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.
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.
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