The story goes that the other great apes, gorillas, chimps and orangutans, possess 24 pairs of chromosomes. Humans possess only 23. This situation suggests that, if we share a common ancestor, humans probably lost one pair instead of the other guys all gaining one. When the human and chimp genomes were sequenced, scientists were able to poke around and see if the genes on the extra ape chromosomes appeared somewhere in the human DNA.
Well, it turns out that they do indeed exist. It is quite clear that two pairs of chromosomes merged sometime after humans and chimps diverged from their common ancestor. You see, not only did they find the gene sequences from the missing chromosome but they found the original telemeres (bits of DNA that form a sort of "cap" on the ends of chromosomes) right in the middle of the human chromosome 2. Plus, they found the extra centromere (a bit of DNA at the center of the chromosome), deactivated, sitting exactly where it was leftover from the extra chimp chromosome.
This way cool. If God did it, he was very messy, leaving extra bits in their like some sort of appendix - but that's another story.
Check out this video explanation