Back in the Paleolithic period, after a long and exhausting chase, wouldn't you take a quick fix of the animals blood before lumping the carcass back to the village for cooking? Don't be embarrassed, no shame in that. The animal would be ***t scared and full of adrenaline so it's plausible that the effects of Adrenochrome would be well know in hunting societies for thousands of years. The old and decrepit shaman would realise the only way to continue getting it would be through ceremonial sacrifice - so it ends up in that side of the culture too, and unfortunately seems to have remained.