Human intelligence came at the cost of mental disorders, study finds. (Reuters)

Human intelligence evolved around 500 million years ago thanks to a "genetic accident" that resulted in animals producing extra copies of brain genes, a new study has revealed.

The study, published in the journal Nature Neuroscience, found that while the increase of brain genes improved our intelligence, it also resulted in brain disorders. Previous studies has found that over 100 brain diseases are caused by genetic mutations.

John Williams, head of neuroscience and mental health at the Wellcome Trust, which part funded the study, said: "This research makes a significant step forward in our understanding of how sophisticated behaviours emerged in humans and other animals

"[It] has implications for how we understand the emergence of psychiatric disorders and will offer new avenues for the development of new treatments."

Study leader professor Seth Grant, from the University of Edinburgh, said: "Our work shows that the price of higher intelligence and more complex behaviours is more mental illness."

Researchers studied the mental capabilities of mice and humans and compared tasks that involved identifying objects on touch-screen computers.

They then combined the results with information from the genetic codes of various species, to identify when different behaviours evolved.

Higher mental capabilities of mice and humans were controlled by the same genes, the researchers found. These genes were in place 100 million years ago, when humans and mice last shared a common ancestor.

However, it also found that when these genes were damaged or mutated, higher mental functions were impaired.

Tim Bussey from Cambridge University, who was also involved in the research, concluded: "We can now apply genetics and behavioural testing to help patients with these diseases."