In this lesson we briefly covered the construction of the geologic timescale and discussed two of the five great mass extinction events that happened since life began on the planet. These topics are controversial in two important ways. First, scientists still argue about the exact cause of mass extinction events on Earth. Impacts have been invoked for both of the two we discussed but it is interesting how much contention there is, especially for the Permian/Triassic extinction. Second, anytime biology and geology meet, the subject of the age of the Earth and the processes of evolution and natural selection naturally come up. These are not scientifically controversial topics but are often represented that way by nonscientists.
Reminder—Review the Lesson 3 Overview
You have reached the end of Lesson 3! Double-check the list of requirements on the Lesson 3 Overview page to make sure you have completed all of the activities listed there.