In the last century, humans have landed a man on the moon, sequenced the genome, and created the Internet — but, surprisingly, we may be slowly evolving to be less intelligent than our ancestors.

That's because a series of mutations affecting the estimated 5,000 genes controlling human intellect have crept into our DNA, says Gerald Crabtree, a geneticist at Stanford University, whose findings were published in the journal Trends in Genetics.

"I would wager that if an average citizen from Athens of 1000 BC were to suddenly appear among us, he or she would be among the brightest and most intellectually alive of our colleagues and companies, with a good memory, a broad range of ideas, and a clear-sighted view of important issues," he writes.

Because human beings have evolved to live in a society, as opposed to fending for themselves, deficiencies in intellect haven't made it impossible for reproduction, he says. Humans no longer (or rarely) die because they were unable to outwit a predator. Humans were much more likely to die due to "lack of judgment" thousands of years ago, he says.

"Intelligence doesn't play as significant a selection in our present, supportive wonderful society," he says. "I don't think we should revert back to the terrible times of extreme selection" where only the strongest survive, he adds.

Crabtree says that simple math makes it likely that humans are getting dumber. In the past 3,000 years, about 120 generations, Crabtree estimates that random, naturally occurring mutations have likely occurred in nearly every human.

"It's worth mentioning these changes are very slow. You wouldn't see an effect in 20 or 100 years," he says. "It's happening so slowly that it's not something that anyone alive should worry about."

So why are modern humans seemingly light years ahead of their ancestors? Crabtree says it comes down to education and the growing sum of human knowledge. Modern society allows people to focus on becoming an expert in one thing — people no longer need a wide breadth of knowledge or even cognitive ability in order to thrive.

"Education makes it so the strongest insights of one individual can be rapidly distributed through our society," he says.

Though he says he's fairly certain human cognition is in decline, modern society's ability to solve problems means we're likely to come up with a solution if humans start acting more like Neanderthals.

"We have a long time to solve it. People 300 years ago had no idea where we'd be scientifically now," he says. "We'll be able to deal with this problem with a range of humane and ethical solutions."

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