Similar to our primate cousins, we humans have unusually large and evolutionarily costly brains which consume as much as 20% of energy while accounting for only 4% of total body mass.
While there are many different explanations for why that is, most researchers today lean towards the social environment as the critical factor – the necessity of navigating increasingly complex social structures is thought to have led to hypertrophy of cerebral tissue.
Another popular hypothesis argues that it’s actually the environment that’s behind the recent explosion in brain size. Learning to hunt and gather food in a seasonally changing savannah, storing it to be eaten later and other similar problems are difficult nuts to crack for a species lacking sophisticated modern technology.
Typically, the ecological hypothesis is tested by correlating brain size to specific problems – e.g., do primates and other animals with large brains have a diet that is challenging to find but nutritionally rewarding?
Even though many studies have, indeed, found evidence for such correlation, they often fail to provide a means of disentangling cause from effect. In other words, we don’t know whether sizeable brains evolved to solve difficult problems or whether they evolved for different reasons and only later enabled their owners to tackled hard problems.
In a new study out in the journal Nature, researchers from the University of St. Andrew’s in the UK took the “dry” mathematical approach by calculating the amount of energy an individual should use for growing her brain as function of the balance between energy costs and potential advantages for problem-solving.
“By varying the amount of ecological or social challenges faced by the individuals, we could work out how large the brain could evolve to be under such different conditions,“ wrote the researchers in a press release.
Interestingly, social challenges were found to decrease brain size, potentially because more intense cooperation takes the pressure off any one specific individual by distributing problem-solving across the group.
Hard ecological circumstances and the ability to learn new complex skills from other fellow humans throughout the lifespan — with the latter explaining why other animals lack the kind of brains humans have — were found to strongly correlate to larger brain sizes.