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I found an explanation here, which is basically saying that sometimes the centromeres are duplicated, breaking a chromosome into two halves. But how could this propagate? I don't think inbreeding is efficient enough for this mutation to become dominant, and it should take a lot of pregnancies for the mutants and the regular ones to produce an healthy offspring. Is the explanation in the link backed by any evidence? Thank you!

By the way, do we know how many chromosomes did the latest common ancestor of human and chimpanzee have, and when did chimpanzee gain the extra pair of chromosomes (or when did we lose it)?

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    $\begingroup$ We cannot have direct evidence as the DNA of common ancestors have either not been found or are degraded to a point where it cannot be analyzed. However when looking at modern genomes we can do syntenic analysis and look at the synteny between chromosomes. Whether you believe it or not, the evidence is within our genome and many other genomes. All you need for it to take hold is an isolated population that is put under selective pressure. $\endgroup$ – AMR Oct 23 '15 at 9:41
  • $\begingroup$ @arax There are two different questions in your post. A post should always be limited to one question. I suggest that you get rid of the second one. You may ask it later. $\endgroup$ – Remi.b Oct 23 '15 at 14:55
  • $\begingroup$ @arax Speciation due to change in chromosomal number by hybridization between two species, yielding to a third species has typically be quite nicely studied. Would such an example answer your question? $\endgroup$ – Remi.b Oct 23 '15 at 14:59
  • $\begingroup$ Note btw that a change in chromosomal number is often (not always) associated with a shift in the ecological niche toward more extreme environments. Why is it so? Last time I heard of it, I have been told that nobody really knows why. $\endgroup$ – Remi.b Oct 23 '15 at 15:01
  • $\begingroup$ Someone asked and then deleted their comment, but to clarify, "direct evidence" in my comment would be intact, sequenceable genomic DNA from a Common Ancestor. Syntenic analysis provides inferential evidence of the events of translocation that have survived in evolution, but without the ability to compare to the actual data of the common ancestor, we can only infer the events. $\endgroup$ – AMR Oct 23 '15 at 16:21

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