Contradiction said:
Yeah true... but... The discrepancies in chromosome changes still remain, regardless of whether it was a shared descendant or the chimpanzee directly.
This here http://en.wikipedia.org/wiki/Colchicine is a current example of how a toxin can create subspecies with way more chromosomes than their father.
As I've seen in genetics class, most of today's wheat is for example mutants obtained with such toxins (caffeine does that too on a lower scale) so that it has some most of its chromosomes in 4 different copies or even more. This implies among other things that cells and plants will grow bigger than "wild" wheat who only has 2 copies of each.
The thing is the mutated "egg cells" obtained by this method will be of two kind : one with too many copies, one with not enough. The one with too many might be fertile and healthy, obviously the one with not enough is likely to die (or unable to be born actually).
These toxins are 100% natural and can be found in the wild. Every living creature is exposed to toxins and radiation that cause more mutations than simple copy error.
Sometimes these mutations won't work (sterile offspring or unborn ones) sometimes they will. Once again these things happen in millions of years !