I've been reading some mythology, and the sun god is usually honored for bringing crops and such. How did people know that plants needed light before we knew about photosynthesis? It could be chalked up to "they don't grow in caves", but why not attribute it to the stars, clouds or moon then?
It's pretty easy to deduce. Plants don't grow well in the shade, plants wither during the winter when there is less sun, and some plants like sunflowers even track the sun... remember ancient people weren't stupid, they just didn't have the cumulative knowledge we had. They still had deductive reasoning skills.
I'd assume this was discovered before agriculture. Maybe you could get a better answer from /r/AskAnthropology?
You can observe and appreciate a causal relationship without understanding why or how.