An international team of researchers has found evidence challenging existing views about the timescale of two major events in human evolution: the first migration out of Africa, and the dating of "mitochondrial Eve,' the last common ancestor of all humans along the matrilineal line.

They estimate that the first migration out of Africa occurred less than 95,000 years ago, and that mitochondrial Eve lived about 160,000 years ago. This contradicts previous findings, which suggested that the split between modern non-Africans and modern Africans occurred as early as 125,000 years ago, and that the last common ancestor for human mitochondrial lineages lived about 200,000 years in the past.

An international team, led by Johannes Krause of the University of Tübingen and researchers from the Max Planck Institute for Evolutionary Anthropology, published the findings in the journal Current Biology.

The researchers used new measurements of mutation rates- the rate at which offspring show DNA changes their parents did not have- to calculate their dates. Specifically, they looked at mitochondrial DNA (mtDNA), a specific type of DNA in cells that is passed on from mother to offspring. All mtDNA is passed on through multiple generations without recombining during reproduction- which means it stays more or less intact, with minimal mutations.

The team reconstructed over ten mitochondrial genomes (mtDNAs) from modern human fossils found throughout Eurasia, from a period spanning 40,000 years in prehistory. Some of the ancient fossils were the oldest modern human remains on record, like the triple burial from Dolni Vestonice in the Czech Republic, and skeletons found in Oberkassel near Bonn in Germany.

Analysis of the fossils' mtDNA showed that hunter-gatherers from pre-ice age Europe carry mtDNA related to modern humans from after the last ice age, about 20,000 years ago. The findings suggest that pre-ice age populations continued throughout the last ice age.

The researchers also used the ancient fossils' radiocarbon-dated age to estimate mutation rates backwards in time, over tens of thousands of years. They calculated the number of gene mutations in modern human groups that were absent in the ancient human groups, assuming that those mutations did not yet exist among the ancient population, and estimated novel mutation rates by counting the number of mutations that accumulated over time along descendant lineages after the radiocarbon-dated fossils.

The used those mutation rates to estimate the most recent common ancestor of modern Africans and modern non-Africans- a figure that would indicate when the first migration from Africa occurred. They calculated that date at between 62,000-95,000 years ago.

Current scientific consensus holds that modern humans evolved in Africa, and then split off into branches that either stayed in Africa or migrated north to Eurasia. The results from Krause's team agree with previous calculated mitochondrial dates for the first migration of Africa, but are at odds with other studies that suggest the split between Africans and non-Africans occurred about 30,000 years earlier.

The same mutation rates were used to calculate the date of last common ancestor for human mitochondrial lineages, "mitochondrial Eve," to around 160,000 years ago.

Mitochondrial Eve was the most scientifically important of all our common ancestors- an individual woman who lived far in the prehistoric past who happened to become the matrilineal ancestor all present-day humans. Mitochondrial Eve was not the most recent common ancestor of all humans, and she was certainly not the only woman who was alive at the time. She just happened to be the most recent woman common ancestor to possess the mtDNA from which all existing humans' mtDNA derived.

Calculating dates of human evolutionary events is always tricky, since fossil evidence is inevitable incomplete and DNA degrades over time. Every so often, new evidence or analysis arises which challenges previous views about the timescale of events like the most recent common ancestor or specific human migrations.

"The results from modern family studies and our ancient human DNA studies are in conflict" said Krause in a statement. "One possibility is that mutations were missed in the modern family studies, which could lead to underestimated mutation rates."