Thermal constraints on Middle Pleistocene hominin brain evolution and cognition

High latitude habitats are subject to thermally-driven energetic constraints that make their occupation challenging. This is likely to have had a particularly significant impact on energy-expensive tissue like the brain, especially during periods of lower global temperatures during the Mid-Pleistocene Ice Ages. I analyse data on endocranial volumes for archaic humans (Homo heidelbergensis, H. neanderthalensis and allies) to show (1) that cranial volumes were typically smaller at high latitudes than in the tropics and (2) that they declined during cold phases and increased during warm phases of the Middle Pleistocene Ice Ages. Within this broad pattern, there is a significant uplift in cranial volumes after 400 ka that seems to coincide with widespread presence of hearths at high latitudes, suggesting that hominin populations might have gained at least partial release from this constraint through cultural control over fire. While this might pinpoint the time at which hominins first began to cook on a regular basis, fire offers other important benefits (notably warmth and extending the length of the working day) that might have played an equally important role in buffering populations against thermal stresses. The larger brain sizes that this made possible have implications for social cognitive capacities like mentalising, that in turn have implications for language skills, cultural behaviour and social group size.