It would be pretty hard for earliest humans to live and survive off of just non-meat foods. The only food they could get in this scenario would be through gathering (Neolithic Revolution had not happened yet).
Of course it is a common fact know that the earliest humans, like ourselves today, also ate meat (Man has been hunting since the dawn of our species). They wouldn't be called "hunter-gathering" societies if they never hunted.
I don't see why they wouldn't eat meat when even the earliest humans knew that said food can help to keep themselves and their community alive, and they knew their body allowed for it.
It's just common sense.
Especially if they were living in areas ripe with animal life. It would be stupid to ignore that resource. After all, apart of the reason animals came into being on this planet is to be one of the food sources for us.