Why do we know that humans and chimps evolved from a common ancestor and not each other?

I'm writing a paper and i need to discuss: Why do we know that humans and chimps evolved from a common ancestor and not each other? What characteristics show our connection to the common ancestor? How are primates different than other hominids? Why are the Ardipithicus raminus the oldest hominid? And which Australopithicines won and which lost the game of life? What patterns of hominid evolution are evident? What does this mean? Some answers to any of these questions would be greatly appreciated.