Eight new studies in the past three years, and an older study, have all calculated the mutation rate directly. This is sort of the result of new high-throughput genome sequencing methods that give you high-quality coverage of the entire genome. So we’re able to get the more precise rate, which we sort of said is about an average of 36 mutations in each newborn. That’s something like a chance of getting 1.2 mutations per nucleotide site per 100 million years, okay? So when you think about spreading 36 mutations over three billion nucleic acids or bases in your genome, it comes out to not very many mutations per generation. This is the average rate in modern humans per generation, and it can be converted into a rate per year. Now there’s a little debate about how you do that because you have to know exactly how long each generation is. But new studies done by Linda Vigilant and her team – a number of primatologists in Germany – have studied the actual generation times using DNA and observations in the field of chimpanzees and gorillas, and we know them in modern humans. What this comes out to is about half the rate that researchers have been using for the past 15 years. One study by David Reich at Harvard and his colleagues comes up with a slower rate, but it isn’t half the rate. And that raises some questions about whether the new genome methods are actually catching all the mutations. We’re sort of at the limits of their resolution. I think most geneticists think that the rate is definitely slower. There is still some debate about precisely how much slower. Is it half or a little bit less?
Yes. So if you apply the new mutation rate, you get a human-chimpanzee split of about 8.3 million to about 10.1 million years ago, instead of 4-7 million years ago. So that’s quite a bit older. And the earliest fossils of the human family only are about 6-7 million years, so there’s a problem there. The human-Neandertal split used to be 250,000 to 350,000 years ago. Now it’s about 400-600 thousand years ago. That fits with fossils that look like they’re ancestral to Neandertals that show up around 500,000 years ago in Europe. So that’s a little better fit. And finally, we date the out-of-Africa migration to earlier, that we have our modern human ancestors coming out of Africa 90,000-130,000 years ago instead of less than 60,000 years ago. That would mean some of the fossils that have been discounted as modern human ancestors – especially in North Africa and Arabia – might actually be ancestral to modern humans if that’s accurate. There will be some debate. I would say at this point anthropologists and paleogeneticists who use these dates are quite confused, and they’re taking a wait-and-see attitude to see what geneticists end up deciding about applying these dates back in time.One good thing to come out of the coming upheaval, as anthropologists scramble to update their models, is that the appearance of modern symbolic behavior and art. during the Upper Paleolithic will finally be decoupled from the Out-of-Africa event.
This will help us understand both: the ancestors of non-Africans did not come forth fully formed, like Athena from Zeus's head, having spent millennia of perfecting their craft and honing their minds by perforating shells and scratching lines in some South African cave. Instead, they may been plain old-style hunter-gatherers who stumbled into Asia by doing what they always did: following the food. At the same time, the UP/LSA revolution may not have been effected by a new and improved type of human bursting into the scene and replacing Neandertals and assorted dummies, but rather as a cultural revolution that spread across a species that already had the genetic potential for it, and was already firmly established in both Africa and Asia.
Science 12 October 2012:
Vol. 338 no. 6104 pp. 189-191
Turning Back the Clock: Slowing the Pace of Prehistory
Researchers have used the number of mutations in DNA like a molecular clock to date key events in human evolution. Now it seems that the molecular clock ticks more slowly than anyone had thought, and many dates may need to be adjusted. Over the past 3 years, researchers have used new methods to sequence whole human genomes, allowing them to measure directly, for the first time, the average rate at which new mutations arise in a newborn baby. Most of these studies conclude that the mutation rate in humans today is roughly half the rate that has been used in many evolutionary studies since 2000, which would make genetic estimates of dates older than previously believed. The question now is how much older?