Metabolically, human brains are way expensive. They take a lot of energy to run and keeping our sophisticated grey matter going comes at an evolutionary cost.

A new study has found that an immediate trade-off occurs inside us when we have to think fast and work hard at the same time i.e. our 'selfish brain' is always prioritised over the rest of our body.According to the researchers, the findings suggest "preferential allocation of glucose to the brain," which they argue is likely to be an evolved trait -- as prioritising quick thinking over fast moving, for example, may have helped our species survive and thrive.Scientists from the University of Cambridge's PAVE (Phenotypic Adaptability, Variation and Evolution) research group conducted a test on 62 male students drawn from the university's elite rowing crews, with an average age of 21.

The rowers were made to perform two different tasks - one memory, a three minute word recall test, and one physical, a three minute power test on a rowing machine.They then performed both tasks at once, with individual scores compared to those from previous tests. As expected, the challenge of rowing and remembering at the same time reduced both physical and mental performance.However, the researchers significantly found that change in recall was less than the change in power output.Their mental performance fell by an average of 9.7 percent, while power fell by an average of 12.6 percent. Across all participants the drop in physical power was on average 29.8 percent greater than drop in cognitive function.

The researchers revealed the results of their new study add evidence to the 'selfish brain' hypothesis: that the brain has evolved to prioritize its own energy needs over those of peripheral organs, such as skeletal muscle.Lead author, Dr Danny Longman from the PAVE team in Cambridge's Department of Archaeology said, "A well-fuelled brain may have offered us better survival odds than well-fuelled muscles when facing an environmental challenge.”"The development of an enlarged and elaborated brain is considered a defining characteristic of human evolution, but one that has come as a result of trade-offs,” Longman continued. "At the evolutionary level, our brains have arguably cost us decreased investment in muscle as well as a shrunken digestive system.

He also said that human babies also have more stored fat than other mammals, acting as an energy buffer that feeds our high cerebral requirements."On an acute level, we have now demonstrated that when humans simultaneously experience extremes of physical and mental exertion, our internal trade-off preserves cognitive function as the body's priority," said Dr.Longman.Dr. Longman says a limited supply of blood glucose and oxygen means that, when active, skeletal muscle becomes a 'powerful competitor' to the brain.'This is the potential mechanism for the fast-acting trade-off in brain and muscle function we see in just a three minute window', he said.However, the author also points out that a selfish brain comes at a cost.'The selfish nature of the brain has been observed in the unique preservation of brain mass as bodies waste away in people suffering from long-term malnutrition or starvation, as well as in children born with growth restriction.'The study appears in the journal of Scientific Reports