Human evolution led to greater use of brains over brawn, but this adaptation may be more complex than previously believed.
The new study involved researchers from the CAS-MPG Partner Institute for Computational Biology in Shanghai and the Max Planck Institutes in Germany. Investigators studied metabolites, which encompasses amino acids, vitamins and sugars, among other chemicals essential for metabolism.
Animals including the rhesus macaque, chimpanzees and mice were studied, and metabolite use was recorded. Researchers wanted to know how these chemicals were utilized by other species, and how energy was divided between mental and physical functions.
The team discovered metabolite concentrations dramatically increased in early humans, feeding both brains and muscles. Most researchers believed increased levels would be found in brains, but muscle concentrations were higher than expected.
Chimpanzees and humans split off from each others genetic lines 6 million years ago. Rhesus macaque monkeys divided from our genetic line 25 million years in the past, 50 million years after our distant ancestors parted ways with rodents. Investigators found a correlation between these dates and metabolite usage. Those species more closely related to humans also possess similar metabolic systems.
In the frontal cortex of human brains -- the region directing higher functions like math, science and art -- metabolite usage rose four times faster than in chimpanzees. Strangely, concentrations in our muscles also changed, by eight times, lowering muscle mass.
Human brains contain around 100 billion neurons -- that is 14 for every person on Earth. Each of these cells can connect with up to 10,000 of its neighbors, creating one million billion connections, called synapses. This highly complex organ consumes 20 percent of the total energy used by the body.
In addition to research on the evolutionary development of humans, investigators also conducted strength tests, comparing our species and two other primates. Both humans and other animals were measured during weight lifting trials.
"The results suggest that, while humans are characterized by superior cognition, their muscular performance might be markedly inferior to that of chimpanzees and macaque monkeys," researchers wrote in the article detailing their study.
Investigators also subjected chimpanzees to two months of a sedentary lifestyle, poor food and high stress to test how they reacted to a "couch potato" lifestyle. The animals were little changed by the experience.
Humans are significantly weaker than our chimpanzee cousins, who can lift twice the weight of a professional athlete. Researchers believe as humans evolved, brain power was favored over physical strength.
"It is a very simple explanation, and it could be completely wrong," Philipp Khaitovich of the Chinese Academy of Science, told Live Science.
Study of the relationship between neural and physical development was detailed in the online journal Plos Biology.
