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Will a girl’s 12,000-year-old skeleton help solve a mystery?

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  • Divers Alberto Nava and Susan Bird transport the Hoyo Negro skull to an underwater turntable so that it can be photographed to create a 3-D model in an underwater cave in Mexico's Yucatan Peninsula in June 2013. (Paul Nicklen/National Geographic/via Associated Press)

  • Diver Susan Bird, working at the bottom of Hoyo Negro, a large underwater cave, brushes the skull of a teenager found at the site. (Paul Nicklen/National Geographic/via Associated Press)

  • This 12,000-year-old skull of a teenage girl is shown as it was discovered in an underwater Mexican cave in 2007, resting against the skeleton's left upper arm bone. (Courtesy of Daniel Riordan Araujo)

  • DNA from this upper right third molar recovered from the girl's skeleton connects first Americans to Native Americans (Courtesy of James Chatters)

  • Victor Polyak, a research scientist at the University of New Mexico, holds a device like those used in the dating process. (Greg Sorber/Albuquerque Journal)

  • Yemane Asmerom, a University of New Mexico professor with the Department of Earth and Planetary Sciences and director of the Radiogenic Isotop Lab, left, and research scientist Victor Polyak dated a 12-000- to 13,000-year-old skeleton found in a Yucatan Peninsula undersea cave. The two scientists use a uranium-thorium dating system in their work. (Greg Sorber/Albuquerque Journal)

  • Calcite growths known as "florets" grew on the skeleton more than 10,000 years ago. Shown here are two samples, which grew like stalagmites on the bones before the Hoyo Negro cave was flooded with sea water. (Greg Sorber/Albuquerque Journal)

  • In this Oct. 25, 2013 photo made available by Roberto Chavez Arce, divers use lights to illuminate Hoyo Negro, an underwater cave in Mexico's Yucatan Peninsula where the remains of "Naia," a teenage girl who lived 12,000 to 13,000 years earlier, were found. (Roberto Chavez Arce/Science/via Associated Press)

  • In this June 2013 photo provided by National Geographic, divers make their way toward Hoyo Negro, an underwater cave in Mexico's Yucatan Peninsula where the remains of "Naia," a teenage girl who lived 12,000 to 13,000 years earlier, were found. (Paul Nicklen/National Geographic/via Associated Press)

The question has long puzzled scientists: Who were the first Americans?

Genetic evidence has consistently supported the idea that the first humans to populate the Western Hemisphere came across a land bridge from Siberia between 26,000 and 18,000 years ago. But there has always been nagging doubt, in large part because of distinct facial, cranial and dental differences between the oldest American skeletons on one hand and modern Native Americans, Siberians and other northeast Asians on the other. This doubt has led to speculation that perhaps the first inhabitants and Native Americans came from different Eurasian homelands.

Complicating the puzzle has been the difficulty of finding intact paleoamerican skeletons to study.

Now, according to an article by James Chatters and colleagues in today’s issue of the journal Science, the answer is in, thanks to the discovery of a nearly complete 12,000- to 13,000-year-old human skeleton and scientific dating techniques provided by two scientists at the Radiogenic Isotope Laboratory at the University of New Mexico.

And the answer based on an analysis of the skeleton of a young woman dubbed “Naia,” is that the first Americans and modern Native Americans are, indeed, descendants of common ancestors who crossed the Bering Sea land bridge many thousands of years ago.

“This is one of the greatest discoveries in archeology, and we feel very lucky to have played a crucial role (the dating part),” said one of the UNM scientists, Professor Yemene Asmerom, in an email to the Journal.

Naia

Mexican research divers discovered the skeleton of a late-Pleistocene 15- or 16-year-old female deep in an underwater cave called Hoyo Negro on Mexico’s Yucatán Peninsula in 2007. The young woman probably fell into the cave long before melting glaciers flooded it some 10,000 years ago. In the cave, divers also found the bones of several species of large animals, some of which are now extinct – saber-toothed cats, pumas, bobcats, sloths, a bear. They, too, in all likelihood, fell in.

Asmerom, of UNM’s Department of Earth and Planetary Sciences, was, if anything, even more effusive in a follow-up interview in his Northrup Hall office this week. “We’re talking about one of the oldest full skeletons in the world, not just here,” he said. “This is the Olduvai Valley of the Western Hemisphere.”paleoamerican

(Olduvai Valley, or Gorge, a ravine in Tanzania, is arguably the most important anthropological site in the world, and has provided skeletal remains of the earliest known hominids ever to walk the earth.)

Naia’s discoverers from the Mexican Institute of Anthropology and History, aware of the significance of their find, called on American colleagues to help. One was Douglas Kennett of Pennsylvania State University. He and Asmerom had worked on another project, and he contacted his friend at UNM.

Kennett initially used radio carbon dating in an attempt to determine Naia’s age. But radio carbon is tricky, and the results can be suspect, especially when the sample material is bone. At UNM, Asmerom and research scientist Victor Polyak have at their disposal a state-of-the-art laboratory – “second-to-none” Asmerom describes it – so, about 18 months ago the two joined the Hoyo Negro project, on which they worked, on and off, for about a year.

“We were excited,” Polyak said. “It started when we were sent a tooth and asked to date it with uranium-thorium.”

The results of the uranium-to-thorium decay-dating method confirmed averages arrived at through carbon dating. “That was very promising,” Polyak said.

The two UNM scientists were able to fix the skeleton’s age at between 12,000 and 13,000 years. In addition to testing Naia’s tooth enamel and dentine, they tested crystals that had formed on her skeleton and on those of the large animals in Hoyo Negro. Called florets, the crystals consist mostly of calcite. Luckily, the florets contained enough uranium for the team to conduct nine separate uranium-thorium tests.

The florets – similar to stalagmites – formed before the sea flooded the cave, which today lies under 140 feet of brackish water. When the water began seeping into Hoyo Negro, the florets stopped growing, and the water served as a preservative.

The significance of Naia’s skeleton, besides its completeness, is that it was adult enough to provide an excellent morphology – the structure and peculiarities of the bones – and fresh enough for clear DNA testing.

“When I looked at the tooth, it was as fresh as if it were extracted yesterday,” Asmerom said.

“There are several facets to this very nice story,” Polyak said. “How things change, how we change, human migration, human evolution.” During that time period, the earth’s climate was changing rapidly, and many large North American animals would soon become extinct. Also, the sea level was rising rapidly, so that “when this young woman went into the cave, the sea level was almost 200 feet lower than it is today.”

Genetic testing of Naia’s bones and teeth by other members of the scientific team determined her DNA is of a type common to other Native Americans whose ancestors came across the Bering Sea land bridge. The scientists who conducted the DNA analysis “feel really good about the results,” Polyak said. “The DNA is really good evidence of a link between Native Americans and ancestral Americans.”

So scientists may finally have a definitive answer. And the facial, dental and cranial differences between the oldest American skeletons and modern Native Americans “probably resulted” from evolutionary changes – after they settled in the western hemisphere, according to the scientists involved in the project.

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