Some time in humanity’s past, a small group of Homo sapiens migrated out of Africa before spreading out to every possible corner of the Earth. All the women of that group carried DNA inherited from just one woman, commonly known as mitochondrial Eve, whose DNA was inherited by all humans alive today. But the exact timing of this migration is not clear, and it has sparked debate among geneticists. Now, new research published in Current Biology may help calm both sides.
Studies of evolutionary events often involve the use of molecular clocks based on changes in DNA that accumulate over time. To accurately calibrate a clock, it helps to have a measure of the rate of mutations.
In 2012, UK Researchers used a method of analysis that involves DNA from the nucleus of present day humans. Armed with data from parents and their offspring, they estimated a new, much lower rate of DNA mutation. Based on their results, it would seem that human DNA may change much more slowly than was previously thought. The slow mutation rate puts the date of human migration out of Africa at somewhere between 90,000 and 130,000 years ago.
"This was very surprising," says Alissa Mittnik, a researcher at the University of Tübingen in Germany. "It contradicts what we know from fossil studies."
Those fossil studies have used mitochondrial DNA (mtDNA), which is easily preserved in old fossils, to estimate the mutation rate. The mutation rate was then used to calibrate events in humanity’s past.
This data spills into other areas of research. For example, when combined with evolutionary models, this information can help predict which humans were able to build the various things that have been dug up by archaeologists. Carbon dating can give the precise age of the objects, but depending on the mutation rates, the species that made and used the object could be, for instance, modern humans or the Neanderthals.
In Current Biology, Mittnik and her colleagues report a new mutation rate. This rate may help researchers find a middle ground on the period when the African migration happened.
Their estimate relies on mtDNA too, but they claim it is much more reliable, in part because it uses mtDNA from ten different fossils, ranging from 700 to 40,000 years old. The new estimated rate is higher than the UK researchers got using nuclear DNA, but lower than older estimates of mtDNA studies. The lower rates reported by UK researchers, Mittnik says, could be because of their use of “too stringent” filters—their analysis missed out on mutations that might have actually occurred (technically called “false negatives”).
Mittnik admits that, if it were possible, she would have studied nuclear DNA of fossils. That is because mtDNA only has 37 of the roughly 20,000 human genes—the rest are in nuclear DNA. But each cell has only two copies of nuclear DNA. Whereas it has hundreds of copies of mtDNA because it has many mitochondria in each cell, and each of them have multiple copies of the mtDNA. This makes it much harder to study nuclear DNA from fossil remains.
Based on Mittnik's results, the suggested range for the time when Homo sapiens moved out of Africa has been set to 62,000 to 95,000 years ago. Thus, there’s common ground with other estimates between 90,000 and 95,000 years ago. Mittnik says she won't be surprised if the precise date of migration was later than archaeologists currently believe, but it certainly is earlier than what the UK researchers claim.