Yes, the story of tracing the Lewis and Clark expedition by the mercury levels is true. Oddly Mercury oxide was a commonly perscribed medicine for most ailments in the early 1800's. It had certain effects which seemed to treat most sickness. They were certainly not aware of the long terms effects of mercury poisoning.

Strangely enough during the War between the States, because the southern doctors couldn't obtain the "Proper medicines" they used what they had, which were the homeopathic medicines of roots, berrys, bark, moss, and herbs. They were quite effective including making a narcotic as effective as morphine out of garden lettuce.

http://www.henriettesherbal.com/eclectic/kings/index.html

As a results the Confederate doctors actually had a better rate of curing sickness that the Union doctors in the North. It wasn't much better, but it was better.

One of the after effects of the War was what was called "old Soldiers Disease". This, it seem most commonly, was what we now call drug addition and the effects of mercury poisoning.