What it really meant when people talk about living fossils etc is that the morphology of the creature has changed very little. Genetically and on a cellular level there would definitely be a lot of changes from their earlier forms millions of years ago. DNA just doesn’t stick around for more than a couple hundred thousand years maximum it seems so it’s difficult for us to chart those changes that aren’t visible in morphology. Creatures that seem to not change drastically in such long scale time usually have niches and environments that haven’t changed drastically in their existence.
Mercury alone is safe enough to handle a bit where you would be mostly worried about breathing too much of its vapor. Organic compounds of mercury on the other hand can kill you within days of a drop touching your skin.