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My first thoughts reading your post are 1) You start WAY TOO LATE IN THE GAME. You are essentially talking about altruism as a conscious choice which means you are well into the higher mammals.
Virtually every sexually reproducing creature devotes resources to reproduction that could have been conserved for individual survival. As you move up in complexity, you have animals feeding their young and performing other services for them. As would be expected with all evolved cooperation, the energy and cost you expend raising your young produces a more survivable young and so is net cost effective at getting the next generation going, which is pretty much what spreads genes.
How big of a leap is it from a mama bird regurgitating food into her baby's mouth to you helping your neighbor hunt for wooly mammoth?
If you were the first organism to get the gene to feed your babies or do whatever expanded their survivability, then obviously that is how that gene propagates, your babies have the gene.
As you get to the more complex forms of altruism of primates and humans, you also get to strong feedback mechanisms against non-cooperators and free-riders. The system may not be perfect but I think it allows a path from feeding babies or burying eggs in the sand to modern altruism in humans where no wierd "how do we start this" behaviors bump up to stop things.