“Within any (relatively) closed system, that element which has the greatest variety of responses available survives best.”

In evolutionary systems, we can notice that with the development of a new system, the old one generally remains functional with the body, and fades away only slowly if it is entirely unused. An example might be the dual nervous system we humans have in our body. The older (in evolution) nerves fire slow and tell us things like ‘dull ache’. The newer nerve system fires faster and tells us things like ‘sharp pain.’ Both co-exist, because both are useful.

Likewise we have two separately functioning digestive processes, one for a hervibore diet and one for a carnivore diet. They interfere with each other somewhat, but having both let us adapt to available foods back in times when that mattered.