It is said that we share 99.9% of our genes with one another, 95% with chimpanzees, and 60% with bananas. (It is also said that this isn’t quite right about bananas, but reading about that did not quell my confusion.)

It is further said that I share 50% of my genes with my brother, and that this is why I like him.

And yet, I seem to be more closely related to my brother than to a banana.

A natural way to resolve this incongruity is to suppose that my brother and I share 50% of the 0.01% of genes that might possibly vary between two humans, which we call ‘50% of my genes’ as a shorthand.

But this raises a problem. The usual explanation for familial affection evolving is that since my brother shares half of my genes, from a genetic perspective it is half-selfish for me to look after his welfare. So genes that make me care about my brother tend to spread.

But on this new model where ‘50%’ is shorthand for ‘50% of the way between virtually all and all’, taking care of my brother is about as good as taking care of myself (genetically). And a banana is worthy of my kin affection more than I thought my brother was. All living creatures are my family, and all humankind is myself.

This does not describe my feelings about bananas, and to learn of a basic biological drive for selflessness across vast swathes of the tree of life would be a bit of a surprise.

How does this actually work?