The human brain may have hit its limit because of a lack of energy and oxygen required for mankind to become any smarter, research indicates.
The Daily Mail reported research by Cambridge University on the structure of brain and how much energy its cells consume.
The neurobiologists said brain capacity is critically linked to powers of deduction, demanding energy to constantly correlate information from different sources.
But it is physically impossible for humans to provide extra amounts of energy and oxygen, meaning there is a limit to human intelligence, the researchers suggest.
The human brain's capacity depends on speedy interlinks among different parts of brain, referred to as "wiring."
"You pay a price for intelligence. Becoming smarter means improving connections between different brain areas but this runs into tight limits on energy, along with space for the wiring," said Ed Bullmore, a professor of psychiatry at Cambridge.
As increases in brain power require disproportionate increases in energy consumption, human intelligence faces tight constraints, the study said.