This is the last question on my homework that is due tomorrow morning. Please help asap. What changes in human evolution enabled the genus Homo to ‘pay’ the cost (in calories) for a big brain?

It appears that major expansion of brain size in the human lineage is the product of synergistic-ally interacting dietary/nutritional and social forces. Although dietary change was not being the sole force responsible for the evolution of large brain size, the exploitation of high-quality foods likely fueled the energetic costs of larger brains and necessitated more complex behaviors that would have selected for greater brain size.