A US biologist is of the opinion that human brainpower peaked thousands of years ago, and that our smarts have been declining ever since.

"I would be willing to wager that if an average citizen from Athens of 1000 BC were to appear suddenly among us, he or she would be among the brightest and most intellectually alive of our colleagues and companions," Stanford University professor Gerald Crabtree writes in a pair of articles published on Tuesday in Trends in Genetics, "Our fragile intellect Part I" and "Part II".

"The larger the number of genes required" to carry out everyday tasks, Crabtree writes, "the more susceptible we are as a species to random genetic events that reduce our intellectual and emotional fitness."

Recent advances in genetic research, he says, have shown that "the number of genes required for normal human intelligence and abilities might be surprisingly large" – between 2,000 and 5,000 needed for full intellectual and emotional function.

Crabtree cites studies that have shown that genetic mutations are more common than previously thought, and that "a gene need not be human or brain specific in its function to be essential for our specific human intellectual abilities," due to the fact that genes function as links in a chain, and that "failure of any one of the links gives rise to deficiency."

Those deficiencies add up "Within 3000 years or about 120 generations," he says, "we have all very likely sustained two or more mutations harmful to our intellectual or emotional stability."

Crabtree disputes the idea that nature is a self-improving system. "One could argue that anything that occurs in Nature must be good for us," he says, "but this line of reasoning is quite incorrect."

So, if we're losing our intelligence, how did we get it in the first place? "Needless to say," he writes, "this is one of the most important questions of modern anthropology and the subject of much investigation and debate."

Although Crabtree freely admits that proposing answers to that question is "outside my comfort zone," he speculates that humans' expanding prefrontal cortex allowed the development of profound intellectual abilities earlier than is commonly thought.

"We seem to be forced to the conclusion that life as a hunter gather required at least as much abstract thought as operating successfully in our present society," he believes.

"Surprisingly," Crabtree writes, "it seems that if one is a good architect, mathematician or banker, these skills were an offshoot of the evolutionary perfection of skills leading to our ancestor's survival as nonverbal, dispersed hunter-gathers."

Well, then – if we humans were so smart back then, when did we start losing our intellectual and emotional abilities through gene mutation? According to Crabtree, the slide began when we began living in cities, and genetic selection began to focus more on such things as disease resistance rather than the improvement of abstract thought. As support for this argument, he cites the genetic principle that as an organism selects highly for one trait, other traits are selected against.

"It is also quite likely," he says, "that the need for intelligence was reduced as we began to live in supportive, high-density cities that made up for lapses of judgment or failure of comprehension."

In other words, our intellectual and emotional capabilities decayed because they weren't as important for survival.

"It seems too obvious to state," he says, "but the tautology applies: our brains are good at the things they have been selected to be good at." And, Crabtree argues, those things are the spatial and abstract-reasoning capabilites that we developed between 50,000 and 500,000 years ago.

One should not fret, however, that our brains are slipping. We should not worry that we're falling so far and so fast that we're not smart enough to figure out a way to compensate for our negative genetic mutations. "Nor do we need to have visions of the world's population docilely watching reruns on televisions that they can no longer understand or build," he believes.

Advances in science, he says, will eventually enable our species to figure out how to continue to maintain our civilization despite a decline in raw brainpower. "But in the meantime," he says, "I'm going to have another beer and watch my favorite rerun of 'Miami CSI' (if I can figure out how to work the remote control)."

Alternatively, Crabtree could chug that beer while watching Mike Judge's 2006 film, Idiocracy. ®