By nature, there isn't really - but many species have progressed over the centuries to be able to eat and digest
foods other than what they are "built by nature" to consume.
Bears are a good example of this; despite a mouth full of fangy, pointy meat-tearing teeth, they've adapted over
eons to be able to eat about anything in order to survive.
Pigs and goats are others along that vein; both originally herbivores by nature, they've progressed to being able
to benefit and survive pretty much off anything - despite a lack of incisors or canine-type teeth.
The difference between humans and most of the rest of the animal kingdom is that we, at least in the last
couple thousand years, have come equipped with all the types of teeth naturally, so humans have the history
by tooth morphology (uniquely in the animal kingdom) of being the only critters to be natural omnivores.