To learn about how old the LDG is, we’ll need to study data on ancient life forms — evidence that comes in the form of fossils. This leads to two problems:

First, because individual species can be hard to identify in fossils, scientists studying the history of life sometimes look at diversity in terms of the number of genera — which are groups of closely related species. It’s a bit like taking a census, but counting the number of families instead of the number of individuals. So when we study data from fossils, we’ll sometimes be looking at numbers of species and sometimes at number of genera.

Second, figuring out where a long-extinct organism lived can be tricky. Over the course of Earth’s history, the continents have moved. Four hundred and fifty million years ago, Antarctica was far from the frigid place it is today — it was at the equator! With all this change, scientists have to do a lot of work to figure out where different organisms lived in the past.

Despite these challenges, scientists have put together some data that give us snapshots of diversity at different points in life’s history. Now let’s look at some of those data…