In a controversial
new study, scientists cite artifacts dating the event to more than
26,000 years ago
A stone point from Chiquihuite
cave (Ciprian Ardelean)
Surprisingly old stone points found in a Mexican cave are the latest
intriguing discovery among many to raise questions about when humans
really arrived in the Americas.
For most of the 20th century archaeologists generally agreed that
humans who had crossed the Beringia land bridge from Siberia to
North America only ventured further into the continent only when
retreating ice sheets opened a migration corridor, about 13,000
years ago. But a few decades ago, researchers began discovering
sites across the Americas that were older, pushing back the first
Americans' arrival by a few thousand years. Now, the authors of
study at Mexico's Chiquihuite cave suggest that human history
in the Americas may be twice that long. Put forth by Ciprian
Ardelean, an archaeologist at the Autonomous University of Zacatecas
(Mexico), and his colleagues, the new paper suggests people were
living in central Mexico at least 26,500 years ago.
Ardelean's work was published in Nature and paired with another
study that presented a broader look at 42 known early human sites
across North America from the Bering Strait to Virginia. Data from
those sites were used to model a much earlier peopling of the Americas,
and help scientists reimagine not only when but how the first people
reached and populated the New World. The model features a number
of archaeological sites, including Chiquihuite cave, which are intriguing
but controversial enough, as experts disagree whether the sites
actually evidence human occupation.
Chiquihuite cave is perched high in the Astillero Mountains, 9000
feet above sea level and 3,280 feet higher than the valley below.
Excavations there were launched when a 2012 test pit unearthed a
few stone artifacts that suggested a human presence dating back
to the Last Glacial Maximum between 18,000 and 26,000 years ago.
More extensive excavations detailed in the new study were carried
out in 2016 and 2017, unearthing some 1,900 stone points or possible
tools used for cutting, chopping, scraping, or as weapons.
The artifacts were dated by 46 different radiocarbon samples of
adjacent animal bones, charcoal, and sediment samples. To the team,
they represent a previously unknown technological tradition of advanced
flaking skills. More than 90 percent of the artifacts were of greenish
or blackish stone, though those colors are less common locally,
suggesting to the authors that they were singled out as desirable.
The bulk of the material is from deposits dating to between 13,000
and 16,600 years ago, leading the scientists to hypothesize that
the humans may have used the cave for more than 10,000 years.
Ardelean knows that Chiquihuite's very old dates will raise most
archaeologists' eyebrows. "As soon as you cross the limit into the
Last Glacial Maximum (LGM), that's when it gets tricky," he says.
"We have a sort of mental blockage just thinking about getting into
a new continent in the middle of a glaciation."
Yet he suggests that if people were visiting this cave during the
Last Glacial Maximum they likely entered America even earlier, more
than 30,000 years ago, before glaciers blocked the way from Beringia.
"It takes centuries, or millennia, for people to cross Beringia
and arrive in the middle of Mexico," Ardelean says. "Even coastal
arrivals wouldn't have been landing on the Mexican coastit's
just way too far. You need many years of previous presence to make
them arrive there if they came by sea or by land."
Sunlight shines into
Chiquihuite cave (Devlin A. Gandy)
Ardelean sees the site as one point on an emerging new timeline
for humans in the Americas. "This site alone can't be considered
a definitive conclusion," he admits. "But with other sites in North
America like Gault (Texas), Bluefish Caves (Yukon), maybe Cactus
Hill (Virginia)it's strong enough to favor a valid hypothesis
that there were humans here probably before and almost surely during
the Last Glacial Maximum."
As would be expected, the site has drawn scrutiny from the archaeological
community. In a Nature "News & Views" article accompanying the
studies, Ruth Gruhn, professor emerita at the University of Alberta,
said that since the idea of an American entry date more than 30,000
years ago is double the currently popular date of about 16,000 years
ago it "will be very hard for most archaeologists specializing in
early America to accept."
Southern Methodist University archaeologist David
Meltzer questions why the stone tool traditions described at
the site haven't been seen anywhere else in the region, and why
their technology remained unchanged for so many thousands of years.
Davis, an Oregon State University archaeologist, says most of
the artifacts appear to have been produced by a single blow or fracture.
Might they actually be broken rocks, created by natural actions
like rockfall from the ceiling? "Humans don't have a monopoly on
the narrow band of physics required to break rocks," Davis says.
"I'm open to being convinced. If I'm wrong about what I seeing in
their reporting, I might change my mind."
Davis also laments a lack of evidence for domestic life in the
cave. "We usually see things like butchering animals and making
food," he says. "They did find lots of animal bones but they say
there is no evidence of butchering and that's really strange. There's
also an absence of things like fire pits, or pits in the ground
for storing things, or unusual distributions of objects."
Ardelean believes some of those features might lie tantalizingly
close by, yet be difficult or impossible to uncover. The current
excavation is taking place far inside the large cave. "Most activities
like cooking and eating happened right at the entrance," he says.
"And that entrance isn't accessible, it's buried under tons of debris
that has fallen from the top of the mountain."
That cave's mountainous location, thousands of feet above the valley
floor, has David Meltzer
asking another question. "Why keep coming back to that same place
on a relatively constant basis over such a long period of time?"
he says. "I find that curious. Not many sites have that kind of
long-term repeated occupation, unless there is something quite useful
or available at the spot to attract people to it."
Scientists work in Chiquihuite
cave (Mads Thomsen)
The study shed some light on environmental conditions that existed
over many centuries at the cavea shifting landscape of mixed
forests and grasslands revealed by plant samples from 31 DNA extractions
from the surrounding soil materials. But while tests of cave sediments
revealed lots of ancient plant and animal DNA, scientists recovered
no unambiguous signal of ancient people. Ardelean says the lack
of verifiable human DNA up to this point is a disappointment. "Until
we would have DNA available, there's nothing to tell us who these
people were or where they came from," he says.
In recent years archaeologists working on various New World sites
have stacked up evidence to refute the once-ubiquitous theory that
the Clovis People, with their distinctive points, were America's
oldest culture. Scientists estimated that they passed through a
corridor between Canada's great ice sheets some 13,000 years ago.
Caves in Oregon people made an entirely different type of projectile
point and left behind fossilized poop at least 14,000 years ago.
Butchered mastodon bones and stone tools in a sinkhole suggest humans
had reached Florida by at least 14,500 years ago. Evidence suggests
humans made tools and butchered animals at
Cooper's Ferry in Idaho 16,000 years ago and made it all
the way to tip of South America at Monte Verde, Chile by 14,500
Many of those sites are represented
in the second study, co-authored by Lorena Becerra-Valdivia,
an archaeological scientist at the University of Oxford and the
University of New South Wales, and Thomas Higham, an archeological
scientist at the University of Oxford. They compiled radiocarbon
and luminescence dating data from 42 archaeological sites across
America, creating a model that maps scenarios of human distribution
across the continent in time and space, from the Bering Strait to
Virginia. When the oldest pre-Clovis sites are plugged in, the model
suggests that humans populated the Americas before and during the
Last Glacial Maximum some 19,000 to 26,500 years ago. That would
mean that humans not only arrived in the Americas earlier than is
commonly believed, but that they somehow circumvented the era's
massive ice sheets.
A theory that these peoples migrated
by travelling down the Pacific coastline 14,000 to 15,000, or
even as long as 20,000 years ago, has been steadily gaining support
as excavations turn over more evidence, though uncovering their
tracks is complicated due to past changes in sea levels. Another
possibility is simply that people entered the Americas by land before
the glaciers blocked the route into the continent's interior. The
model also suggests that a second, more widespread peopling of the
Americas unfolded during a period of sudden and dramatic warming
about 12,900 to 14,700 years ago. Becerra-Valdivia says this is
evidenced by a spike in archaeological sites and the emergence of
stone tool traditions like Clovis. Genetic research, she adds, also
"points to marked population growth between around 15 to 16 thousand
If the Americas begin to look more heavily populated by distinct
groups of people after these dates, Ardelean believes the earlier
pre-Clovis sites, each with distinct types of technologies or artifacts,
tell a different tale.
"I think that the human presence during the Last Glacial Maximum
was extremely diverse, and there were multiple arrivals from multiple
directions," he says. "I believe humans were culturally diverse
and potentially genetically diverse. There was no such thing as
a single arrival."