University scientists say they have found strong proof that the human brain is still evolving.
The changes presumably confer a survival benefit
By comparing modern man with our ancestors of 37,000 years ago, the Chicago team discovered big changes in two genes linked to brain size.
One of the new variants emerged only 5,800 years ago yet is present in 30% of today's humans, they believe.
This is very short in evolutionary terms, suggesting intense selection pressures, they told Science.
Each gene variant emerged around the same time as the advent of so called "cultural" behaviours.
The microcephalin variant appeared along with the emergence of traits such as art and music, religious practices and sophisticated tool-making techniques, which date back to about 50,000 years ago.
It is now present in about 70% of humans alive today.
The other, called the ASPM variant, originated at a time that coincides with the spread of agriculture, settled cities and the first record of written language.
Researcher Dr Bruce Lahn said the big question was whether the genetic evolution seen had actually caused the cultural evolution of humans or was merely chance.
Their hunch is that it might have something to do with the important role that these genes play in brain size, but stressed that did not necessarily mean better intelligence.
"Just because these genes are still evolving doesn't necessarily mean they make you any smarter," said Dr Lahn.
But he added: "Our studies indicate that the trend that is the defining characteristic of human evolution - the growth of brain size and complexity - is likely still ongoing.
"If our species survives for another million years or so, I would imagine that the brain by then would show significant structural differences from the human brain of today."
The researchers said the next step was to examine whether biological differences imparted by the genetic differences caused natural selection to favour that variation over others.
They must have conferred some evolutionary advantage, such as a desired change in cognition, personality, motor control or resilience to neurological or psychiatric diseases, they said.
Dr Geraint Rees, a cognitive neurologist at University College London and Wellcome Trust senior research fellow, said: "It's very interesting.
"I do find it surprising that they can pinpoint these changes to a point relatively recently in evolutionary history.
"It gives us a clue to perhaps follow up on and try and understand why they emerged at that time and what the consequences were."
He said it would be too big a leap now to conclude that the genetic changes were responsible for some of the cultural changes we have seen, such as the emergence of agriculture. "But that's a tantalising prospect," he said.