WARNING: SCIENCE CONTENT
Now, we all know about chromosomes: they're the things in our cells which hold our genes. Now, all the great apes have 24 pairs of chomosomes. This is fair enough, but (according to evolution) we are just as closely related to the great apes as they are to each other.
So we should also have 24 pairs of chomosomes, correct?
Now, this isn't a massive deal: chromosome counts differ greatly within genus, and sometimes even within species. The plant genus Clarkia, for example, has species with chromosome counts of n = 5, 6, 7, 8, 9, 12, 14, 17, 18, and 26 (Lewis 1993) (referenced from TalkOrigins). But chromosomes don't just disappear: when they do, the victim of the mutation is sterile at best, stillborn at worst. So what happened to the 24th human chomosome?
Upon discovering this, a prediction was made: at some point in our ancestory, two chromosomes must have joined. This isn't a harmful mutation: the gene's themselves are still viable, but they are now part of a single chromosome, not two.
Now, chromosomes have easily recognisable sequences at the ends, called telomeres, and another recognisable type of sequence in the centre called centromeres (0). Centromere's are where the chromosome divides during reproduction.
So, geneticists realised that if humans really are decended from apes, one of their chromosomes should have two (X's) in the middle of it, with a pair of (0's) flanking them. Keep in mind that if this isn't found, common ancestory is in big trouble: falsified, even.
-- Kenneth Miller