Mercury compounds do originate from natural sources.
However, most of the mercury found in fish comes from human activity. It’s a byproduct of burning fossil fuels, particularly coal.
Further, citing that natural sources of mercury exist does not make mercury harmless or alter its extreme toxicity against neurons. Toxicity effects are cumulative. The more mercury you take in, the more neurons will be degraded or killed.
Read up on mercury toxicity. Here, I’ll help you get started:
Mercury in any form is poisonous, with mercury toxicity most commonly affecting the neurologic, gastrointestinal (GI…emedicine.medscape.com
It’s odd of you to point out that elemental mercury is less dangerous than other forms (it isn’t easily absorbed in the human digestive tract), because the elemental form isn’t what is being placed into vaccines, which is the topic of the article, and because the mercury compound in vaccines bypasses the human digestive tract altogether; it goes right into circulating blood.