The argument over when our lineage split from chimps is about to be settled, with colossal consequences for prehistory
LINE them up in your head. Generation after generation of your ancestors, reaching back in time through civilisations, ice ages, an epic migration out of Africa, to the very origin of our species. And on the other side, take a chimp and line up its ancestors. How far back do you have to go, how many generations have to pass, before the two lines meet?
This is one of the biggest and hardest questions in human evolution. We know that at some point we shared a common ancestor with chimps, but exactly when - and what that ancestor was like - have been maddeningly hard to pin down. Palaeontologists have searched for fossil remains, and geneticists have rummaged through the historical documents that are human and chimp DNA. Both made discoveries, but they did not see eye to eye.
No more. New estimates for when our lineage and chimps went their separate ways suggest that some of our established ideas are staggeringly wrong. If correct, they demand a rewrite of human prehistory, starting from the very beginning.
When was that beginning? The obvious first place to look for answers is in the fossil record. But fossil humans - or more strictly hominins, the group that includes us and all our extinct relatives from after the split - are notoriously thin on the ground and difficult to interpret.
Geneticists have more to work with. DNA contains telltale traces of events in a species' past, including information about common ancestry and speciation. In theory, calculating the timing of a speciation event should be straightforward. As two species diverge from a common ancestor their DNA becomes increasingly different, largely due to the accumulation of random mutations. The amount of genetic difference between two related species is therefore proportional to the length of time since they diverged. To estimate when the human-chimp split occurred, geneticists can simply count the differences in matching stretches of chimp and human DNA and divide it by the rate at which mutations accumulate. This is known as the molecular clock method.
But there's a catch. To arrive at the answer you have to know how fast the mutations arise. And that leads you back to square one: you first need to know how long ago we split from chimpanzees.
To get around this catch-22, geneticists turned to orang-utans. Fossils suggest that they split from our lineage between 10 and 20 million years ago. Using this fudge, geneticists arrived at a mutation rate of about 75 mutations per genome per generation. In other words, offspring of humans and chimps each have 75 new mutations that they did not inherit from their parents.
Fossils or DNA
This number rests on several big assumptions, not least that the orang-utan fossil record is a reliable witness - which most agree it is not. Even so, it led to a guess that human ancestors split from chimpanzees between 4 and 6 million years ago.
When fossil-hunters hear this number, they cry foul. The lower end of the estimate is particularly hard to swallow. Australopithecus afarensis - an early hominin from east Africa - already has distinctly human characteristics yet dates back at least 3.85 million years. Its canines were small, for instance. And it walked upright.
Both of these traits are considered hominin, meaning they evolved in our lineage after the split and did not appear on the chimp side. And yet it is hard to see how they could have evolved so quickly, in perhaps as little as 150,000 years after the split.
"Geneticists ignored the palaeontologists completely," says Owen Lovejoy of Kent State University in Ohio. "We would get estimates around 4 million years, and yet there are unmistakable and highly evolved hominins that go back almost 4 million years. To claim a 4 million year divergence date is just silly."
Even a 5 to 6-million-year split was met with scepticism. That's largely because of three recently discovered fossils from Africa dating from around the same period. All three predate Australopithecus, but still bear unmistakable marks of humanity. Though the interpretation of the remains is controversial, many regard them as being post-split.
Simply put, the palaeontologists were sure there was little chance that the DNA results were accurate. Humanity, they affirmed, had to be older than the geneticists claimed.
History looks set to prove them right. In the past three years, researchers studying human populations have for the first time been able to observe mutations almost as they happen. And that makes all the difference. Instead of relying on an estimate based on rare fossils, we can now watch the molecular clock ticking in real time. "Until we were able to compare genomes of children with their parents, we could not estimate the mutation rate in humans," says Aylwyn Scally of the Wellcome Trust Sanger Institute in Cambridge, UK.
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