A new genetic study deals a blow to claims that humans reached
America at least 30,000 years ago - around the same time that people
were colonising Europe.
The subject of when humans first arrived in America is hotly
contested by academics. On one side of the argument are researchers
who claim America was first populated around 13,000 years ago,
toward the end of the last Ice Age. On the other are those who
propose a much earlier date for colonisation of the continent -
possibly around 30,000-40,000 years ago.
The authors of the latest study reject the latter theory,
proposing that humans entered America no earlier than 18,000 years
They looked at mutations on the form of the human Y chromosome
known as haplotype 10. This is one of only two haplotypes carried by
Native American men and is thought to have reached the continent
first. Haplotype 10 is also found in Asia, confirming that the
earliest Americans came from there.
The scientists knew that determining when mutations occurred on
haplotype 10 might reveal a date for the first entry of people into
America. Native Americans carry a mutation called M3 on haplotype 10
which is not found in Asia. This suggests it appeared after people
settled in America, making it useless for assigning a date to the
But a mutation known as M242 looked more promising. M242 is
found in Asia and America, suggesting that it appeared before the
first Americans split from their Asian kin.
Knowing the rate at which DNA on the Y chromosome mutates -
errors occur - and the time taken for a single male generation, the
scientists were able to calculate when M242 originated. They arrived
at a maximum date of 18,000 years ago for its appearance.
This means the first Americans were still living in Asia when
M242 appeared and could only have begun their migration eastwards
after this date.
"I would say that they entered [America] within the last 15,000
years," said Dr Spencer Wells, a geneticist and author who
contributed to the latest study. In 1997, a US-Chilean team
uncovered apparent evidence of human occupation in 33,000-year-old
sediment layers at Monte Verde in Chile.
They claimed that burned wood found at the site came from fires
at hunting camps and that fractured pebbles found there were used by
humans to butcher meat. But the interpretation of these remains has
been questioned by several experts.