Where did the earliest Americans come from? 

Speculation has pointed to Eastern Asia, Western Asia, Japan, Beringia and even Europe. Differences in cranial form between today's Native Americans and the earliest known Paleoamericans have lent credence to all possibilities but the analysis of a nearly complete Paleoamerican skeleton with Native American DNA that dates close to the time that people first entered the New World may have some answers to part of the puzzle.

The skeleton of the teenage girl, given the name Naia by the scholars, was found in Hoyo Negro ("black hole"), a deeply submerged chamber in the Sac Actun cave system in the eastern Yucatan Peninsula, Mexico.

Alberto Nava Blank and a team of science divers discovered the skeleton along with many extinct animal remains deep inside this inundated cave in 2007. The Hoyo Negro project is led by Pilar Luna and the Instituto Nacional de Antropología e Historia of Mexico, Nava and James Chatters, owner of Applied Paleoscience, with funding from the National Geographic Society.

 Douglas Kennett, professor of environmental archaeology at Penn State and postdoctoral fellow Brendan J. Culleton were originally asked to directly date the skeleton but after traditional direct-dating methods failed because the bones were mineralized from long emersion in warm salty water within this limestone cave system, which helped to build a geochronological framework for Naia using a unique combination of techniques to constrain the age of the skeleton to the end of the ice age.

It won't be without some controversy. To build the case for a late Pleistocene age they collaborated with Yemane Asmerom and Victor Polyak from the University of New Mexico using global sea level rise data to determine when the cave system, which at the time Naia and the extinct animals entered was dry, filled with water.

The site where Naia lies is now 130 feet below sea level and sea level rise would have raised the groundwater level in the cave system and submerged everything between 9,700 and 10,200 years ago. So initial estimates of the latest that animals and humans could have walked into the cave system was 9,700 years ago.

At the same time, the researchers experimented with uranium thorium dating the skeleton directly. Asmerom and Polyak tried to directly date Naia's teeth using this method, but that also did not work well.

The bones were found deep below today's ground surface in a collapsed chamber connected to the surface via a web of now flooded tunnels that Naia once walked along to fall to her untimely death. Because the caves are limestone, mineral deposits continued to form while the cave was largely dry. Working with Patricia Beddows, Northwestern University, Chatters noticed accumulations of calcium carbonate -- tiny rosettes of calcite deposited by water dripping off the cave roof -- which could be accurately dated using the uranium thorium method. Because these drip water deposits formed on top of Naia's bones, their date must occur after she fell in the cave. The oldest one dated so far is 12,000 years old.

Naia's tooth enamel was also radiocarbon dated to 12,900 years ago by Kennett's lab.

"Unfortunately, we can't rule out that the tooth enamel is contaminated with secondary carbonates from the cave system, but we removed potential contaminates using standard techniques and Tom Stafford, Stafford Research Laboratories, produced a comparable age," said Kennett. "We consider this a maximum age and when combined with the uranium thorium dates from the adhering speleothems, we argue that the skeleton dates between 12,000 and 13,000 years ago. Well placed as a Paleoamerican."

Morphologically, Naia does not look like a contemporary Native American, but mitochondrial DNA testing -- maternally inherited DNA -- carried out by Brian Kemp, Washington State University, and his collaborators shows that she has a D1 haplotype. This is consistent with the hypothesis that her ancestors' origins were in Beringia, a now partially submerged landmass including parts of Siberia, Alaska and the Yukon. Early humans moved into this area from elsewhere in Asia and remained there for quite some time. During that time they developed a unique haplotype that persists today in Native Americans. Genetically, Paleoamericans have similar attributes as modern Native Americans even if their morphology appears different.

"More work is needed," said Kennett. " There are still carbonate deposits on the bones of Naia and other animal bones in the cave. The Instituto Nacional de Antropología e Historia and the scientific diving community have nicely preserved the site so the next step will be to date additional samples to constrain the age of Naia and associated extinct animals further."
Also working on this project were Eduard Reinhardt, McMaster University, Ontario; Joaquin Arroyo-Cabrales, INAH; Deborah A. Bolnick, University of Texas at Austin; Ripan S. Malhi, University of Illinois; Dominique Rissolo, Waitt Institute; and Shanti Morell-Hart, Stanford University.