Because evolutions are random genetic mutations, so don't affect an entire species at once. Also environmental pressures play a part in shaping evolution. So, f

Because evolutions are random genetic mutations, so don't affect an entire species at once. Also environmental pressures play a part in shaping evolution. So, for example, apes under one set of circumstances might have to survive in a more demanding environment than apes under another. Thus [over aeons] that particular group might self-select for individuals with more dexterity / better problem-solving abilities, etc.