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Anyways, we can now ask why anything causes us to feel pleasure or pain, and evolution by natural selection provides that answer.

From Charles Darwin to Thomas Malthus, to John Calhoun, to Bill Hamilton, to Richard Dawkins, to Robin Dunbar, to Robert Putnam, to Robert Trivers, to Martin Nowak, to the most cutting edge thinkers in evolutionary psychology.

The most important ethical concept here being the selfish gene:

We are a an organism, which is a set of genes being expressed throughout a series of environments.

Our genes would like to be immortal but they cannot create an organism that is likely to live forever, and so they instead replicate themselves to extend the duration of the existence of genes identical or similar to themselves through the creation of new organisms that were created from copies of themselves, and carry them forth to make other organisms in turn that will likewise be vectors for replication of that gene.

Our genes are optimized over the course of generations of success vs failure to replicate to become the sets of DNA with the best chance at replicating themselves.

Now, you can factor in things like aging (an evolution that meant older generations don't have to compete with newer ones), mutation (an evolution that meant successive generations can adapt to new or changing environments and become better replicators over the course of generations), and sex (an evolution that allowed good genes sharing an organism with bad genes to end up in another organism with better genes that compliment the advantage the gene can give to replication via sex).

But one development has been that we feel pain to help us avoid the things that reduce put chances at replication by making us averse to them, and pleasure to incentivize us to seek out the things that increase out chances at replication, highest amongst these being a sexual partner or multiple sexual partners who we could use to replicate our genes with via reproduction.

So therefore the closest thing to an objective sense of morality is the maxim that we must reproduce as much as we could, and assist those with whom we share genes in common in reproduction, and also prevent those who have different genes that are in competition with ours from reproducing.