The study, published today (Nov. 12) in the journal Trends in Genetics, argues that humans lost the evolutionary pressure to be smart once we started living in dense agricultural settlements several thousand years ago.
"The development of our intellectual abilities and the optimization of thousands of intelligence genes probably occurred in relatively non-verbal, dispersed groups of peoples [living] before our ancestors emerged from Africa," said study author Gerald Crabtree, a researcher at Stanford University, in a statement.
Since then it's all been downhill, Crabtree contends.
The theory isn't without critics, with one scientist contacted by LiveScience suggesting that rather than losing our smarts, humans have just diversified them with various types of intelligence today.
Life or death situations
Early humans lived or died by their spatial abilities, such as quickly making a shelter or spearing a saber-toothed tiger. Nowadays, though almost everyone has the spatial ability to do ostensibly simple tasks like washing dishes or mowing the lawn, such tasks actually require a lot of brainpower, the researchers note.
And we can thank our ancestors and the highly tuned mechanism of natural selection for such abilities. Meanwhile, the ability to play chess or compose poetry likely evolved as collateral effects.
But after the spread of agriculture, when our ancestors began to live in dense farming communities, the intense need to keep those genes in peak condition gradually waned.
And its unlikely that the evolutionary advantage of intelligence is greater than it was during our hunter-gatherer past, the paper argues.
"A hunter-gatherer who did not correctly conceive a solution to providing food or shelter probably died, along with his/her progeny, whereas a modern Wall Street executive that made a similar conceptual mistake would receive a substantial bonus and be a more attractive mate. Clearly, extreme selection is a thing of the past," the researchers write in the journal article.
Anywhere between 2,000 and 5,000 genes determine human intelligence, and these genes are particularly susceptible to harmful changes, or mutations, the researchers write. Based on knowledge of the rate of mutations, the team concludes that the average person harbors two intelligence-stunting genetic changes that evolved over the last 3,000 years.
The hypothesis is counterintuitive at first. After all, across the world the average IQ has increased dramatically over the last 100 years, a phenomenon known as the Flynn Effect. But most of that jump probably resulted from better prenatal care, better nutrition and reduced exposure to brain-stunting chemicals such as lead, Crabtree argues.
But just because humans have more mutations in their intelligence genes doesn't mean we are becoming less brainy as a species, said psychologist Thomas Hills of the University of Warwick, who was not involved in the study. Instead, removing the pressure for everyone to be a superb hunter or gatherer may have allowed us to evolve a more diverse population with different types of smarts, he said.
"You don't get Stephen Hawking 200,000 years ago. He just doesn't exist," Hills told LiveScience. "But now we have people of his intellectual capacity doing things and making insights that we would never have achieved in our environment of evolutionary adaptation."