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.
"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: scientificamerican.com - 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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