[See]

[Bigger Brains: Complex Brains for a Complex World] "Brain size increases slowly - From 6–2 million years ago - During this time period, early humans began to walk upright and make simple tools. Brain size increased, but only slightly. - Brain and body size increase - From 2 million–800,000 years ago - During this time period early humans spread around the globe, encountering many new environments on different continents. These challenges, along with an increase in body size, led to an increase in brain size. - Brain size increases rapidly - From 800,000–200,000 years ago - Human brain size evolved most rapidly during a time of dramatic climate change. Larger, more complex brains enabled early humans of this time period to interact with each other and with their surroundings in new and different ways. As the environment became more unpredictable, bigger brains helped our ancestors survive."

Although these authors opine that the factor driving brain growth was climate variability, the timing of most rapid brain growth also coincides with the emergence of village life, i.e. gender roles. Whether larger brains caused village life to emerge, or the emergence of village life rewarded brain enlargement, is an open question.

[Q&A: What is human language, when did it evolve and why should we care?] "Combining these genetic hints with the differences in symbolic and cultural behaviour that are evident from the fossil record suggests language arose in our lineage sometime after our split from our common ancestor with Neanderthals, and probably by no later than 150,000 to 200,000 years ago."