Footprints of 'first Americans'
By Paul Rincon BBC News science reporter
Published: 2005/07/05 10:17:53 GMT © BBC MMV
Human settlers made it to the Americas 30,000 years
earlier than previously thought, according to new evidence.
A team of scientists came to this controversial conclusion by dating
human footprints preserved by volcanic ash in an abandoned quarry in
Mexico.
They say the first Americans may have arrived by sea, rather than by
foot.
The traditional view is that the continent's early settlers arrived
around 11,000 years ago, by crossing a land bridge between Siberia and
Alaska.
Details of the latest findings were unveiled at the UK Royal Society's
Summer Science Exhibition.
Ancient lake
Dr Silvia Gonzalez of Liverpool's John Moores University and her
colleagues found the footprints in the quarry, some 130km (80 miles)
south-east of Mexico City, in 2003. But they have only finished dating
them this year.
The footprints were
preserved as trace fossils in volcanic ash along what was the
shoreline of an ancient volcanic lake. They were soon covered in more
ash and lake sediments and, when water levels rose, became as solid as
concrete. Dr Gonzalez was under no illusions that the finding
would be controversial: "It's going to be an archaeological bomb," she
told the BBC News website, "and we're up for a fight." The
team used several methods to date a variety of material from the site
near Puebla, Mexico, in order to be sure they were right about the
age. "We have materials that have been dated below the
footprint layer, the footprint layer itself and on top of the
footprint layer. Everything is making sense," said Dr Gonzalez.
The researchers used
radiocarbon dating on shells and animal bones in the sequences and
dated mammoth teeth by a technique called electron spin resonance. The
sediments themselves were dated by optically stimulated luminescence.
"Some lake sediments were incorporated into the ash and were baked.
They look like small fragments of brick and these were the ones we
dated in the footprint layer. They gave us a result of 38,000 years,"
Dr Gonzalez.
Land crossing
Under the traditional view, humans trekked from Siberia to Alaska
across a land bridge that linked these land masses at the end of the
last ice age (between about 10,000 and 12,500 years ago).
Central to the theory, called the Clovis First model, are Clovis
points - the tools these settlers used to hunt large beasts, or
megafauna, such as mammoths and mastodons.
"The existence of 40,000-year-old human footprints in Mexico means
that the Clovis First model of human occupation can no longer be
accepted as the first evidence of human presence in the Americas,"
said coinvestigator David Huddart, of Liverpool John Moores.
Dr Michael Faught, an expert in early American archaeology, was
reserving judgment until evidence was published: "It would be
significant if it were demonstrated, but usually those (early) sites
don't hold up well," But, he added: "There's more and more evidence
that Alaska was not the only place people came into the continent."
Dr Gonzalez is a proponent of the Coastal Migration Theory. This
proposes that people arrived on the west coast in boats, hugging the
coastline from North to South. But where these settlers
came from is still a mystery, she says. Some have proposed that
the earliest humans to reach the continent could have come from
south-east Asia or even Australia.
Genetic studies of present-day Native American populations support a
recent arrival from north-east Asia, which agrees well with an entry
through the Beringian land bridge at the end of the last Ice Age.
Dr Gonzalez suggests that the earliest settlers may have become
extinct, leaving no genetic legacy today. She thinks these hunters may
have been highly mobile, living in small groups, perhaps explaining
why they left scant trace of their presence.
Dr Gonzalez and ancient DNA expert Alan Cooper, of the University of
Adelaide in Australia, have managed to extract genetic material from
three molars belonging to Peñon Woman III, a 13,000-year-old partial
skeleton from Mexico. The analysis is still underway. |