viernes, 16 de mayo de 2014

Mexican skeleton gives clue to American ancestry

Photograph by Paul Nicklen for National Geographic

In a flooded cave in Mexico, divers transport a skull for 3-D scanning. Between 12,000 and 13,000 years old, the skull is part of the most complete skeleton of such antiquity yet discovered in the Americas.
The oldest complete skeleton of its kind ever found, dating to more than 12,000 years ago, is helping solve a mystery about the differences in body types between the first humans to arrive in the Americas and later Native Americans, scientists announced Thursday.

Anthropologists have long puzzled over why Native Americans don't look more like their ancient ancestors, who migrated into the Americas during the Pleistocene, the epoch that encompassed the last ice age and that ended about 12,000 years ago.

The ancient skulls are larger, their faces are narrower and more forward-projecting, and they more closely resemble native peoples of Africa, Australia, and the southern Pacific Rim than they do their supposed American descendants.

Were those differences the product of evolutionary changes in the founding populations? Or were the Paleoamericans, the first arrivals to the Americas, displaced by later migrations of people with features more like those of Native Americans?

 Photograph by Paul Nicklen for National Geographic
A diver shines a light on the newly discovered skull.

Chatters described Naia's face as narrow with wide-set eyes and a low, prominent forehead; a low, flat nose; and outward-projecting teeth—"about the opposite of what Native Americans look like today." To see those features coupled with genetic markers indicating a common lineage with Native Americans is highly significant.

"This is the first time that we have genetic data from a skeleton that exhibits these distinctive skull and facial features," said Deborah Bolnick, an anthropological geneticist at the University of Texas at Austin and one of the study's co-authors.

The find from Hoyo Negro comes on the heels of the recent genomic sequencing of the 12,600-year-old remains of an infant found at the Anzick Clovis site in Montana, which also revealed a shared ancestry with Native Americans.

Genetic analyses of modern Native Americans indicate they descend from a founding population that originated in Asia. They were isolated from other population groups for several thousand years somewhere in or near the region known as Beringia, a broad swath of land that reached from Siberia to Alaska during the last glacial maximum.

It was there that this founding population developed its unique genetic markers. But until the Anzick discovery, little genetic data had been available from Paleoamerican skeletal specimens, leaving their relationship to Native Americans poorly understood.

The genetic data from the Anzick find is superior to that of Hoyo Negro's because it was derived from mitochondrial and nuclear DNA, providing a much more comprehensive lineage history than mitochondrial DNA alone, which traces only maternal lineages. But the downside of Anzick is that the specimen itself is much less complete: just four bones and the braincase portion of a cranium.
Michael Waters, director of the Center for the Study of First Americans at Texas A&M University in College Station, said the Anzick and Hoyo Negro finds complement one another.

"Now we've got two specimens, both from a common ancestor that came from Asia," he said. "Like Hoyo Negro, the Anzick genome shows that Paleoamericans are genetically related to native peoples, so the latter cannot be a replacement population. Their differences have to be a result of evolutionary change. What drove that change, we don't know."

 Photograph by Paul Nicklen for National Geographic 

Divers search the walls of Hoyo Negro, the underwater cave on Mexico's Yucatán Peninsula where the ancient skeleton was found.

From Hunters to Homemakers?

Chatters speculates that ancient Americans' morphology may have changed as their living conditions changed. As highly mobile hunter-gatherers became more settled, evolutionary processes may have selected for more domestic traits and temperaments, resulting in the softer, rounder features seen in the faces of Native Americans.

"You start seeing these more domestic forms when females have more control over the food supply, when they're not so dependent on aggressive men," Chatters said. He added that this process of neotenization—the retention of some juvenile traits—can be seen in populations across the Northern Hemisphere between the late Pleistocene and modern times.

Speculation about the potential drivers of evolutionary change is not part of the team's study. Even so, some scientists warn against drawing too many conclusions from the Hoyo Negro find.

"We know there's a tremendous variation in physical form, and the sample of crania we have from that time period is so tiny," said David Meltzer of Southern Methodist University, Dallas, Texas. "Imagine plucking a dozen skulls from New York City; they wouldn't look a lot alike. We've got to be really careful about drawing conclusions based on relatively small samples. That's true for the skeletal anatomy, and it's true for the genetic record as well."

On Thursday, a team led by archaeologist James Chatters reported in the journal Science that they'd found a big piece of the puzzle: the most complete skeleton of such antiquity ever found in the Americas, between 12,000 and 13,000 years old. The skeleton contains both the craniofacial features of ancient Paleoamericans and mitochondrial DNA possessed by latter-day Native Americans.

Tracing a DNA Trail

The skeleton, dubbed "Naia" (an ancient Greek name related to water nymphs) by her discoverers, belonged to a teenage girl who fell more than 100 feet to her death nearly a half mile inside an elaborate network of karst caves that were largely dry at the end of the Pleistocene. Divers who found Naia in the cave on Mexico's Yucatán Peninsula named her watery grave Hoyo Negro ("Black Hole" in Spanish).


DNA from the skeleton shows similarities to modern Native Americans, while its skull structure matches those of Paleoamericans that came across the Bering land bridge 

The near-intact skeleton of a delicately built teenage girl, who died more than 12,000 years ago in what is today’s Mexico, could help to solve the riddle of how the Americas were first populated.

Cave divers discovered the skeleton seven years ago in a complex of flooded caverns known as Hoyo Negro, in the jungles of the Yucatan Peninsula. They called her Naia, after the naiads, the water nymphs of Greek mythology. She lies in a collapsed chamber together with the remains of 26 other large mammals, including a saber-toothed tiger, 600 meters from the nearest sinkhole. Most of the mammals became extinct around 13,000 years ago.

Analysis of the remains, most of which are still lying in the submerged cave where they were found, suggests that modern Native Americans are the descendants of the earliest Paleoamericans, who migrated from Siberia towards the end of the last glacial period. An alternative theory held instead that a mysterious, more recent influx had brought in new populations from Eastern Asia.

“Naia, and the other animals, would have slipped through a hidden sink hole and fallen 30 meters into a shallow pool,” says paleontologist James Chatters of Applied Paleoscience in Bothell, Washington, who led the study, published today in Science. “There would have been no way out.” The broken pelvis of Naia’s otherwise near-perfect skeleton is likely a result of the fall, he says.

Reconstructing Naia

It was impossible to safely recover the body from the cave location, so the research team dove to the cave and made bone measurements in situ. They placed Naia’s skull on a rotating tripod, and set a camera on a second tripod next to it. Turning the skull slowly, they snapped pictures every 20 degrees. Later the team used the photographs to reconstruct a three-dimensional image (see video at bottom).

Naia, they calculated, was approximately one and a half meters tall. Her skull, with its small, projecting, angular face and pronounced forehead, was similar to those of the earliest fossils of Paleoamericans dating from more than 10,000 years ago, most of which have been found in the Pacific Northwest. Her teeth and bone development suggest she was 15 or 16 years old.

The divers also recovered two teeth, a rib and sample of mineral deposits that had grown onto the surface of the bones. Using two independent methods to date the remains, the authors carbon-dated the tooth enamel and measured the ratio of uranium and thorium in the mineral deposits. Naia must have been between 12,000 and 13,000 years old, they concluded. The mitochondrial DNA for their genetics analysis came from one of her teeth.


DNA story
Naia's mitochondrial DNA reveals genetic signatures in common with modern Native Americans, despite her very different skull shape.

“You can never exclude that Native Americans have more than one group of ancestors,” says Chatters. But his team’s data, he points out, are consistent with the idea that Native Americans evolved from Siberian ancestors.

“It helps support the consensus view, from archaeological, genetic and linguistic evidence, that the Americas were initially peopled 15,000–20,000 years ago from Siberia,” says human geneticist Chris Tyler-Smith from the Wellcome Trust Sanger Institute near Cambridge, UK.

According to this widely held theory, the Americas were populated by Siberian ancestors who crossed the Bering land bridge that back then linked Eurasia and Alaska. The migration is thought to have started during the Pleistocene ice age — which ended around 14,000 years ago — and continued over the next several thousand years as these populations moved south.

Yet researchers have puzzled over why the more-than-10,000-year-old Paleoamerican skulls unearthed so far have such different morphology from those in more recent finds and from modern Native Americans. Scientists wondered whether other Native American ancestors had arrived in a later migration. The new DNA results indicate that the very different skulls of modern Native Americans have evolved on North American soil.

Paleoamerican remains are few and far between, because the nomadic tribes did not always build tombs for their dead. This is the first full skeleton to be found, and the first major set of remains to be unearthed so far south.

Source: - This article is reproduced with permission from the magazine Nature. The article was first published on May 15, 2014.
Photographs by Paul Nicklen for National Geographic


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