All apes are omnivorous. Chimps will eat monkey and even other chimps. Gibbons and ourangs will eat birds. The gorilla is the most nearly vegetarian of the apes, but even they regularly eat insects. It would be truly astounding is humans were the only ape that didn’t eat meat.
As foolsguinea said, human can’t synthesis some of the B vitamins. Modern vegetarians get these from cultured micro-organisms in pill form. Without such supplements humans will die. If we assume that people 100, 000 years ago didn’t have access to modern pharmaceuticals then that fact alone makes nonsense of any claim that people are naturally vegetarian. Of course a vegetarian can get B vitamins form milk, but the act of squeezing milk form the glands of a domestic cow is about as unnatural as I can imagine.
Then we have the problem of explaining all those stone spears that we find. If people are ‘naturally’ vegetarian then what exactly were they doing using thousands of spear heads. Are we to believe that people are naturally murderous, but unwilling to kill animals? Perhaps we are expected to believe that people made huge quantities of spears to protect themselves from animal attack, but that they never thought to eat the animals they had already killed.
Then we need to explain how exactly all those animal bines got into prehistoric middens. Are we expected to believe that people were collecting animal carcasses and burying them ceremonially?
The whole idea is just silly. One of the bigger problems is how we define natural. All extant or recently extinct hunter gatherer cultures are highly omnivorous. Surely that is natural, or are we to believe they too are corrupted descendants of Cain? If so then what is natural. Neanderthals ate meat. Homo erectus ate meat. Even H. habilis ate meat. What is this person’s standard for defining what is natural? When and where did this natural population exist? The only known vegetarian populations belong to agricultural societies, and agricultural by definition is unnatural.