Not to imply that I necessarily challenge any of what you said, but when I see ‘the only explanation’, my antennae go up. (‘Most likely explanation’ is another matter altogether, but that’s not what you said.)
You may have an answer for this – I haven’t read the book – but I immediately thought that the same damage could occur in both species because they were both subject to the same forces that caused the damage. Unless you KNOW what all those forces were, how can you say this isn’t a logically possible – however unlikely – explanation?
Let me see if I can illustrate the scenario more clearly.
First, some background: Humans have 23 chromosomes whereas chimps have 24 – the reason for this is that after humans split off from chimps, two of the human chromosomes merged into one. We know this because human chromosome 2 has two “centers”, one of which isn’t active, and has in its middle the telltale signs of an end-to-end fusion. So, human chromosome 2 corresponds to two chimp chromosomes, and the rest of the human chromosomes pair up on a 1-to-1 basis with the rest of the chimp chromosomes.
Now, I don’t have the book handy, but my recollection is that the scenario goes something like this: There is a particular gene which has about 18 copies of itself located throughout the human genome. One of them is a “tandem duplicate” that lies right next to the original. The rest are “retro duplicates” that are located on different chromosomes – these particular duplicates are a kind form from the original and can then float during cell division to anywhere in the human genome. A particular one of these “retro duplicates” is incomplete – something went wrong during either the copying or insertion process.
We know that the duplicates are indeed duplicates and are not more originals because when a gene duplicates there is a “signature” that gets attached to the duplicate gene in the process. So what we have in the human genome is a single original gene that, over time, generated 18 copies of itself – 1 “tandem” and 17 “retro”, and of the 17 “retro” copies, the one that landed on, say, chromosome 12 got broken.
Now, “tandem duplicates” usually end up alongside their originals, so the fact that both the chimp and human genomes each contain the original with a “tandem duplicate” beside it is really nothing all that special. “Retro duplicates”, however, once they are made can land on any chromosome in any place on that chromosome. Let’s say that each chromosome has 1000 places where a “retro duplicate” could land (and that’s probably a ridiculously conservative estimate), the odds – and I had to use teacher-allowed “cheat sheets” in my college statistics class, so don’t be surprised if I get this wrong – of a “retro duplicate’s” landing in a specific point on a specific chromosome is 1/24000. Therefore, the odds of a “retro” copy landing in the same place on the same chromosome in two totally unrelated genomes is (1/24000)^2 = 1/576000000. The odds of this happening 18 times in two unrelated genomes already breaks my home calculator, but even if we accept this extremely remote possibility as having happened by chance, we still have the broken “retro” copy, which is broken in the same way in both genomes, to bump our odds even further into the realm of impossibility.
The alternative is much cleaner: All these duplicates occurred in an ancestor species common to human and chimps, and after all these copies were created, then the ancestor species split into the human and chimp lineages – each lineage carrying its own “inheritance” of copies.
–Mike
