ABSTRACT

BACKGROUND: THE CAMBRIAN EXPLOSION Although multicellular animals had only appeared at the very end of the Precambrian (Chapter 2), their evolution at the beginning of the Cambrian was so rapid that this event is known as the ‘Cambrian Explosion’. In an astonishing orgy of evolution, a period of little more than 10 million years at the beginning of the Cambrian saw the appearance of almost every animal phylum and body plan known today, along with some other bizarre forms which soon became extinct, suggesting that this was an experimental phase of evolution. Approximately 35 different animal phyla are known today; in the Cambrian seas there were undoubtedly several more and some authorities would claim up to 100.