It doesn't take many genetic differences at all to make two organisms
look extremely dissimilar, if the differences are in key developmental
regulation genes. For example, maize and teosinte are very different
looking plants: maize is a single tall stalk with ears attached to it,
whereas teosinte is a stem with long branches sprouting off it, and
tassels on those branches--the seeds also look very different. For
quite a while no one realized that these are essentially the same
species (or next door to it) -- it turns out that you can get maize
that looks almost exactly like teosinte by changing just five genes.
The human-chimp difference could equally be due to just a very few
developmental genes: one statement that's often made is that humans
look like young chimps, so genes that affect the timing of development
(say, prolonging childhood and childlike patterns of growth) could play
a major role.
Mary Kuhner mkkuhner at genetics.washington.edu