For the Want of a Chromosome Pair

Slate has an article on how we ended up with 46 chromosomes. Despite the colloquial writing style, there is some good information about how we got one less set of chromosomes than Chimpanzees, our closest relatives. Sam Kean writes:

Around a million years ago, in some fateful man or woman in Africa, what were the 12th and 13th human chromosomes (and still are the 12th and 13th chromosomes in many primates) got entangled at their tips. Instead of separating cleanly, 12 and 13 fused together, like one belt buckled onto another. This amalgam eventually became human chromosome 2.

Fusions like this are not uncommon—one in every 1,000 babies has some sort of chromosomal fusion—and most go unnoticed because they don’t upset anyone’s health. (The ends of chromosomes contain few genes, so often nothing gets disrupted.) However, a fusion by itself can’t explain the drop from 48 to 46. A fusion leaves a person with 47 chromosomes, not 46, and the odds of two identical fusions in the same cell are remote. And even after the drop to 47, the person still has to reproduce to pass on the trait, a serious barrier.

He would have to find a person also with 47 chromosomes and, despite the possibility of many miscarriages, one in thirty-six would inherit both fused chromosomes and have 46. This must have had some sort of selective advantage because that is the form in which we find ourselves today. It has been suggested that there was a genetic bottleneck (Venema also writes about this) and it is possible that this was the point that the 46 variant took over. it is thought-provoking, if speculative. One thing is certain: we have fewer chromosomes than chimpanzees and it is because of a chromosomal fusion. The key here is that we and chimpanzees share a last common ancestor.