Re: knowing what you don't know
Ocean life produces small amounts of methyl chloride, larger amounts of methyl bromide, and huge amounts of methyl iodide. Also amounts of other halogenated hydrocarbons small compared to human production. Natural emissions of methyl iodide outweigh human emissions by at least one order of magnitude. (It's a major component of the "smell of the sea" that you can detect a few miles inland with a breeze from that direction).
Fortunately, methyl iodide has a shortish half-life in the ground-level atmosphere, because iodine would otherwise be a far more potent destroyer of ozone than chlorine.
Methyl iodide is an extremely potent greenhouse gas as well as an ozone eater. It's long been an evolutionary puzzle as to what it was that kept the Earth from freezing, back when the sun was young (so considerably cooler than today) and when the atmosphere did not contain oxygen. Methane and CO2 aren't thought to be sufficiently potent greenhouse gases for back then. My theory is lots of methyl iodide, stable in a reducing (methane, nitrogen, CO2) atmosphere. It's created by cyanobacteria in today's oceans, and they are amongst the most ancient and radiation-tolerant of life-forms, so why not the same back then? When they'd finally emitted enough photosynthetic oxygen to convert all the planet's near-surface Iron-II into Iron-III, the oxygen built up in the atmosphere, the methyl iodide levels dropped to today's low level, and (not coincidentally?) the "snowball earth" episode occurred. Which in turn seems to have driven the emergence of higher life-forms. But maybe that's the unlikely event, and a possible answer to the Fermi paradox - most other places, simple life drives itself to extinction when it finally converts its atmosphere to oxygen?