The Human Brain Has been Getting Smaller Since the Stone Age


EXCERPT: I don’t mean to alarm you, but the average human brain size is shrinking. And we can’t blame reality T.V. or twitter. No, this decline began tens of thousands of years ago. It’s something of a well-known secret among anthropologists: Based on measurements of skulls, the average brain volume of Homo sapiens has reportedly decreased by roughly 10 percent in the past 40,000 years. This reduction is a reversal of the trend of cranial expansion, which had been occurring in human evolution for millions of years prior (see chapter 17).

[...] From every region with data, there seems to have been a roughly half cup decrease in endocranial volume that began when the Ice Age gave way to the Holocene, the most recent geological epoch, which is characterized by a comfortable, stable climate. Since this pattern was first noticed in the late 1980s, researchers have proposed a number of possible explanations.

Some say the decrease came from from a slight reduction in body size and robustness, related to the warmer conditions of the Holocene. Bigger bodies were better during the Ice Age, and then became disadvantageous as the climate warmed. But anthropologist John Hawks has countered this idea by showing that the documented brain reduction is too great to be explained by simply having slightly smaller bodies. Other researchers point to the fact that brains are energetically costly organs. Though the modern human brain is only 2 percent of our body weight, it consumes almost one quarter our energy input. By inventing ways to store information externally — cave art, writing, digital media — humans were able to shed some brain bulk, according to one proposal.

But perhaps the most convincing hypothesis is that Homo sapiens underwent self-domestication, a proposal that stems from our understanding of animal domestication. [...] The self-domestication hypothesis — or what anthropologist Brian Hare called “survival of the friendliest” — suggests we also did this to ourselves. The idea is, within Stone Age societies, cooperative, level-headed individuals were more likely to survive and reproduce than combative, aggressive ones. Those pro- or anti-social inclinations were influenced by genes regulating hormones, which also affected physical traits, including body and brain size. Overtime, “survival of the friendliest” led to humans with slighter builds and brains on average. (MORE - details)

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