This is the controversial hypothesis of a leading geneticist who believes that the immense capacity of the human brain to learn new tricks is under attack from an array of genetic mutations that have accumulated since people started living in cities a few thousand years ago.
Professor Gerald Crabtree, who heads a genetics laboratory at Stanford University in California, has put forward the idea that rather than getting cleverer, human intelligence peaked several thousand years ago and from then on there has been a slow decline in our intellectual and emotional abilities.
Although we are now surrounded by the technological and medical benefits of a scientific revolution, these have masked an underlying decline in brain power which is set to continue into the future leading to the ultimate dumbing-down of the human species, Professor Crabtree said.
His argument is based on the fact that for more than 99pc of human evolutionary history, we have lived as hunter-gatherer communities surviving on our wits, leading to big-brained humans. Since the invention of agriculture and cities, however, natural selection on our intellect has effectively stopped and mutations have accumulated in the critical "intelligence" genes.
"I would wager that if an average citizen from Athens of 1,000BC were to appear suddenly among us, he or she would be among the brightest and most intellectually alive of our colleagues and companions, with a good memory, a broad range of ideas and a clear-sighted view of important issues," Professor Crabtree says in a provocative paper published in the journal Trends in Genetics.
"The basis for my wager comes from new developments in genetics, anthropology, and neurobiology that make a clear prediction that our intellectual and emotional abilities are genetically surprisingly fragile," he says.
A comparison of the genomes of parents and children has revealed that on average there are between 25 and 65 new mutations occurring in the DNA of each generation.
Professor Crabtree says that this analysis predicts about 5,000 new mutations in the past 120 generations, which covers a span of about 3,000 years.
Some of these mutations, he suggests, will occur within the 2,000 to 5,000 genes that are involved in human intellectual ability.
However, other scientists remain sceptical. "At first sight this is a classic case of Arts Faculty science. Never mind the hypothesis, give me the data, and there aren't any," said Professor Steve Jones, a geneticist at University College London.
"I could just as well argue that mutations have reduced our aggression, our depression and our penis length but no journal would publish that," Professor Jones said.