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Americans claim First sparks controversy

ExperimentFossils and rocks

Image copyright
San Diego Natural History Museum

Image caption

Purported stone tools can be seen alongside broken mastodon bones at the excavation site near San Diego

A study that claims
humans reached the Americas 130,000 years ago – much earlier than has
previously been suggested – has run into controversy.

Humans are
thought to have arrived in the New World no earlier than 25,000 years
ago, so the find would push back the first evidence of settlement by
more than 100,000 years.
The study is based on analysis of animal bones and tools from California.
But many experts contacted by the BBC said they doubted the claims.
Thomas
Deméré, Steven Holen and colleagues examined material from the Cerutti
Mastodon site near San Diego. The site was originally uncovered in 1992,
during the construction of a road. Possible stone tools were discovered
alongside the smashed up remains of a mastodon (Mammut americanum) – an extinct relative of mammoths and living elephants.
The
researchers behind the latest study were unable to carry out
radiocarbon dating on the remains, so they used a technique called
uranium-thorium dating on several bone fragments, coming up with a date
of 130,000 years.
The team members found that some of the bones
and teeth bore a characteristic breakage pattern known as spiral
fracturing, considered to occur when the bone is fresh. Additionally,
some of the bones showed characteristic signs of being smashed with hard
objects.
Rocks found alongside the mastodon remains show signs
of wear and being struck against other objects, the researchers say.
They conclude that these represent hammerstones and anvils – two types
of stone tool used by prehistoric cultures across the world.

Mastodon bone

Image copyright
Tom Deméré, San Diego NHM

Image caption

This close-up of a mastodon thigh bone shows spiral fractures

Dr Deméré, curator of palaeontology at the San Diego
Natural History Museum, said the totality of evidence at the site had
led team members to the conclusion that “humans were processing [working
on or breaking up] mastodon limb bones using hammerstones and anvils
and that the processing occurred at the site of burial 130,000 years
ago”.
Dr Steve Holen, co-director of the Center for American
Paleolithic Research in South Dakota, commented: “We have conducted two
experiments breaking elephant bones with large rock hammers and anvils.
We produced exactly the same kind of fracture patterns as we found on
the Cerutti mastodon limb bones.”
He added: “We can eliminate all
of the natural processes that break bones like this. These bones were
not broken by carnivore chewing, or by other animals trampling on this
bone… the distribution patterns of the fractured pieces of bone right
around the anvils is fairly conclusive evidence because we see that
experimentally also.”
It’s not entirely clear what the humans were trying to achieve at the location.
“We
have no evidence that this is a kill or butchery site, but what we do
have evidence of is that people were here breaking up the limb bones of
this mastodon, removing some of the big thick pieces – probably to make
tools out of – and they may also have been extracting the marrow for
food,” said Dr Holen.
But if the team’s conclusions are correct,
people could have reached the Americas from Asia via a land bridge
across the Bering Strait that periodically emerged during cold periods,
when ocean water was locked up as ice, and disappeared when the climate
warmed up again and sea levels rose.
The earliest widely accepted
evidence for humans in the Americas dates to roughly 15,000 years ago.
This is a field where fierce debate has raged over rolling back ages of
human occupation by one or two thousand years, let alone 100,000.

Possible hammerstone

Image copyright
Tom Deméré, San Diego NHM

Image caption

A boulder from the Cerutti Mastodon site that could have been used as a hammerstone

Dr Deméré and colleagues are not the first
scientists to posit much earlier dates for people settling in the
Americas. What distinguishes the latest work is that it has been
published in one of the most prestigious peer-reviewed science journals
in the world – Nature.
However, other experts remain thoroughly
unconvinced by the new evidence. Prof Michael R Waters, from Texas
A&M University in College Station, described the new paper as
“provocative”.
He told BBC News the study “purports to provide
evidence of human occupation of the Americas some 115,000 years before
the earliest well established evidence”.
Prof Waters explained: “I
have no issues with the geological information – although I would like
to know more about the broader geological context – and the likely age
of the locality. However, I am sceptical of the evidence presented that
humans interacted with the mastodon at the Cerutti Mastodon site.”
“To
demonstrate such early occupation of the Americas requires the presence
of unequivocal stone artefacts. There are no unequivocal stone tools
associated with the bones… this site is likely just an interesting
paleontological locality.”
Echoing that view, Prof Tom Dillehay, from Vanderbilt University in Nashville, told BBC News the claim was not plausible.
Another
authority on early American archaeology, Prof David Meltzer from
Southern Methodist University in Dallas, said: “Nature is mischievous
and can break bones and modify stones in a myriad of ways.

Possible hammerstoneImage copyright
Kate Johnson, San Diego NHM

Image caption

Experimental breakage was able to re-produce the same patterns seen at the ancient site

“With evidence as inherently ambiguous as the broken
bones and non-descript broken stones described in the paper, it is not
enough to demonstrate they could have been broken/modified by humans;
one has to demonstrate they could not have been broken by nature.
“This is an equifinality problem: multiple processes can cause the same product.”
Chris
Stringer, from London’s Natural History Museum, said that “if the
results stand up to further scrutiny, this does indeed change everything
we thought we knew about the earliest human occupation of the
Americas,” adding: “If true, the results may well mean that archaic
people like the Denisovans or Neanderthals were the first colonisers of
the Americas, rather than modern humans.”
He explained that
“extraordinary claims require extraordinary evidence – each aspect
requires the strongest scrutiny,” but Prof Stringer observed: “High and
concentrated forces must have been required to smash the thickest
mastodon bones, and the low energy depositional environment seemingly
provides no obvious alternative to humans using the heavy cobbles found
with the bones.”
The dating method used by the researchers to
assign an age to the material works by measuring the radioactive decay
of uranium that becomes incorporated into the bones over time.
“The
type of samples that are most widely dated with this technique are ones
that contain uranium as a primary substitution in their structure, such
as inorganic carbonates, like cave carbonates, or corals, which take in
uranium as they take calcium out of seawater,” Dr Warren D Sharp, an
expert in isotope dating from the Berkeley Geochronology Center in
California, told BBC NeImage copyright
San Diego NHM

Image caption

The site was originally excavated during a highway expansion in the early 1990s
Experiment

“What they’ve done in this paper is applied it to
bone. That can be challenging because bones don’t contain significant
amounts of primary uranium. They acquire the uranium when they become
buried – they take it up from soil porewaters.”
He added: “That
said, I think the dating is sound. They have done a very careful job.
They have dated multiple samples and obtained similar results. The
systematics of the concentrations of uranium in profiles across the
bones are what you’d expect for reliable dates. And the bones that
they’ve dated seem to be an integral part of the site, so their age
should be relevant to the rest of the observations.”
Prof Meltzer
said the history of the material from the site meant it would be
difficult to ever say with any certainty that humans broke the bones. He
explained: “[The evidence] comes from a site that was excavated
[approximately] 25 years ago as a salvage project during a highway
expansion.
“The kinds of detailed information necessary to
understand how these bones and stones came to be… is simply not
available. The authors do what they can with the extant collections, but
they necessarily have to rely more on generalisations about what could
(or could not) account for the evidence – which gets us back to the
equifinality problem.”

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