Imagine a Martian is setting up a human zoo exhibit and they want to figure out what is the best diet for humans. They could ask us directly what we like to eat but after hearing our requests for donuts and pizza they might be a little skeptical. Next they might turn to what nutritionists have to say but then they would find many warring, political factions not able to agree. So the logical thing would be to look at what has worked for us as a species. What has worked for us historically?