This was a new one on me, I’d never heard it before. The answer seems so utterly obvious to me that I don’t know why anyone would bring it up, still, it’s started to come up from more than one source, so here’s the simple answer.
‘If there’s no god, how do you account for our inherent survival instinct?’
The survival instinct is an evolved trait. The survival benefit of… wanting to survive, should be blatantly obvious. Those creatures that want to survive pass on the tendency to want to survive, and they survive more often because they want to survive.
Creatures with a lesser survival instinct or no survival instinct at all, are – obviously – less likely to survive and so don’t pass on their suicidal tendencies.