If those genes don't work, babies are born with severely small brains, called microcephaly.
In fact, the variations were so common they couldn't be accidental mutations but instead were probably due to natural selection, where genetic changes that are favorable to a species quickly gain a foothold and begin to spread, the researchers report.
Lahn offers an analogy: Medieval monks would copy manuscripts and each copy would inevitably contain errors — accidental mutations.
'It made me think of my outrage at our false reasons for going to war in Iraq and the scandalous way in which the conflict was sold to the American people. Bush, then just sworn into his second term in office, and I decided to call the image "Four More Years".'As any new parent will tell you, the future suddenly becomes a very serious concern.
As ecstatic as I was at the birth of my daughter, I felt selfish bringing her, and later my son into our screwed-up world.
"We, including scientists, have considered ourselves as sort of the pinnacle of evolution," noted lead researcher Bruce Lahn, a University of Chicago geneticist whose studies appear in Friday's edition of the journal Science.
"There's a sense we as humans have kind of peaked," agreed Greg Wray, director of Duke University's Center for Evolutionary Genomics.
So suggests new research that tracked changes in two genes thought to help regulate brain growth, changes that appeared well after the rise of modern humans 200,000 years ago.
That the defining feature of humans — our large brains — continued to evolve as recently as 5,800 years ago, and may be doing so today, promises to surprise the average person, if not biologists.
'It's as if they believe the apocalyptic End Time is near, therefore protecting the earth and the future of all our children is futile.