CHAPTER SEVEN

THE CAMBRIAN EXPLOSION

The fossil record

At the end of the 18th century, William Smith, an English engineer who was engaged in canal building, discovered an empirical rule for comparing the rocks of different geographical areas. The idea was to identify the sedimentary rocks by their fossils, because Smith had noticed that each stratum contains fossils that are never found in higher or in lower strata. Hence the idea that if two rocks have the same fossils, they also have the same geological age, even if one is at the bottom of a valley and the other on the top of a mountain. The fossils became in this way the key for reconstructing the past movements of the Earth’s crust, and Smith used them to draw the first geological map of the United Kingdom.
Smith’s discovery, however, had implications that were going far beyond geology. The fact that the fossils of a stratum do not appear in all other strata means that the organisms which lived in that geological age were different from those that lived in all other ages. It means that life on Earth has gone through a long history of changes, and that sedimentary rocks are still keeping a record of that history.
But how accurate is the fossil record? What we see in it is all that remains of the ancient inhabitants of the Earth, but the remnants could be imperfect, and even deceitful, documents. The sedimentary rocks contain, sometimes within a few centimeters, materials that were deposited to the bottom of ancient seas for millions of years, and their sharply discontinuous structure seems to suggest that organisms appeared suddenly on the face of the Earth only to disappear, some time later, with the same abruptness.