Changes in diets and climates are one of the main causes of evolution, fool.
And basically what Whiplash said.
Think of the primate period of our evolution as a tree, there's the trunk, which could be considered a common ancestor, and then on one branch you have an earlier species of us (homo erectus/habilus/sapiens), and on another branch are monkeys and stuff. Obviously things which evolved from a closer ancestor are going to bare more likeness to us.
Female breasts as we know them (heh) on the other hand are (possibly) a direct result from some time in our evolution where we met criteria that required them.