Immortality vs Survival
In a world with no dangers, the first critter could perhaps "succeed" by being immortal, but it would soon be beaten by other critters that DID reproduce - it would become outnumbered.
In a world WITH dangers, mortality is more or less assured and any critter MUST reproduce so that in the future there will still be critters of its kind.
After that, it becomes a Darwinian arms race...