Evidence on why the change in chromosome 21 between chimps and humans happened after the human line split from

the ape line.

Sorry I can't provide picture of the two chromosomes but basically the chimp and human chromosome 21 are identical in banding pattern but the chimps splits into two chromosomes.

I think you are referring to the human chromosome 2 (not 21). Sequencing, not just banding, showed it is a fusion of two chromosomes, now designated 2A and 2B, found in chimps, gorillas and orangutans. The conservation among the other great apes indicates that this the fusion is an event distinct to the human line.