestry of the earliest Americans is still debated because the facial features of the oldest American skeletons don't look much like those of modern Native Americans. This has led some researchers to suggest that the Americas were colonized by separate migration events from different parts of Eurasia.
"Modern Native Americans closely resemble people of China, Korea, and Japan, but the oldest American skeletons do not," said James Chatters, an archaeologist at Applied Paleoscience and the lead author of the study.
These Paleoamerican skeletons have longer, narrower crania than later Native Americans, and smaller, shorter faces, more closely resembling modern peoples of Africa, Australia, and the Southern Pacific Rim. "This has led to speculation that perhaps the first Americans and Native Americans came from different homelands," Chatters continued, "or migrated from Asia at different stages in their evolution."
"The divers are the astronauts of this project; we scientists are their mission control."
The human skeleton discovered by Chatters and colleagues was hidden deep in a submerged chamber in the Sac Actun cave system on Mexico's Eastern Yucatán Peninsula.
Divers search through the darkness of Hoyo Negro, finding Naia's skull at the cave bottom. | Filmed and edited by Alberto Nava Blank
"Hoyo Negro is a more than 100-foot-deep, bell-shaped, water-filled void about the size of a professional basketball arena deep inside a drowned cave system," Chatters said. "Only technical cave divers can reach the bottom. First they must climb down a 30-foot ladder in a nearby sinkhole, then they swim along 200 feet of tunnel to the pit rim before making a final 100-foot drop. The divers are the astronauts of this project; we scientists are their mission control."
Like nearby caves in Pleistocene times, Hoyo Negro was accessible only via sinkhole; the fossils found inside are from people and animals that fell in and were trapped. Then, starting about 10,000 years ago, nearby glaciers melted and filled the caves with water. In addition to the near-complete human skeleton, the researchers found the remains of 26 large mammals, including sabertooths and elephant-like animals called gomphotheres.
The nearly-intact skeleton was that of a small female about 15 or 16 years old that the dive team named "Naia." Based on radiocarbon dating of tooth enamel and analyses of mineral deposits on her bones, the researchers estimate her remains to be at least 12,000 years old.
Chatters described Naia's last moments in a 14 May teleconference for reporters: "In looking at Naia's skeleton, her pelvis is broken. It appears she fell quite a distance and struck something hard enough to fracture it. It's possible she was there looking for water because the region was extremely dry. She may have been trying to get water out of the little puddle that was intermittently at the bottom of Hoyo Negro."
She possesses the unique craniofacial morphology of the earliest Americans, but to understand more about her ancestry and its potential linkage to modern Native Americans, the researchers extracted DNA from one of her molars. "We tried a DNA extraction on the outside chance some fragments might remain," Chatters said. "I was shocked when we actually got intact DNA."
He and his colleagues analyzed the girl's mitochondrial DNA (mtDNA), a useful tool for examining the relatedness of populations. Their analysis revealed an mtDNA genetic signature common to modern Native Americans. This genetic signature occurs only in the Americas, likely having developed in Beringia after populations there split from other Asians.
Critically, it shows that despite differences in craniofacial form, this early American woman was related to modern Native Americans. The differences in skull shape are probably best explained as evolutionary changes that happened after the divergence of the Paleoamericans from their Siberian ancestors, the authors say.
"We hope that this project will set an example on how to approach, in a respectful and professional way, other sinkholes and inundated caves that exist by the thousands in the Yucatán Peninsula," said co-author and archaeologist Pilar Luna of Mexico's National Institute of Anthropology and History. "Many of them contain invaluable prehistoric and pre-Hispanic remains."