Nature
PUBLISHED
Deep in a submerged cave in the Yucatan, researchers have found a near-complete human skeleton with an intact skull and preserved DNA from as early as 13,000 years ago. They call her Naia, after water nymphs in Greek mythology, and she could help solve a decades-long debate: Who were the first Americans?
It’s been a challenge trying to decipher the ancestry of the first people who populated the Americas. Because of the differences in skull shape and facial features between the oldest known Paleoamerican skeletons and modern Native Americans, archaeologists and anthropologists have suggested separate origins for each -- despite genetic evidence to the contrary. Based on DNA, modern Native Americans descended from the earliest Americans, or Siberians who moved across Beringia (a land bridge that once connected Asia and North America) between 26,000 and 18,000 years ago, and then spread southward.
Even with genetic data, the ancestry of earliest Americans is still debated. "Modern Native Americans closely resemble people of China, Korea, and Japan," study leader James Chatters of Applied Paleoscience says in an AAAS news release. Specifically, rounder skulls with flatter faces. “But the oldest American skeletons do not," he adds. These skulls are longer and narrower and seemed to belong to humans who came over from Australia, Europe, or Southeast Asia. Now, this new fossil finding directly links Paleoamericans with modern Native Americans.
Naia was found alongside 26 extinct mammals (including sabertooth cats, giant ground sloths, and mastodon relatives), who also fell down the watery sinkhole and became trapped in the Sac Actun cave system on Mexico's Eastern Yucatán Peninsula. She suffered a broken pelvis. The cave, Hoyo Negro, is a 40-meters-deep, “bell-shaped, water-filled void about the size of a professional basketball arena deep inside a drowned cave system,” Chatters describes. The limestone caves filled with water around 10,000 years ago when glaciers around the world started melting. He adds: “The divers are the astronauts of this project; we scientists are their mission control.”
And this is how Alberto Nava Blank of Bay Area Underwater Explorers describes what they saw in 2007, according to Science: “a human skull resting on the top of a small ledge. It was lying upside down, carrying a perfect set of teeth and with dark eye sockets looking back at us.” They couldn’t recover the skeleton from the collapsed cave, so a team went back to take measurements and tons of photos right there. They took pictures every 20 degrees of the skull on a rotating tripod, and you can see a 3D reconstruction in this video.
The skeleton belonged to a human female about 15 or 16 years old and 4’10’’ (about a meter and a half) tall. Based on radiocarbon dating of tooth enamel and uranium thorium analyses of the rose-shaped calcite mineral deposits on her bones, the fossil dates to between 13,000 and 12,000 years ago -- in the late Pleistocene, at the end of the ice age. She had many cavities, indicating how her diet was likely heavy in fruit or honey, and her delicate size suggests she ate little protein. “She went hungry a lot and much of her food came in the form of plant sugars," Chatters tells New Scientist. "This is a long way from the big game hunters -- at least successful ones -- we expect to see in this early time."
Naia possesses the unique craniofacial morphology of the Paleoamerican fossils: a projecting, angular face with a pronounced forehead. But when the researchers extracted genetic material from her molars and analyzed her mitochondrial DNA (maternal lineage), they found a genetic marker -- called mtDNA haplogroup D1 -- that’s common to modern Native Americans. This genetic signature occurs only in the Americas and probably developed in Beringia.
Turns out, Naia is a unique combination: Paleoamerican craniofacial characteristics with Beringian-derived mitochondrial DNA. The findings suggest she’s an early American relative of modern Native Americans, and the differences in craniofacial form likely resulted from changes that happened after Beringians diverged from their Siberian ancestors.
"There are lots and lots of theories about the peopling of the Americas and who was where when," Ripan Malhi of the University of Illinois at Urbana-Champaign says in an Illinois release. "And it seems like genetic research is slowly falsifying a lot of these theories."
So, it doesn’t look like the Americas were colonized by separate migration events from different parts of the Old World; rather, the earliest Americans represent an early population expansion out of Beringia. These findings suggest that Paleoamericans and contemporary Native Americans come from a single source population who must have “evolved in place.” And that’s what led to their morphological differences -- not separate ancestry.
"The challenge, to date, has been finding a fossil of an adult complete enough to do the morphology work, preserved enough to have mitochondrial DNA, while at the same time having appropriate material for dating," Yemane Asmerom of University of New Mexico says in an UNM release. "Naia, the most complete human skeleton found, meets those requirements.”
The huge international collaboration also includes researchers with various expertise from Penn State, Northwestern, Washington State, University of Texas-Austin, with funding from the National Geographic Society. Watch a video of the discovery.
The work was published in Science this week.
Images: Paul Nicklen/National Geographic (top, second), Roberto Chavez Arce (third), Daniel Riordan Araujo (below)
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