“There is evidence that large fauna was abundant when early humans evolved," anthropologist Miki Ben-Dor, who recently published a study in Yearbook of Physical Anthropology, told SYFY WIRE. "At some stage very large species were replaced by smaller (though still large) species with a decline in the richness of species. I believe that humans' preference for large prey was responsible for their demise.” There are other primates which are technically omnivores but spend most of their days hanging from trees and feeding. For humans, that was too much effort for too little energy, so they hunted. Ben-Dor and Barkai argue that brain size actually began to decline near the end of the Pleistocene as people started eating more plants and let some of the meat they had previously been after run away from their spears. They also believe that human brain size was at its height when when our own many-times-great-grandparents and other Homo species were consuming things that had more fuel—but they had to eat a few mammoths first.