The session today was focused on encephalization in the middle Pleistocene. This is the time period during which Homo heidelbergensis lived - the putative last common ancestor of both Neanderthals and humans. Encephalization refers to evolutionary increases in brain size, this occurred significantly in primates and to a more extreme extent in the Homo genus. We began the session with a short introduction to the Homo phylogenetic tree and possible placements of Homo heidelbergensis and its relationship to the high ‘encephalization quotient’ we see in Homo lineages. This session the group, in pairs, presented different theories of the factors that led to encephalization around half a million years ago. The theories that were presented were fire use, diet, more complex social networks, and tool use.  We discussed the likelihood of each and whether one particular factor may have preceded the others. For example, perhaps not having an advanced form of communication would preclude the widespread use of fire, or at least the teaching of fire control across generations. Regarding the social nature of early humans, we discussed how adaptive it might have been to be a better ‘politician’. Lastly, we discussed tool use and what a proto-language with ‘context-free’ grammar might have looked like. The format of the session allowed for each group member to contribute meaningfully, however, having targeted discussion questions may help to narrow down the focus of the session on a few topics. It was agreed that a more detailed deeper dive would be more productive than discuss too many ideas at once, especially given the controversial and complex nature of the subject matter.