How many mass extinctions do paleontologists theorize have occurred since life developed on Earth?

And 11,600 years ago, at the end of the last ice age, during the Younger Dryas period. Our most recent civilisation was wiped out by a series of cataclysmic comets, sending the few survivors back thousands of years, to essentially start again. There are plenty of ancient ruins left on every continent on earth. One only has to reject the ridiculous mainstream theory on these structures to see how truly advanced they were.