Scientists challenged 62 crew members who were asked to perform a three-minute word recall memory task and demonstrate their muscle power on a rowing machine.
When both tasks were repeated at the same time, it was the rowers' physical performance that suffered most.
While multi-tasking, their energy resources were mostly directed towards their brains.
The findings support the "selfish brain" theory that says humans survived because they prioritising thinking ability over brute force.
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Lead researcher Dr Danny Longman, from the university'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.
"At the evolutionary level, our brains have arguably cost us decreased investment in muscle as well as a shrunken digestive system."
The energy-hungry human brain competes with muscles for a limited supply of blood glucose and oxygen, Dr Longman said.
During the multi-tasking test, the drop in physical power was on average 29.8 per cent greater than the reduction in memory function across all participants, said the researchers writing in the journal Scientific Reports.
