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.
By Paul Rincon
The subject of when humans first arrived in America is hotly contested by academics.
Kennewick Man, a 9,300-year-old American
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 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 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 first migrations.
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.
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 to claimants.
Some archaeologists argue that the remains of early Americans are sufficiently different from their descendents to be exempt from Nagpra.
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 Native Americans.
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."
The latest research is to be published in the American Journal Of Human Genetics.