As other comments have stated the issue is with the question not my answer. It's not possible to answer in the way it was asked. There wasn't really a before. Lead comes from the decay of uranium. Hippocrates know about its dangers somewhere around 300 BC, Nikander described results of lead poisoning in 250 BC. Maybe I'm wrong but I don't think OP was asking why they made lead necklaces in Anatolia around 6000 years ago. Although it was probably the last time people did use lead without knowing it was toxic.