Science - mutation-rates

Mar

2013

1:07pm, EDT

Another chapter written in 'Out of Africa' story

F. Demeter

A new genetic analysis suggests that humans left Africa no earlier than 95,000 years ago, pushing the date of that migration back more than 100,000 years.

By Tia Ghose
LiveScience

Our early human ancestors may have left Africa more recently than thought, between 62,000 and 95,000 years ago, suggests a new analysis of genetic material from fossil skeletons.

The new findings are in line with earlier estimates, but contradict a more recent study that put humans' first exodus from Africa at least 200,000 years ago.

The new results "agree with what we know from archaeology," said study co-author Alissa Mittnik, a biologist at University of Tübingen, in Germany.

Hot debate
Exactly when the first humans emerged from Africa to colonize the world has been a topic of heated debate. [Photos: Our Closest Human Ancestor]

All of the estimates hinge on one number: the gene mutation rates. By knowing how often genes change, and then counting up the number of genetic differences between different species or groups of people, scientists can create a "molecular clock" to decipher how long ago they shared a common ancestor.

Early studies used genetic differences in mitochondrial DNA — genetic material inside the cells' energy-making structures that gets passed on from mother to child — between chimpanzees and humans.

But since that technique is based on the number of mutations divided by the time since the two shared a common ancestor, it requires an estimate of when the common ancestor of chimpanzees and humans lived.

Newer research estimated the mutation rate in modern human families based on DNA from the nucleus, which involved another way of getting at the common ancestor timing. That method suggested humans were racking up genetic mutations at half the rate — meaning to reach the genetic differences we see today humans would've had to leave Africa more than 200,000 years ago.

Fossil DNA dated
But that didn't jive with archaeological and other evidence, Mittnik told LiveScience.

For instance, the slower mutation rates that were previously reported had several implications, including "much earlier dates for the separation of the lineages of chimps and humans, and of Neanderthals and Homo sapiens, and earlier dates for so-called "African Eve" and the exit of modern humans from Africa," Chris Stringer, an paleobiologist at the Natural History Museum in London, who was not involved in the study, wrote in an email.

It seems unlikely that all of those dates are wrong. To sort out the problem, the researchers extracted mitochondrial DNA from 11 ancient human fossil skeletons from Europe and Asia. Using radioactive carbon dating, the oldest was estimated to be 40,000 years old, while the most recent came from medieval times.

The team found a mutation rate suggesting that humans left Africa between 62,000 and 95,000 years ago.

Method questioned
The researchers estimate that in its effort to avoid false positives (mutations that weren't really mutations), the nuclear DNA method missed quite a few real mutations. That would lead to an underestimate of the mutation rate and a longer estimate for when humans left Africa, diverged from Neanderthals, and other things.

Right now, it's not clear which method is most reliable, Peter Visscher, a quantitative geneticist at the University of Queensland, who was not involved in the study, wrote in an email to LiveScience.

"This debate will continue a bit longer, but soon there is likely to be a consensus on what mutation rates are in the present, because there is so much sequencing being done around the world," Visscher wrote.

The findings were published Thursday in the journal Current Biology.

Follow Tia Ghose on Twitter @tiaghose. Follow LiveScience @livescience, Facebook and Google+. Original article on LiveScience.com.