It is true that human has 23 chromosomes, in contrast to chimps who have 24 chromosomes,and lack any pair resembling the long human chromosome 2 pair and instead chimps have two pairs of medium sized chromosome, This would be explained by the fact that the common ancestor of both humans and chimpanzees had 24 chromosomes, however at some point in the human lineage, two chromosomes fused end to end, providing some selective advantage and thus humans ended up having 23 chromosomes instead of 23.
