What is the evidence that an asteroid collision has happen before?

The first clue that an asteroid had struck earth at around the same time as the mass extinction was the amount of iridium around the K-T boundary. Another clue was that they found that most animals that lived in the cretaceous died when the asteroid crashed and they found bones of animals from the cretaceous.

i don't know. if i were you, i would search it up online : d

i think its white blood cells

i hope this