Pure speculation. Like I mentioned before in response to Rama Set. Yes, meteorites do contain Iridium-191 and -193... but so does a nuclear blast, and traces of these can be found all over the Earth in all sorts of places... so either a) several meteorites hit the Earth throughout history or b) advanced ETs periodically nuked the Earth for whatever reason (I cannot particularly fathom their motives). Either theory is perfectly valid, it just depends on which one you subscribe to. I, for one, think it was nukes. Why? Because ancient texts across the world talk about nuclear weapons, way before nuclear weapons even existed according to modern science.
Ancient India's religious texts support the nuclear weapon theory. Here is an expert from the Mahabharata:
"The Earth shook, scorched by the terrible heat of this weapon. Elephants burst into flames and ran to and fro in a frenzy, seeking a protection from terror. Over a vast area other animals crumpled to the ground and died. The waters boiled, and the creatures residing therein also died. From all points of the compass the arrows of the flame rained continuously! "
It goes on to state that: "a single projectile charged with all the power of the universe!"
Later in the text, we can read these words:
"An incandescent column of smoke and fire, as brilliant as ten thousand suns rose in all its splendour. It was the unknown weapon, the iron thunderbolt, .....a gigantic messenger of death!"
To me, that sounds like a description of nuclear weapons. Now, how would ancient people from India be able to describe a nuclear blast so accurately? I feel like that is much more than a coincidence. I am also glossing over the numerous other references in ancient books that describe very similar events. The Bible mentions a great famine that Prince Joseph is warned of by 'God'. The famine described sounds very much so like a nuclear winter caused by fallout, as it covered the entire Earth and killed off many species of animal.