2003-07-22 14:09:15 UTC
Date limit set on first Americans
By Paul Rincon
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
The subject of when humans first arrived in America is hotly contested
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
The authors of the latest study reject the latter theory, proposing
that humans entered America no earlier than 18,000 years ago.
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
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 first
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
"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.
The debate over the biological origins of the first Americans has
wide-ranging political and racial implications.
In the US, the Native American Graves Protection and Repatriation Act
(Nagpra) has resulted in the handover of many scientific collections
Some archaeologists argue that the remains of early Americans are
sufficiently different from their descendents to be exempt from
For example, a 9,300-year-old skull from Washington State known as
Kennewick Man has been interpreted as looking European due to its
long, narrow (dolichocephalic) skull shape. More recent American
populations tend to have short, broad skulls.
Dr Wells said individuals such as Kennewick Man looked this way
because Europeans and early Americans had a common origin around
35,000-40,000 years ago in south-central Asia.
"[Dolichocephaly] is a general feature of very early skulls," Dr Wells
told BBC News Online.
He said a later migration into America from East Asia 6,000-10,000
years ago associated with the spread of Y chromosome haplotype 5 could
have been responsible for the Asiatic appearance of many present-day
But Dr Wells acknowledged the possibility that even more ancient
American populations carrying unidentified Y chromosome haplotypes
could have been swamped by later migrations, resulting in their
genetic legacy being erased.
"We can't rule that out," he said, "but in science we have to deal
with what's extant."