Brain over brawn for 'weedy' humans
Humans evolved to be weedy in the energy battle between brains and brawn, research suggests.
A study found that over the course of evolution, essential molecules had changed even more rapidly in human muscle than in the brain.
The rate of change in muscle was 10 times that seen in our closest animal cousin, the chimpanzee.
In contrast, the molecules in the human brain had evolved four times faster.
The scientists believe human muscle needed to alter drastically to free up energy for the brain, becoming puny as a result.
Lead researcher Dr Kasia Bozek, from the CAS-MPG Partner Institute for Computational Biology in Shanghai, said: "Our results suggest a special energy management in humans that allows us to spare energy for our extraordinary cognitive powers at a cost of weak muscle."
The scientists focused on the evolution of metabolites - molecules such as sugars, vitamins, amino acids and neurotransmitter nerve signalling chemicals with key roles in the way the body functions.
"Metabolites are more dynamic than the genome (genetic code) and they can give us more information about what makes us human," said co-author Dr Philipp Kaitovich, also from the Shanghai team.
"It is also commonly known that the human brain consumes way more energy than the brains of other species; we were curious to see which metabolic processes this involves."
The researchers carried out tests on macaque monkeys to show that the muscle changes seen were not merely the result of human lifestyle.
Simulating human living by feeding the monkeys fatty and sugary food for several weeks and keeping them in small indoor enclosures only had a small effect on their muscle "metabolome", the collection of metabolites in muscle tissue.
The weakness of human muscle was demonstrated with a weight-pulling contest which showed that chimps and macaques had more than twice the strength of even professional athletes.
This was despite all of the animals being kept in captivity and not receiving any training.
"For a long time we were confused by metabolic changes in human muscle, until we realised that what other primates have in common, in contrast to humans, is their enormous muscle strength," said Dr Bozek.
The results are published in the online journal Public Library of Science Biology.
Dr Patrick Giavalisco, who led the metabolome measurement work at the Max Planck Institute for Molecular Plant Physiology in Golm, Germany, said: "The world of human metabolomics is just starting to open up its secrets to us.
"Such human-specific metabolic features we find could be related not only to physical or cognitive performance but also to common human metabolic diseases."