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."