In the 1830s, a group of Scottish and English naturalists were pursuing a new and unpopular idea: that the Earth's features could be explained by natural processes, acting over vast stretches of time. Studying the rocks of the British Isles, their attention was drawn to a particular variety of fossilized corals. From lower to higher rocks, the corals grew more complex. Anticipating Darwin, they inferred that they were observing the development of a life form. And they inferred something else: the fossil record could help interpret the rock record.

Two limestones may be similar in composition. But fossil remains are distinct. Wherever they're found, rocks that contain the same distinctive ...