Altruism is used in discussions of ethics as an alternative to ethical egoism: the belief that people ought to be selfish.
Altruism leads us to find a middle ground between being purely self interested and devoting our entire lives to the group. We learn that it’s not preferable to be too self interested, but can be harmful to devote all your energy to other people. A little too much self interest can be as harmful as too much virtuousness towards others. As an ethical egotist, what you might learn from altruism is; there are certain situations where it could pay not to be self interested. A good example would be several people stuck on a deserted island.
If everyone left destined for their separate survival, surly only a few would live very long. If the people banded together, shared the work and supplies, surely most or all would survive longer. This shows that there are situations you can handle better when you chose to be in a group, then on your own. In conclusion I think any person should carefully consider before deciding to act in their own interest or act for a group that they are part in. People should try and think of what might be more beneficial for them in the long term as well as immediately.
If the worst is true; and humans are purely selfish beings, what people should do is substitute a little altruistic philosophy in their thinking. This would help incorporate every self interested person into a group that functions better than any people could individually. It would probably help expand the available choices for people who feel they cannot make a decision. I think it can also lead the world in a more tolerant direction.