The Mystery of the Missing Chromosome


There’s something fascinating about our chromosomes. We have 23 pairs. Chimpanzees and gorillas, our closest living relatives, have 24. If you come to these facts cold, you might think this represented an existential crisis for evolutionary biologists. If we do indeed descend from a common ancestor with great apes, then our ancestors must have lost a pair after our lineage branched off, some six million years ago. How on Earth could we just give up an entire chromosome.

A close look at our genome and the genome of our close relatives reveals that we didn’t. We just combined a couple of them. Every now and then, chromosomes fuse. This fusion occurs as sperm and eggs develop, as pairs of chromosomes fold over each other and swap chunks of DNA. Sometimes two different chromosomes grab onto each other and then fail to separate.

Scientists have observed both humans and mammals with fused chromosomes. Chromosomes typically have distinctive stretches of DNA in their center and at their ends. From time to time, scientists will find an individual that’s short a chromosome, but one of the chromosomes it retains now has an odd structure, with chromosome endings near the middle and other peculiar features.

This might seem like a fantastic mutation–something like a human and a horse being joined into a centaur. Remarkably, however, fused chromosomes are real, and there are surprising number of normal, healthy people carrying them.

If humans and apes did indeed share a common ancestor, then it would make sense that two chromosomes fused in our ancestors. The rise of genome sequencing allowed them to test that hypothesis. They found that human chromosome two bears the hallmarks of an ancient chromosome fusion, with remnants of chromosome ends nestled at its core. In 2005, it became possible to test the hypothesis again, when a team of scientists sequenced the chimpanzee genome and could compare it to the human genome. The chimp genome team were able to match human chromosome two to two unfused chromosomes in the chimpanzee genome.

Ken Miller, a biologist at Brown who was an expert witness in the 2005 Dover creationism trial, includes this research in his lectures on evolution. Here’s a video of one of those lectures, where he lays out some of the evidence with impressive clarity.

What makes evolutionary biology such a fun subject to write about is that it does not stay still. While Miller’s description is entirely accurate, the past five years have rendered it obsolete. Last month, Evan Eichler at the University of Washington and his colleagues published a study in the journal Genome Research in which they deeper into the history of our missing chromosome.

They were able to do so thanks to the publication earlier this year of the gorilla genome. A comparison of the human, chimpanzee, and gorilla genomes confirms that the ancestors of gorillas branched off from the ancestors of chimpanzees and humans about ten million years ago. Humans and chimpanzees then branched apart later. A comparison between all three species provides a clearer picture of what our chromosomes looked like before they fused, and how they’ve changed since.

Written By: Carl Zimmer
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  1. “What makes evolutionary biology such a fun subject to write about is that it does not stay still. While Miller’s description is entirely accurate, the past five years have rendered it obsolete”

    A fine example of why science is the best path to truth. It’s dynamic nature follows the evidence and not the presuppositional dogma of, hmmm, lets say religion.

  2. Some of these ID/ Creationist guys are apparently highly intelligent, educated people (as seen from my perspective). So, what do they get from pursuing a cause that becomes more futile with every passing day- can the Faith virus really be that powerful or are there big bucks in pseudo science?

    O/T- just read Deepak Chopra’s review of ‘The Magic of Reality’. He comes across as a spiteful, small minded, ignorant bigot, using many well known logical fallacies. Again, why does he bother?

  3. $20.00 to read that article updating the talk Miller always gives on this subject.


  4. Another thing that fits in the theory of evolution.I find it funny how aways science exceed our imagination in many ways.There is so much to be discovered and many wonders awaits humanity, its sad that ID or creationism(its the same thing really) slows our progress and wants to drive us back into the dark ages.Its funny how they use logic when it come to spreading their supertition – ID>Creationism>Religion.They dont have any intentions of contributing to science.

  5. Creationism is an industry. They not only run profitable churches and miseducation centers (they call them “museum”), they also have media publishing houses that produce homeschooling material and so on.
    Ray Comfort knows less about science, history, philosophy and numerous other subjects than an average high school student, and he makes more money than a college professor. And Comfort is small fry in the evangelical market.But you asked about the more educated guys. Well, you can get just about anything published in creationism as long as it contains enough Bible verse quotes. Easy to make up, none of it has to be testable. Andrew Snelling, the geologist, actually publishes mutually contradicting articles in credible science journals and creationist literature at the same time.Some people just have no scruples to lie for money.And then there’s guys like Kurt Wise, who know and have publically admitted that there’s no scientific support for creationism, but still stick to it and give up doing science because they can’t shake the religious brainwashing. Emotional blackmail for decades will do that to a brain.

  6. Zimmer:

    There’s something fascinating about our chromosomes.

    Boom.  Just like that.  Using simple understatement, the master science writer says “Ladies and germs, your attention right here, please.”

    Awesome.  Yep, that’s how it’s done, folks.

    (Five bucks says parasite-expert Carl Zimmer has used that Milton Berle line before.)

  7. Klinghoffer: 

    But how about gathering his thoughts after reading the book and then telling us what’s wrong with Science and Human Origins? No, that he will not do.

    Hey Reverend Klinghuckster:  why don’t you quit your insipid mewling and just point us to some evidence?
    Jesus what a retard.  David Klinghoffer has to be on the list of Top Five scientific morons.

  8. This is something the discovery institute either still doesn’t understand, or continues to deny while knowing is true

  9. Neodarwinian: 

    My sole reason for posting was to mention the Kenneth Miller lecture and the errant telomere.

  10. Discovery Institute’s David Klinghoffer arising from a discussion on a
    Facebook page: His issue here was this paragraph: But the idea of such
    an event having occurred at all is itself far from sure

    ANYTHING David Klinghoffer says about science, is likely to be so far from sure that it it could power an infinite improbability drive! (see t HHG-to t- G)

    Zimmer, well-known science writer, has been battling with Discovery Institute  fellow
    David Klinghoffer over the past few days over the evidence for a
    chromosome fusion

    Battling??  Really???   It sounds like a competitive race between a Soyuz supply rocket to the ISS and a joining of hands to perform a levitation at seance!

    Congratulations to CZ on another excellent article and patience in answering comments on the link.

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