did. It's reasonable to conclude that our food had something to do with our increased lifespan.

Sanitation/solid waste disposal probably has the biggest impact in human lifespan. Followed by antibiotics and other medical advances.
Grains are certainly the most efficient way to feed a large population. Most of human history would have loved to have an endless supply of cheap calories. Although aside from being able to easily fulfill the caloric intake required for survival, most grains are relatively low in nutritional value.

You forgot it was STOLEN?!