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

A mass extinction is defined as more than 75% of all life disappearing in a period of time. Paleontologists have theorized 5 mass extinctions since life began on Earth.

Listed from the time period, years ago, % of life loss, and most accepted cause:

1. End Ordovician, 444 million years ago, 86% of species lost - Theoretical Cause: a short, severe ice age.

2. Late Devonian, 375 million years ago, 75% of species lost - Theoretical Cause: algal blooms deplenished the oxygen in the oceans.

3. End Permian, 251 million years ago, 96% of species lost - Theoretical Cause: a series of natural catastrophes that occurred in short periods of time.

4. End Triassic, 200 million years ago, 80% of species lost - Theoretical Cause: No clear cause has been found.

5. End Cretaceous, 66 million years ago, 76% of all species lost - Theoretical Cause: volcanic activity and finalized when an asteroid struck the Earth.