I think the reason why we humans have so varied dietary needs is that we just simply live in so various places. There's practically no local plants for Eskimos to eat, so they do need to eat meat. I've wondered that perhaps they(and a lot of other people too) live in "wrong place"? A human can't survive in such extremes if they don't do something "unnatural" - like clothe themselves, make fire, build houses from concrete and with central heating.
If a lion can't survive in Northern Europe, it doesn't go there. Lions are where they can just live off the Earth. They just hunt where there is food for them. Same with Gazelles and Elephants and other plant eating animals - they live where there is food for them.
Even if humans ate only plants, even then I think there would be vast differences between one and the other. Plant food doesn't just grow for them, they(humans) grow it. Tomatoes wouldn't even grow in Nordic countries, but people build greenhouses(which animals incidentally don't.
). In a way, humans have distanced themselves from Nature. Instead of letting nature nourish them, they use nature to grow food for themselves, by themselves(even if nature didn't really intend it this way). Different people in different places grow different food, hence the need for different diets.
That's all just a pondering. Not even a theory!!