How did our species become the dominant one on Earth? Pinpointing the genetic changes that made this possible is one of the great challenges of evolutionary biology. Now, a team has developed a method that could allow us to reconstruct a detailed, step-by-step genetic history of our species spanning tens of thousands of years.
As a first step, they have identified a mutation that arose in east Asia and then was carried over to North America as our ancestors crossed the Bering Strait and colonised the New World.
"There is an archaeological record hidden in our DNA that can help point us to the traits that have been critical in human survival," says Pardis Sabeti of the Broad Institute in Cambridge, Massachusetts.
Sabeti's technique identifies versions of genes that have been created by random mutation and then retained - because they give their owners some natural advantage over individuals who do not have the mutation.
Gene variants that confer a survival advantage in this way can spread through a population – a process known as natural selection. But genes can also mutate and then spread by random chance even though they provide no specific advantage. In 2010, Sabeti developed a test that distinguishes the two possibilities.
The test works by looking for features that are associated with natural selection. For instance, when a gene spreads through natural selection, neighbouring stretches of DNA tend to also become more common, because they are attached to the favoured gene. So gene variants that have been selected for are often found surrounded by the same stretches of DNA, whereas those that have not been selected do not have consistent genetic neighbours.
Sabeti has now examined the genomes of 179 people from around the world, and used her test to pinpoint 412 DNA regions that have been strongly selected for.
The mutations in these regions represent key changes in human evolution over the last 40,000 years, after our ancestors first left Africa and were going global. The challenge now is to find out when each mutation arose, how it changed us, and why.
Hair and sweat
Sabeti's team made a start on this by examining the gene for the ectodysplasin A receptor (EDAR). Their analysis had revealed that one version of this gene, EDAR370A, is only found in some east Asians and Native Americans – among whom it is much more common than the original EDAR. Based on its modern distribution, Sabeti calculated that the mutation that gave rise to EDAR370A arose in China around 30,000 years ago.
To determine what changes the mutation brought about, she genetically modified mice to express EDAR370A.
This produced a catalogue of transformations. The modified mice had thicker hair fibres – as do Asians with EDAR370A. Their mammary glands also had smaller fat pads, which the team says may correlate with the fact that Asian women tend to have smaller breasts than African women.
Perhaps most importantly, the mutant mice had more sweat glands on the pads of their feet. When the team examined 623 Han Chinese people, they found that those with EDAR370A also had more sweat glands on their fingers.
Helpful mutation
It's not clear why EDAR370A was selected for. Any one of the changes it produces could have been beneficial, or perhaps it was a combination of the effects. "You can come up with a good story for all the traits," says team member Yana Kamberov of Harvard Medical School in Boston.
One possibility, the researchers say, is that the extra sweat glands helped humans to keep cool in the warm and humid Chinese climate of 30,000 years ago. That would fit with what we already know. Hunter-gatherers often bring down prey in long-distance chases, and being able to keep cool is essential for endurance running.
Alternatively, it could be all about sex. Men in Asia may have found small breasts more attractive, and thick hair may have been desirable generally. Such sexual selection can have a powerful effect on populations.
"I personally favour the idea that the traits could all have been acted on at different times," says Kamberov. She points out that the climate has changed dramatically since EDAR370A arose, so different effects of the gene may have been beneficial at different times. "It doesn't make sense to me that it was a one-hit wonder."
It's always tempting to make up just-so stories about human evolution, says Ewan Birney of the European Bioinformatics Institute in Cambridge, UK. "Sabeti didn't do that." He adds that using mice to figure out exactly what the mutated gene does is a key advance on previous studies.
Sabeti's work is the latest to identify genes that were important in the evolution of humans, and our subsequent history. In 2011, a study suggested that deletions of large chunks of DNA played a crucial role when the human family split from the ancestors of chimpanzees.
More recently, the SRGAP2 gene was found to have duplicated 2.5 million years ago. The extra copies may have allowed our brains to grow larger.
And within the last 70,000 years, interbreeding with Neanderthals gave us crucial immune genes that may have helped us go global.
Journal reference: 10.1016/j.cell.2013.01.035 and Cell, DOI: 10.1016/j.cell.2013.01.016
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