Much like our brains, human muscles have evolved several times more rapidly than primate muscles, according to a new study — but that process has made us weaker over time in a process, while brains become more advanced. In other words, human evolution favored brain over brawn, making us much smarter (but also weaker) than our closest mammalian cousins. 

Courtesy of PLOS Biology

In the study, published yesterday in the scientific journal PLOS Biology, researchers from China's Shanghai’s CAS-MPG Partner Institute for Computational Biology and Germany's Max Planck Institutes explain that "metabolic changes in human muscle seem to be paralleled by a drastic reduction in muscle strength," and that "the observed rapid metabolic changes in brain and muscle, together with the unique human cognitive skills and low muscle performance, might reflect parallel mechanisms in human evolution." 

The scientists did not expect to find such a drastic reduction in muscular strength in our evolutionary history. According to Science Daily, the scientists had been out to explore how our brains evolved compared with primate brains, and were using muscle evolution as a control when they made the realization: 

The metabolome [metabolites in tissue] of the human brain has evolved four times faster than that of the chimpanzee. What was more surprising, however, is that human muscle accumulated an even higher amount of metabolic change -- ten times that of the chimpanzee!

The startling realization prompted the scientists to test what has, until this point, been simply observed and reported anecdotally: that the weakest apes are still stronger than the strongest humans. To check the theory, the scientists subjected some monkeys to a sedentary adult lifestyle — i.e., turned them into couch potatoes to see how their muscles fare. A group of macaque monkeys were transported from sprawling outdoor facilities into smaller indoor ones and fed foods rich in fat and sugar, like most humans eat. But the change hardly altered the metabolome in their muscles. The experiment essentially confirmed that our weak muscles are a result of genetics, rather than the way we lead our lives today. To re-check the finding, scientists conducted another round of experiments. Again, Science Daily explains

Researchers involved several chimpanzees, macaques, university students, and even professional athletes in a pulling strength competition. Despite their sweat and determination, all of the human participants of the experiment were outcompeted by their primate opponents by more than two-fold.

Though the scientists were not able to determine a link between our weak muscles and strong brains, they do suspect that one exists. They write in the report:

We hypothesize that metabolic evolution of human muscle and brain metabolomes may have occurred in parallel. Studies demonstrating a connection between aerobic exercise and cognitive performance in humans of different age indicate that these two organs might be metabolically related. Furthermore, it was also previously suggested that rescaling of energetically expensive organs, such as the gut, allowed the development of a larger brain in human evolution. 

In other words, our body redirected our limited store energy to the brain, at the expense of organs and muscles. That was probably a pretty good trade-off, even if it means being able to do fewer pushups.

Our results indicate that the reallocation of energy to energetically costly human brains may have required further decrease in energy expense in the skeletal muscle, at least during peak performance.

Now we can blame our gym aversions on science.