It’s been noted by many anthropologists that the human brain size is shrinking. Or at least that it is actually smaller than our hominid ancestors. And there is no scientific consensus about why that is.
Over the past 20,000 years, the average volume of the human male brain has shrunk from 1,500 cubic centimeters to 1,350 cubic centimeters. That is a loss of 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.
If the human brain keeps shrinking at this rate over the next 20,000 years, it will approach the size of that found in Homo erectus, an early human ancestor with a brain volume of only 1,100 cubic centimeters. And anthropologists and geneticists aren’t exactly sure why.
One theory says that a large factor human encephalization is from having a large musculature. And that the Cro-Magnon ancestors had much more muscles than modern humans did–they lived in a rough and tumble world. Which is why, 20,000-30,000 years ago, the Cro-Magnons also had brain sizes larger than modern humans. That doesn’t necessarily mean that Cro-Magnons were smarter. Now, we know that the evolution of the pre-frontal cortex is strongly responsible for what we often think of as “intelligence”.
In addition, some genetic studies have suggested that even as the brain was shrinking in size, the brain developed more interesting structures and neurotransmitter systems that made the brain “more efficient”. That’s one theory.
Others, have suggested a less palatable “Idiocracy” theory, named after the 2006 Mike Judge film. The suggestion is that as humans have developed agriculture and become sedentary, they have somewhat “domesticated” themselves. As populations grew larger, brain sizes grew smaller–as more activities were offloaded 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, it may have caused a decline in brain volume for modern humans! Whether this is good, bad, ideal, or not is hard to say. But maybe that’s because I have a smaller, modern human brain.
What we can say is that it’s wrong to think that modern humans have had the largest brains. Why? So far, we’re all Not Sure.