CONWAY MORRIS, S.: The Cambrian explosion: what’s the problem?

Behold the donkey approaching; hardly able to move under the impedimenta. Leading it is the bowed figure of Darwin, and on its back a number of luminaries waving manuscripts; while trudging behind, with a large scoop? – well, never mind. Now let us creep closer. What is that Darwin is muttering? “Time to re-write Chapter 9; I was wrong, so very wrong �..” Darwin, ever petrified some observation would demolish his theory, was very uneasy about what we now call the Cambrian explosion. But hadn’t you heard? There was no explosion, or at least some now claim. In reality, current opinion is polarized: molecular clocks suggest a deep ancestry of animals, an observation seemingly at odds with the palaeontological record of Ediacaran diversity, the onset of biomineralization and an obvious diversification in trace fossils, not to mention Burgess Shale-type faunas. Invoking pre-Ediacaran metazoans is unproblematic, but it is unlikely present search images are valid, notably in terms of a meiofauna or larval forms. Presumably such animals were inhabitants of a microbial ecology, analagous to the sophisticated ciliates, but definite fossil evidence is still awaited. To reiterate, what’s the problem? In essence, it is the evidence for pre-Cambrian molecular events that are decoupled from the emergence of body-plans, the latter being a specifically Ediacaran/Cambrian phenomenon. The answer will lie in understanding the evolution of developmental networks, but in terms of tangible evidence the real excitement now is the historical analysis, via the fossil record, of the assembly of bodyplans and functional explanations for the emergence of the deuterostomes, ecdysozoans and lophotrochozoans. As it happens, Darwin had nothing to worry about.