I'm aware there has been quite a bit of research and debate about how it is that humans and other animals have developed a propensity to altruism. I think this is tackled to some extent in The Selfish Gene and Dawkins' subsequent books. I do have a copy somewhere so if I get the chance I'll dig it out.
If the tribe is successful then it is likely that an individual's genes will be propagated, since presumably there was a lot of shared family history within each tribe (maybe that's what you were getting at?).
It is amazing how Darwin's work so accurately corresponds with what we now know about DNA. There was a fascinating TV programme about Alfred Russell Wallace a few weeks ago (hosted by Bill Bailey) which pointed out how unfair it is that he is almost forgotten whereas Darwin is given all of the credit. It does seem that evolution was an idea waiting to be discovered, I find it quite bizarre - not to say a little scary - that there are so many people (particularly in the US) who claim not to believe in evolution.