Intelligence and the capacity for abstract thought evolved in our prehistoric ancestors living in Africa between 50,000 and 500,000 years ago, who relied on their wits to build shelters and hunt prey.
But in more civilised times where we no longer need to fight to survive, the selection process which favoured the smartest of our ancestors and weeded out the dullards is no longer in force.
Harmful mutations in our genes which reduce our "higher thinking" ability are therefore passed on through generations and allowed to accumulate, leading to a gradual dwindling of our intelligence as a species, a new study claims.
Prof Gerald Crabtree, a developmental biologist at Stanford University, explained in the Trends in Genetics journal that a mutation in any one of 2,000 to 5,000 particular genes could lower our intellectual and emotional ability.
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