Humans sacrificed brawn for brains

Humans may be smart because they sacrificed muscle growth for increased brain power, a new study has found.
Humans possess more complex, powerful brains than humanity's closest living relatives, such as monkeys and apes.
A reason behind this jump in brainpower may lie in how much of the human metabolism is devoted to the human brain - it consumes 20 per cent of the body's total energy, researchers said.
The study led by researchers from the Chinese Academy of Sciences' Key Laboratory of Computational Biology in Shanghai compared the metabolisms of humans and animals such as chimpanzees, mice and rhesus monkeys and focused on how much energy each species devoted to the brain and body.

They analysed more than 10,000 compounds known as metabolites, which are small molecules formed by, or necessary to, metabolism, such as sugars and fats; the building blocks of proteins, DNA and cell membranes; and chemical signals given off by cells.
They investigated metabolite levels in the kidney, thigh muscle and three brain regions - the primary visual cortex, which is involved in vision; the cerebellar cortex, which helps coordinate muscular activity; and the prefrontal cortex, which plays a major role in complex mental behaviour, decision making and social behaviour.

They compared how much the metabolisms of these animals differed with how far apart these species are evolutionarily.

Scientists found the levels of differences between the metabolisms of these species matched how far apart they were evolutionarily.
The rate of change in the metabolism of the human prefrontal cortex was about four times faster than that of chimpanzees.
Also, the rate of change in the metabolism of human muscle was more than eight times faster that that of the chimpanzee.
Researchers also conducted experiments to see how much stronger chimps and rhesus monkeys are than humans. They conducted muscle strength tests that involved pulling weights upward.
All of the human volunteers in the experiment - who included professional athletes - were outcompeted by their primate opponents by more than twofold.

"According to our results, an average adult chimpanzee is approximately two to three times stronger than an average adult human," study author Philipp Khaitovich told 'Live Science'.
The fact that metabolic changes in human muscle are parallelled by a drastic reduction in muscle strength leads the researchers to hint that human ancestors may have swapped brains for brawn, researchers said.
The findings are published in the journal PLOS Biology.