The same genes that gave rise to higher mental function are also responsible for a number of brain disorders, the results reveal, suggesting that the evolution of intelligence came at the cost of mental illness.
The discovery pinpoints when in history the genes that enable us to think and reason evolved, giving us the ability to learn complex skills, analyse situations and have flexibility in how we think.
A team led by researchers at the University of Edinburgh used a combination of human and mouse behavioural studies and analysed the underlying genetics to work out how and when certain behaviours evolved.
Their findings trace back the evolution of higher cognitive functions to an increase in the number of copies of specific genes that are involved in signalling processes in the brain. The evidence suggests that the increase occurred as the result of two genetic duplication events in one of our evolutionary ancestors some 500 million years ago. Having the extra copies led descendants of this primitive animal to become more behaviourally sophisticated.
Both humans and mice were used in the study and were given comparative tasks to perform, involving identifying images on computer touchscreen tablets. The research reveals that higher mental functions are controlled by the same genes in both humans and mice, showing that the roles of these genes in cognition were already in place 100 million years ago when the two species last shared a common ancestor.
The study also found that when these genes were mutated or damaged, they interfered with higher mental functions. Professor Seth Grant of the University of Edinburgh, who led the research, explained: "Our work shows that the price of higher intelligence and more complex behaviours is more mental illness."
The researchers had previously shown that more than 100 childhood and adult brain diseases are caused by gene mutations.
"We can now apply genetics and behavioural testing to help patients with these diseases," said Dr Tim Bussey from the University of Cambridge, who was also involved in the study.
"Using the latest molecular and behavioural techniques, this research makes a significant step forward in our understanding of how sophisticated behaviours emerged in humans and other animals," said John Williams, Head of Neuroscience and Mental Health at the Wellcome Trust. "This ground-breaking work has implications for how we understand the emergence of psychiatric disorders and will offer new avenues for the development of new treatments."
The study was funded by the European Union, the Medical Research Council and the Wellcome Trust.
Image: A human brain. Credit: Heidi Cartwright/Wellcome Images
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