The connection between chromosome number and speciation is one kind of re-arrangement that is well known among plant breeders. They’ve known for a long time that by simply increasing the number of chromosomes typical of a species – a phenomenon called polyploidy – that it was possible to obtain a new variety with different characteristics...But while polyploidy is fairly common among plant species, it’s rarely found among animals...Instead, the chromosome re-arrangement most often exhibited in animal species is translocation. This is when non-homologous chromosomes break and exchange parts: one of the two chromosomes in pair A exchange a part with one of the two chromosomes in pair B. If the break in the respective chromosomes occurs near their ends and the long parts are joined together, the short segments sometimes contain so little genetic information that they may be lost. This gives the appearance of two chromosomes having been fused together. This is what happened in our own ancestry, and is the mechanism responsible for the origin of man.
I have reproduced a small part of the diagram given in the article [The editor has redrawn this with limitations in part attributable to the low quality of his copy of Gift of Fire.], showing the human second chromosome on the top, and two chimpanzee chromosomes on the bottom. It is patently obvious that the human second chromosome was created by translocation, or a fusion if you prefer, of two chimpanzee chromosomes. Or to be more precise, that the human second chromosome was created by a translocation of two chromosomes in an animal that was ancestor to both man and chimpanzee.
The sequence of events probably took place something like this. About five million years ago a translocation like that described above occurred in a pithecine male who was the controller of a harem of females. Rather than having 48 chromosomes, which was normal for his species, he had 47. When he mated with members of his harem, who possessed the usual number of chromosomes, half of his offspring would have had 48 chromosomes and half would have had 47. If some of those with 47 chromosomes mated among themselves, or were back-bred to their father, one quarter of their offspring would have had 48 chromosomes, one half would have had 47, and one quarter would have had 46. Those with 46 were the prototype of the new genus Homo. But at this stage they were not yet a new species. At most they can be thought of as a new chromosomal race., probably with great phenotypic difference from their fellows, but still not yet a new species. That had to wait for the appearance of one of the chromosome inversions discussed above. This inversion also probably occurred in a male with a harem and was transmitted in much the same way as the translocation. In this case, however, crosses between individuals with the inversion and those without produced only a few offspring, while matings between inverted chromosomes continued to be fertile, as did those without the inversion. This was the first step in breeding isolation. Suddenly, almost overnight, a new species came into existence.