A freak accident millions of years ago spurred the evolution of thinking genes in mammals - including humans, suggests a study.

Researchers say a simple invertebrate animal living in the sea about 500 million years ago experienced a "genetic accident" resulting in extra copies of some genes. This animal's descendants benefited from these extra genes, leading to behaviourally sophisticated vertebrates - including humans.

Intelligence in humans developed as the result of an increase in the number of brain genes in our evolutionary ancestors, giving us the ability to learn complex skillsand have flexibility in the way in which we think, the journal Nature Neuroscience reports. 

Seth Grant, professor at the University of Edinburgh, who led the research, said: "One of the greatest scientific problems is to explain how intelligence and complex behaviours arose during evolution." 

The research also shows a direct link between the evolution of behaviour and the origins of brain diseases.