Almost all human beings have 46 chromosomes (23 pairs) and being born with an extra or missing one usually signifies that the person will have serious medical problems such as Down syndrome.

On the other hand, our close relatives the chimpanzees have 48 chromosomes (24 pairs). The chimps and us shared a common ancestor about 6-8 million years ago. So how did we end up with fewer? This is because about a million years ago, two of the 24 chromosomes in a human fused together end-to-end to form a single longer chromosome. Since the crucial genetic information in each chromosome was preserved by this fusion process, the organism could survive. The evidence suggests that it was chromosomes #12 and #13 that fused to form the present chromosome #2.

The interesting question is how that mutation might have occurred and why it took hold in the human population so that 46 chromosomes is now the standard.

In this fascinating article (sent to me by reader Fu DaYi), Barry Starr of Stanford University describes a recent discovery in China of a man who seems to have undergone a similar reduction process with chromosomes #14 and #15 becoming fused, and now has just 44 chromosomes (22 pairs). His case sheds light on how the chromosome reduction process might have occurred in our own ancestors.