Large brains are energetically costly. In theory, large brains can pay their overhead costs in spatially and temporally variable environments by using them to adapt to variation using learning and social learning. Social learning (culture) is adaptive over a range of time scales ranging from a fraction of a generation to several tens of generations. At sufficiently long time scales, genetic evolution can track variation well enough and at sufficiently short ones social information is mostly outdated and individual learning is more adaptive than social learning. We have known for some time that the last glacial was characterized by a huge amount of variation at millennial and submillenial time scales. A few high resolution cores going back as far as 8 glacial cycles now exist. They suggest that the amount of millennial and submillennial variation increased during each successive glacial. Brain size and cultural complexity have increased in parallel.

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