Final answer:
The increase in Homo erectus brain size is closely linked to the advent of cooking, leading to greater nutrient intake and the evolutionary advantage of a larger brain. The process allowed for softer foods, reducing dental and jaw structures, and coincided with significant climate change, which necessitated smarter brains for survival.
Step-by-step explanation:
The increase in brain size of Homo erectus is most commonly attributed to a dietary shift including the increased use of fire and the eating of cooked meat. Studies have shown that with the advent of cooking, there was an increase in the calorie and nutrient uptake from food, which in turn supported the energy demands of a larger brain. The 'expensive tissue hypothesis' supports this notion, suggesting that the metabolic demands of maintaining a larger brain necessitated a reduction in gut size, which was made possible by a more nutrient-dense diet and cooking, leading to better digestion and more energy available for brain growth.
Cooking also led to softer foods, reducing the need for large teeth and jaws, thereby making more room for brain expansion. The advent of cooked food not only increased calorie intake but also increased the availability of protein - a crucial nutrient for brain development. Additionally, the social and cultural aspects of gathering around a campfire could have stimulated further cognitive development and brain growth. Indeed, brain size increased rapidly during a period of significant climate change, when having a larger and more sophisticated brain would have been advantageous for survival.