The ancient Greeks had a lovely theory. Certain places on the earth (caves, mostly) were, they thought, gateways to the underworld. Plants growing near these places could absorb the deadly essence of Hades and became poisonous.
Snakes and other venemous creatures got their poison by consuming these plants. And stinging insects got their little doses of poison by feeding off dead snakes.
Isn't that a great narrative? It explains everything, in a nice logical progression. OK, it presupposes what we would call a "supernatural" force as the ultimate origin of poison, but other than that, it's an entirely "scientific" account. In accordance with Occam's Razor, it proposes a single unified process underlying diverse phenomena.
It is, in other words, a perfect scientific theory. It's completely wrong, on every point, but we only know that because we now understand atoms, molecules, chemistry and biochemistry, which the Greeks had no way of knowing. At the time, the Hades theory was surely the best possible theory about where poison came from.
The moral of this story is, beware nice theories based on incomplete data.
Reference: Greek Fire, Poison Arrows and Scorpion Bombs, which I'm currently reading, all about chemical and biological weapons.