[rm445]

I'm curious whether the whole tree of life being related plays some part in cross-species adoption (and even animal behaviour in general???).

In the sense that genes don't 'want' anything, but selection creates a pressure towards overall adaptation and survival... a lion shares a lot of genes with a leopard. Is it pro-genetic-survival in some very broad sense for a lioness to care for a baby leopard? I guess not, if adults of those species compete to the death... but many species don't seem to do a lot of murdering, beyond predation for food. Animals sometimes seem quite incurious about other activity close to them, except when they need to eat. Could there be some small selection pressure towards live-and-let-other-DNA-live?