Imagine if you will a proto-human species with brains small enough that they didn't have the problems that we have with squeezing our baby heads through the birth canal. Imagine that that species has, on account of their limited cognitive capacity, very little in the way of long-term planning capability, and like I would imagine most animal species do, mostly just live their lives day to day, reacting only to immediately observable threats, and then when those threats are gone, going back to blissful ignorance and not worrying about when the next threat is going to come along.

Imagine that a mutation happens in some individual of this species that causes him to have unusually large brain size and comparably increased cognitive capacity. This poor individual is, on behalf of that, cursed with what is effectively anxiety the likes of which none of his species has ever known. Instead of just relaxing and being happy, this individual is constantly thinking ahead about what the next threat is going to be, and planning on how to deal with it in advance. He looks around at all the other members of his species and sees that these people aren't working their asses off like he is, they're just coasting along all happy and innocent, because they have no idea of the signs of danger that he can see, they can't tell safe from threatening at all unless it's right up in their damn faces.

On account of that nigh-paranoid existential dread, this individual does manage to survive and reproduce better than other members of his species. Maybe this mutation came at a time when the changing environment made the caloric requirements of such a big brain and the difficulties in giving birth to such smart babies worth the trade off. In time, more and more of their people are descended from this lineage of big-brained folks. But they haven't speciated yet. There's still the big-brains and the little-brains living together in the same population. And at least the big-brained one can see the difference between them. They know what's a bad sign and what's a good one, while the others don't. As a consequence, they know that death is coming for them eventually, and they feel compelled to work their asses off to ward it off as long as possible, instead of just coasting along blissfully ignorant until unforeseen death takes them. They also notice that their big heads make childbearing a major pain in the... lets say "ass"... compared to their little-brained brethren.

Imagine at least some form of proto-language is around by this time, if not having been recently invented by the big-brained folk. So accounts of this observation were passed down, of the anxious workaholic big-brained folk gradually displacing the blissfully ignorant little-brain folk, their people gradually changing from the old kind to the new as differential survival odds changed the distribution of genes in their population... accounts of that passed down, with the inevitable distortions, over time unmeasured.

Until eventually someone wrote down the account as the first peoples having not known death, until they learned to tell good from evil, becoming as gods in the process thanks to such foresight, but being cursed to work hard their entire lives in exchange, in contrast to their former ignorance; and their women cursed to a painful childbirth as well.

And also something about a talking snake and a pair of trees, but that stuff's irrelevant decoration of the core story.