" we can identify the causes of these revolutions, they’re highly varied: glaciation in the case of the end-Ordovician extinction, global warming and changes in ocean chemistry at the end of the Permian, an asteroid impact in the final seconds of the Cretaceous. The current extinction has its own novel cause: not an asteroid or a massive volcanic eruption but “one weedy species.” As Walter Alvarez put it to me, “We’re seeing right now that a mass extinction can be caused by human "

― Elizabeth Kolbert , The Sixth Extinction: An Unnatural History