When the human mind evolved, it only had to process small numbers, like the number of fingers on a hand.

To this day, we base our counting systems on that and use it to grasp very large numbers—with limited success.

So, when scientists needed to make sense of Earth’s 5 billion–year history, they tied it to something concrete: Earth itself.

We started by realizing that Earth’s history is preserved in layers of sedimentary rock. That younger layers were deposited on top of older layers. And that fossils found in those older layers came from earlier animals.

Then we mapped Earth’s history onto those rock layers and fossil periods.

But misunderstandings still occur. You may have seen dioramas in museums of a T. rex attacking a Stegosaurus.

However, the Stegosaurus went extinct 150 million years ago. And the T. rex lived 65 million years ago. That means there is less time separating T. rex and humans than T. rex and Stegosaurus!

Which is almost impossible to comprehend.

So, scientists also compare geologic time to units we can understand. For instance, imagine a 100-yard football field with all of Earth’s history laid upon it.

Early humans appear in the last one-eighth of an inch. And all of civilization is represented by the width of two human hairs.

Pretty humbling to think we’re just a blink of an eye in Earth’s lifetime.