For Darwin, the sudden appearance of advanced animals in the fossil record was a major problem for his theory of evolution. He attempted to explain the lack of a visible prehistory by calling on the incompleteness of preservation of the crucial rock record. This explanation he himself regarded as unsatisfactory, and we now know that he was on the wrong track: The Cambrian explosion was a real biological event, a radical reconstitution of the biosphere, not a fluke of preservation. Darwin's head-ache is still with us, however, but in other forms. We are still far from a satisfactory explanation of the Cambrian explosion. For example, the origins of multicellularity, non-bilaterians and bilaterians are all contentious problem sets where expectations have frequently forced the interpretation of fossil data. Although our knowledge and understanding of the events leading up to the Cambrian explo-sion are immeasurably richer than Darwin's, his metaphor of the incompletely preserved volumes of books still has relevance.
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