Comes the oxygen
When oxygen first began to accumulate in our oceans and our atmosphere, most of what was alive at the time couldn’t take it. The oxygen was a terrible poison. Even those life forms that could stand the extra O2, had trouble when the oxygen reacted with the iron in the ocean, and took away much of what had been food. If that wasn’t bad enough, a long ice age got started, all because of tiny microbes, too small to see with the naked eye.
Here’s a link to the episode of that other podcast that has an add for this podcast in it.
Here’s a nifty video about the way oxygen caused problems when it first arrived.
And here’s an article about how lakes can switch over to the same sorts of microbes that ruled the world, back before all this troublesome oxygen changed everything.