Our genes are arrayed along 23 pairs of chromosomes. On rare occasion, a mutation can change their order. If we picture the genes on a chromosome as

ABCDEFGHIJKLMNOPQRSTUVWXYZ

a mutation might flip a segment of the chromosome, so that it now reads

ABCDEFGHISRQPONMLKJTUVWXYZ

or it might move one segment somewhere else like this:

ABCDLMNOPQRSTUEFGHIJKVWXYZ

In some cases, these changes can spread into the genome of an entire species, and be passed down to its descendant species. By comparing the genomes of other mammals to our own, scientists have discovered how the order of our genes has been shuffled over the past 100 million years. In tomorrow's New York Times I have an article on some of the latest research on this puzzle, focusing mainly on two recent papers you can read here and here.

One of the most interesting features of our chromosomes, which I mention briefly in the article, is that we're one pair short. In other words, we humans have 23 pairs of chromosomes, while other apes have 24. Creationists bring this discrepancy up a lot. They claim that it represents a fatal blow to evolution. Here's one account, from Apologetics Press:

If the blueprint of DNA locked inside the chromosomes codes for only 46 chromosomes, then how can evolution account for the loss of two entire chromosomes? The task of DNA is to continually reproduce itself. If we infer that this change in chromosome number occurred through evolution, then we are asserting that the DNA locked in the original number of chromosomes did not do its job correctly or efficiently. Considering that each chromosome carries a number of genes, losing chromosomes does not make sense physiologically, and probably would prove deadly for new species. No respectable biologist would suggest that by removing one (or more) chromosomes, a new species likely would be produced. To remove even one chromosome would potentially remove the DNA codes for millions of vital body factors. Eldon Gardner summed it up as follows: “Chromosome number is probably more constant, however, than any other single morphological characteristic that is available for species identification” (1968, p. 211). To put it another way, humans always have had 46 chromosomes, whereas chimps always have had 48.

There's a lot that's wrong here, and it can be summed up up with one number: 1968.

Why would someone quote from a 37-year-old genetics textbook in an article about the science of chromosomes? It's not as if scientists have been just sitting around their labs since then with their feet up on the benches. They've been working pretty hard, and they've learned a lot. And what they've learned doesn't agree with what Apologetics Press wants to claim.

The first big discovery came in 1982, when scientists looked at the patterns of bands on human and ape chromosomes. Chromosomes have a distinctive structure in their middle, called a centromere, and their tips are called telomeres. The scientists reported that the banding pattern surrounding the centromere on human chromosome 2 bore a striking resemblance to the telomeres at the ends of two separate chromosomes in chimpanzees and gorillas. They proposed that in the hominid lineage, the ancestral forms of those two chromosomes had fused together to produce one chromosome. The chromosomes weren't lost, just combined.

Other researchers followed up on this hypothesis with experiments of their own. In 1991, a team of scientists managed to sequence the genetic material in a small portion of the centromere region of chromosome 2. They found a distinctive stretches of DNA that is common in telomeres, supporting the fusion hypothesis. Since then, scientists have been able to study the chromosome in far more detail, and everything they've found supports the idea that the chromosomes fused. In this 2002 paper, for example, scientists at the Fred Hutchinson Cancer Research Center reported discovering duplicates of DNA from around the fusion site in other chromosomes. Millions of years before chromosome 2 was born, portions of the ancestral chromosomes were accidentally duplicated and then relocated to other places in the genome of our ancestors. And this past April, scientists published the entire sequence of chromosome 2 and were able to pinpoint the vestiges of the centromeres of the ancestral chromosomes--which are similar, as predicted, to the centromeres of the corresponding chromosomes in chimpanzees.

Today geneticists sometimes encounter people with fused chromosomes, which are often associated with serious disorders like Downs syndrome. But that doesn't mean that every fusion is harmful. Many perfectly healthy populations of house mice, for example, can be distinguished from other house mice by fused chromosomes. The fusion of chromosome 2 millions of years ago may not have caused any big change in hominid biology--except, perhaps, by making it difficult for populations of hominids with 23 pairs of chromosomes to mate with populations who still had 24. As a result, it may have helped produce a new species of hominid that would give rise to our own.

Just goes to show what 37 years of scientific research can turn up.

Update: Tuesday, 3:30: Thanks to Dr. Paul Havlak for pointing out that some people with fused chromosomes suffer no ill effects. This site at the University of Utah has more information.