However, this would not have influenced reproductive ability since the effects of prolonged stress don't usually strike until later in life. Let's say that you are injured by someone within your band of hunter-gathers. Your survival and the survival of your offspring might be contingent upon you finding a way to live in peace. In fact, the sheer fact of living with others and having to find ways of tolerating or, better, accepting the differences, requires that each person find a way to resolve resentment. Living in close quarters with people who are exceptionally inter-reliant would necessitate the ability to find inner peace with others' shortcomings.
Forgiveness makes sense from an evolutionary standpoint.
However, forgiveness is also self-rewarding. Perhaps it is just from the sharp physiological and psychological contrast that one experiences going from being mentally churned up and physically assaulted by the effects of our autonomic nervous systems while in that condition, that creates the profound sense of love and relief you feel when you forgive. My personal inclination is to believe that subtle energies exist in and around us, and that we and those around us are affected positively by the lightened spirit one encounters within when forgiveness is achieved. Either way, it makes sense to forgive. And there may even be an evolutionary basis for the character building that comes in that moment when a person must choose to forgive.