Our "Missing" Chromosomes

Maggie Koerth-Baker is a guest blogger on Boing Boing. A freelance science and health journalist, Maggie lives in Minneapolis, brain dumps on Twitter, and writes quite often for mental_floss magazine.

Back in February, I went to the American Association for the Advancement of Science conference in Chicago. The whole thing was pretty much a geeked-out blast, but one of my favorite sessions was this four-hour long symposium, the crux of which can basically be summed up as, "Evolution: It Works, Bitches." This particular tidbit, which I originally heard there in a lecture given by Brown University biology professor Kenneth Miller, is just totally nifty and must be shared.

So here's the thing: We have 46 chromosomes. Our nearest great ape relatives have 48. On the surface, it looks like we must have lost two. But that's actually a huge problem. Made up of organized packs of DNA and proteins, chromosomes don't just up and vanish. In fact, it's doubtful any primate could survive a mutation that simply deleted a pair of chromosomes. That's because chromosomes are to the human body what instruction sheets are to inexpensive, Swedish flat-pack furniture. If you're missing one screw, you can still put that bookcase together pretty easily. But if the how-to guide suddenly jumps from page 1 (take plywood panels out of box--uff da) to page 5 (enjoy bookcäse!), you're likely to end up missing something pretty vital. All this left scientists with a thorny dilemma: How could we have a common ancestor with great apes, but fewer chromosomes?

Turns out: The chromosomes aren't missing at all.
Genetic investigators caught the first whiff of the prodigal chromosomes' scent in 1982. That year, a paper published in the journal Science described a very funny phenomenon. Researchers knew all chromosomes had distinctive signatures; patterns of DNA sequences that can be reliably found in specific spots, including in the center and on the ends. These end-cap sequences are called telomeres. Molecular biologist Elizabeth Blackburn says telomeres are like the little plastic tips that keep your shoelaces from unravelling. They protect the ends of chromosomes and hold things together. Given that important function, you wouldn't expect to find telomeres hanging out on other parts of the chromosome. But that's exactly what the 1982 study reported. Looking at human chromosome 2, the scientists found telomeres snuggled up against the centromere--the central sequence. What's more, these out-of-place human telomeres were strikingly similar to telomeres that can be found, in their proper location, on two great ape chromosomes.

This evidence laid the groundwork for a brilliant discovery. Rather than falling apart, the two missing chromosomes had fused together. Their format changed, but they didn't lose any information, so the mutation wasn't deadly. Instead, scientists now think, the fusion made it difficult for our ancestors to mate with the ancestors of chimpanzees, leading our two species to strike out alone. In the two decades since the original study, more evidence has surfaced backing this up, which leads us to 2005, when the chimpanzee genome was sequenced around the same time that the National Human Genome Research Institute published a detailed survey of human chromosome 2. According to Kenneth Miller, we can now see extra centromeres in chromosome 2 and trace how its genes neatly line up with those on chimpanzee chromosomes 12 and 13. It's a great example of evidence supporting the common descent of man and ape.

I'm currently in the process of putting together a mental_floss story looking at this, and several other awesome experiments revolving around the origins of life. I'm not sure yet when it will be published, but, obviously, I'm really excited about it.

Photo courtesy feverblue