It doesn't matter how the ecosystems work, though. It's completely irrelevant here. All that matters for the "competition" discussion is whether or not chimps/humans changed from their ancestor because of a change in the ecosystem. If so, they could exist in different areas and competition doesn't play a huge role, if not, then what caused them to "evolve?"
It sure does matter how ecosystems work. It's absolutely central. I've tried to explain it, and I'm sorry if you don't get it.
Also , competition certainly does play a role, just perhaps not competition between chimps, the common ancestor, and humans,, since each species fills a different niche: eg: one lives in trees, another on the savanah and caves, another elsewhere . Secondly, you are assuming that evolution is somehow causal in nature. Genetic mutations are happening randomly (although we know that God is in control) all the time. Most of the time, they result in things that just don't work, and the abberant individuals die. If a mutation does work well for an individual as it goes about its life,, however, that means that the organism with this "successful" mutation can survive to reproduce. The mutation is then passed on down the genetic line, and evolutionary changes result over a LONG period of time.