What you say is mostly sensible, however it breaks down at certain points:
I actually don't see any real obligation, if I were an atheistic evolutionist, to do anything about the earth. Or, for that matter, to do anything for humanity. Unless I see a distinct benefit in it for me AND I have a desire to reap said benefit.
In your post you go about declaring how being a humanitarian is in your best interest. You do not actually answer the above statement - That doing something about the earth is in your best interests. What do I care if we as a species die out? We wouldn't die immediatly... life would slowly become harder and harder, and more than likely people would have fewer children than is necessary to maintain the population (the earth's carying capacity would fall slowly, and we would die out)
Secondly your argument that humanitarianism is a result of evolution breaks down because being a humanitarian is often not to your advantage. For example, it makes no sense for a species to preserve its invalid members - result of harmful mutations - because this would pollute it's gene pool. However, we build assylums and we have special schools for instance. In these situations, humanitarianism offers no selective advantage versus a "selfish" mentality.
This can be applied to the human universe where we send money to help with famine, poverty, war, disease. While this is not a direct result of mutations, the concept is often the same. Whenever somebody needs something, often they will need it again. Giving relief money just aleviates the symptoms of the problem. The problem being the poor econonmic problems, the people afflicted by them will rarely escape, and relief money just prolongs their suffering.
I do not believe that cooperation, doing things for humanity, whatever you wish to call it, evolved. I believe that the only way for this ammount of cooperation to come about amoung humans is if they were created. If you look at moral reletavism, which is the logical conclusion for the absense of God, you can see that a society that is truely based on these concepts will ultimately break down.
In conclusion a quote: "There are only two possibilities as to how life arose: one is spontaneous generation -- a rising evolution. The other is a supernatural act of God. There is no third possibility. Spontaneous generation was scientifically disproved 120 years ago by Pasteur and others. This leaves us with only one logical conclusion -- that life arose as a supernatural act. I will not accept that, philosophically, because I do not want to believe in God. Therefore, I choose to believe that which I know is scientifically impossible." George Wahl, former Harvard professor and Nobel Prize winner in biology.