The Romans used lead in their pipes their eating utensils and to fortify their wine, but eventually the Incredible works they created fell into ruin and chaos as the empire tumbled down around them. Corruption and madness inflicted the romans, could a buildup of lead in mankinds biology have reduced our intelligence through poisioning like can happen in ecosystems exposed to harmful buildups of chemicals leading to loss of knowledge and heavy dark ages fanatical action?
You may want to refer to this post a few months ago which debunked the myth of lead poisoning causing the fall of Rome. You may also be interested in our FAQ section on the "Dark Ages" which by and large deal with dispelling the myth of it.
There's really no good answer to this question. The "loss of knowledge and heavy dark ages fanatical action" are a later exaggeration and not an accurate reflection of Eastern Europe post-Rome. Here's a good thread on that. Lead poisoning could not have caused the Dark Ages simply because they weren't that dark at all. Furthermore, it's not as if the Late Roman Empire had a monopoly on "corruption and madness." Take the year 69, when four separate emperors held the throne: Otho, Galbe, Vitellus, and Vespasian. And yet the empire survived centuries later. If lead could actually make people that inept, the empire would have fallen at a much earlier crisis.