It's true, we share the common ancestor of the homonid. We didn't evolve directly from the apes we see today.

I think the concept is simple: it's survival of the fittest. The organisms that fit their environment the best or most efficiently, go on to create the most offspring, which go on to create more offspring with those same features. These minute changes lead to large change over time, which accounts for the great variety of species specialized to their environment.