I have NO clue where the concept came from. At the same time, I’m positive it was BEFORE D&D was ever invented. D&D came into being in 1974 & I can remember points being used for Chess back in the mid to late 50’s.

You could argue that it goes back to the caveman era, when you would have to participate in a hunt to gain experience.
And then of course you have cannibalism, who believe that by consuming another human, you obtain their strength.

Agree with @ragingloli – it came from cavemen days when men all thumped their chests and grunted because they killed the bad ole Gigantopithecus.

Even today some hunters cut out and eat the raw heart of the animal they just killed to obtain their spirit as well as honoring their death and giving thanks. Honorable hunters kill as quickly and humanely as possible and use all parts of it.