Many anthropologists think the human brain size is shrinking. Or at least that they are now smaller than our hominid ancestors. Scientists have no consensus for why that is.
Over the past 20,000 years, the average volume of the human male brain shrank from 1,500 cubic centimeters to 1,350 cubic centimeters. Our brains lost the volume about the size of a tennis ball. And, the female brain (which is slightly smaller than the male brain) has shrunk by about the same proportion. And it’s happened worldwide: Europe, Asia, Africa. Brains shrank everywhere.
If human brains shrinking continues at this rate over the next 20,000 years, it will approach the size of that found in Homo erectus. Home erectus sported a brain volume of about 1,100 cubic centimeters. And anthropologists and geneticists aren’t exactly sure why this reversal trend continues.
Brains Were Originally Made to Control Bodies
One theory says that a large factor for human brain growth is from having a larger musculature: lots of brain space controls the physical body. And that the Cro-Magnon ancestors had much more muscles than modern humans did–they lived in a rough and tumble world. So, the thinking goes, 20,000-30,000 years ago, the Cro-Magnons also needed brain sizes larger than modern humans.
But that doesn’t necessarily mean that Cro-Magnons were smarter. Today, we know that the evolution of the pre-frontal cortex controls what we often think of as “intelligence”. In addition, some genetic studies suggest that even as the brain shrank in size, the human brain developed more interesting structures and neurotransmitter systems that made the brain “more efficient”.
Idiocracy theory>: Self-domestication/h3>
Others have suggested a less palatable “Idiocracy” theory, named after the 2006 Mike Judge film. This cynical theory states that humans have developed agriculture and become sedentary, so they have somewhat “domesticated” themselves.
As populations grew larger, brain sizes grew smaller–as more activities were off-loaded to the human collective. Likewise, as humans became more sedentary, aggressive qualities in human beings may have caused a corresponding decrease in grey matter. This is because the simplest way for natural selection to affect aggression is to select for people whose brains develop more slowly.
And, through ordinary evolutionary pressures, brains became smaller for modern humans! Whether good, bad, ideal, or not? We don’t know. But maybe that’s because I carry around a smaller, modern human brain.
We can say modern humans do not possess the largest brains. Why? So far, we’re all “Not Sure”.