Can this be a faithful testimony of what really happened? The fossils’ interpretation problem was forcefully brought to the generale attention when it was discovered the greatest of all discontinuities, the so-called Cambrian explosion. In the 1830s, Roderick Murchison found the geological stratum that was bearing the very first visible fossils of Earth’s history. Whilst all lower strata were, at the naked eye, completely devoid of fossils, in the Cambrian one could see the fossilized remains of creatures that were unmistakingly exibiting the sophisticated structures of highly developed metazoa.
The Cambrian animals were totally different from modern organisms, but Murchison did not see in this a sign of evolution. The most telling fact, to him, was the suddennes of their appearence, and he concluded that the abrupt arrival of complex animals could only be explained by an act of creation. “The first signs of living things – he wrote in 1854 – announcing as they do a high complexity of organization, entirely exclude the hypothesis of a transmutation from lower to higher grades of being. The first fiat of Creation which went forth, doubtlessly ensured the perfect adaptation of animals to the surrounding media”.
Darwin could reply to this argument only by invoking the imperfection of the fossil record, and almost a century had to pass before George Simpson could point out that a geologically sudden event has a time span of a few million years, and therefore it is not at all sudden from a biological point of view. Today, Simpson’s argument has been largely confirmed by the experimental data, but it should not be forgotten that the Cambrian explosion is still waiting for an explanation. And what has to be explained is not only its physical time span but also, and above all, its biological mechanism.

The experimental data

It has been very difficult to obtain reliable data about the Cambrian, and many conflicting proposals have been made both on the dating of its geological strata and on its division into subperiods.