Since modern humans emerged from the evolutionary brambles of our ancient ancestry, our bodies and minds have been transforming under the pressures of natural and sexual selection. But what of human intelligence? Has our cognitive ability risen steadily since our forebears knapped the first stone tools? Or are our smartest days behind us?

Gerald Crabtree, a geneticist at Stanford University in California, bets on the latter. He believes that if an average Greek from 1,000 BC were transported to modern times, he or she would be one of the brightest among us. Our intellectual prowess has probably been sliding south since the invention of farming and the rise of high-density living that it allowed, he claims.

In two articles published in the journal Trends in Genetics, the scientist lays out what might be called a speculative theory of human intelligence. It is, he admits, an idea that needs testing, and one that he would happily see proved wrong.

At the heart of Crabtree's thinking is a simple idea. In the past, when our ancestors (and those who failed to become our ancestors) faced the harsh realities of a hunter-gatherer lifestyle, the punishment for stupidity was more often than not death. And so, Crabtree argues, enormous evolutionary pressure bore down on early humans, selecting out the dimwits, and raising the intellect of the survivors' descendants. But not so today.

As Crabtree explains in the journal: "A hunter-gatherer who did not correctly conceive a solution to providing food or shelter probably died, along with his or 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 scientist draws on recent studies to estimate a figure for the number of genes that play a role in human intellectual ability, and the number of new mutations that harm those genes each generation. He settles on a suite of 2,000 to 5,000 genes as the basis for human intelligence, and calculates that among those, each of us carries two or more mutations that arose in the past 3,000 years, or 120 generations.

All of which leads to the conclusion that humans reached our intellectual height in the dim and distant past. "We, as a species, are surprisingly intellectually fragile and perhaps reached a peak 2,000 to 6,000 years ago," Crabtree writes. "If selection is only slightly relaxed, one would still conclude that nearly all of us are compromised compared to our ancient ancestors of 3,000 to 6,000 years ago," he adds.

The idea may not survive in the face of experiments, or even close scrutiny from other geneticists. The kind of enormous evolutionary pressure Crabtree talks of perhaps isn't necessary to maintain human intelligence.

Whichever it turns out to be, Crabtree ends on a positive note: the human race is not hurtling towards cognitive oblivion, doomed, as he puts it, to watch reruns on televisions we can no longer build. "Remarkably it seems that although our genomes are fragile, our society is robust almost entirely by virtue of education, which allow strengths to be rapidly distributed to all members."