What likely happened is, since humans started living on the ground after they left trees, they had enough sunlight exposure; so we evolved sunlight to be a star

What likely happened is, since humans started living on the ground after they left trees, they had enough sunlight exposure; so we evolved sunlight to be a start marker for some important metabolic activities, purely by randomness. It just so happens that too much sunlight exposure causes cancer. But since cancer happens later in life, it wasn't selected against and we just ended up with this weird biochemistry: UVB is necessary for health but it also causes cancer.