I don’t know enough about evolution or (l’havdil) the Torah views on evolution to know if it’s complete kefira to believe in any aspect of evolution.
But in terms of the above comments referring to the concept of humans and animals sharing a common ancestor (which I believe is what most people are referring to when they talk about evolution being kefira): The problem with this theory is that humans and animals are two inherently different beings. Humans have a Neshama and are created b’tzelem Elokim. We have intelligence and bechira which animals don’t have. So a human could never have been an animal! Adam Harishon might have looked different than people do today, but he had a neshama from the time He was created and G-d gave him life. And it is part of our beliefs that all humans descend from Adam Harishon.