When NA-KAA had a hand in creation he was very fond of his beautiful garden and his two magnificent trees.
When it came to humans he decided to keep them segregated to avoid all the normal human hassle. So each sex (there were only two at the time) were allocated their own tree to sit under, not knowing there was another tree occupied by the opposite sex.
About to set off on an urgent business trip, NA-KAA foolishly called them altogether to give them gardening instructions whilst he was away. The final words were "Back to your own trees. Do not mix or mingle. Do not look back. Stay with your own tree. Ladies to the Left. Gentlemen to the right. Off you go — quick march — no dawdling."
Well — they had seen each other. Mad with excitement and curiosity they all ran under the same tree the moment the boss was out of sight. It would have been the nearest one. It was the wrong choice.
When Na returned he coldly informed them they had made a bad choice and they had chosen the Tree of Death, and they would all eventually die, whereas the other Tree was the Tree of Life but it was too late now.
We think he was just making this up because he was riled, as it has never been made clear which sex was fobbed off with the Death Tree in the first place. Typical of Gods. They just can’t stand the idea of humans competing in the immortality stakes.