Answer

You are conflating the question of how our tendencies were formed with the question of whether and why it is right to act that way. Those are two different questions. It is entirely possible that evolution created an altruistic tendency, but that does not mean altruism is right or worthy. Our conclusion that it is right, if indeed there is such a conclusion, is based on one philosophical assumption or argument or another. In other words: the fact that altruism is beneficial for survival is a fact. The claim that one ought to be altruistic is a norm. A norm cannot be derived from facts (the naturalistic fallacy).
There are those who argue that since evolution created this, there is reason to doubt whether it is really correct, and perhaps it was simply implanted in us. But that is a skeptical question, and you can raise similar doubts about any claim, not only about values. For example, the principle of causality, induction, mathematical propositions, and laws of nature are all the result of reasoning and observation—and those too are capacities that evolution implanted in us. So why believe them? That very question itself, of course, is also being asked using those same tools.