My take is that cooperation for mutual advantage is based on self interest. There is nothing mysterious about it, other animal species cooperate with the pack or with family members. That has a perfectly good Darwinian explanation: to protect, preserve, and propagate one's own genetic information.
The atheist Richard Dawkins wrote s book about it called The Selfish Gene, lol.
What is noteworthy is Oscar Schindler or Georgio Perlasca risking torture and death to save thousands of complete strangers, or the Good Samaritan sacrificing his own resources to save a stranger from a rival tribe that hates the Samaritans, without any expectation of mutual advantage or reciprocity.