Darwin's dilemma: the realities of the Cambrian ‘explosion’

Abstract

The Cambrian 'explosion' is widely regarded as one of the fulcrum points in the history of life, yet its origins and causes remain deeply controversial. New data from the fossil record, especially of Burgess Shale-type Lagerstätten, indicate, however, that the assembly of bodyplans is not only largely a Cambrian phenomenon, but can already be documented in fair detail. This speaks against a much more ancient origin of the metazoans, and current work is doing much to reconcile the apparent discrepancies between the fossil record, including the Ediacaran assemblages of latest Neoproterozoic age and molecular 'clocks'. Hypotheses to explain the Cambrian 'explosion' continue to be generated, but the recurrent confusion of cause and effect suggests that the wrong sort of question is being asked. Here I propose that despite its step-like function this evolutionary event is the inevitable consequence of Earth and biospheric change.

Cell evolution and Earth history: stasis and revolution

“…I do not agree that animal origin was inevitable (Conway Morris 2006). Our presence owes much to rare and unique historical and evolutionary accidents.…”

“…Peterson & Butterfield (2005) imply that the anus evolved at the Early Ediacara (570 Myr). However, like Conway Morris (2006), I am unconvinced that any Ediacara fauna are bilateria; nor can I place them in any protist or plant group. Most likely they are early animals related to sponges and cnidaria, but fossils have too little detail to show if they belong within either or are a distinct phylum; whichever makes little difference to the suddenness of the animal Cambrian radiation and its contemporaneity with the origin of most protist phyla.…”

“…Most likely they are early animals related to sponges and cnidaria, but fossils have too little detail to show if they belong within either or are a distinct phylum; whichever makes little difference to the suddenness of the animal Cambrian radiation and its contemporaneity with the origin of most protist phyla. The Ediacaran/Cambrian explosion of protists and animals is a fundamental feature of eukaryote evolution, not a preservational artefact (Peterson & Butterfield 2005;Conway Morris 2006). Given the initial bilaterian pattern of a continuous-flow through gut and associated developmental mechanisms, all feasible animal bilateral body plans would inevitably be tried out without further delay by minor developmental modifications to basic animality, leading to an immediate explosive production of all bilateral animal phyla within a few tens of millions of years, each involving rapid quantum evolution and the entry to a major animal adaptive zone.…”

Dynamical patterning modules: a "pattern language" for development and evolution of multicellular form

“…Animal body plans of virtually all the modern Metazoan types emerged relatively suddenly ("compressed in time"; Rokas et al, 2005) during the "Cambrian explosion" about 550-530 million years ago (Ma) (Conway Morris, 2006). The metazoans, in turn, may have had roots (Erwin, 2008) in the Ediacaran period, 575-542 Ma (Shen et al, 2008), derived, in part, from simpler sheetlike, or budded, segmented, tube-like multicellular biota found in deposits of the earlier period (Narbonne, 2004;Droser and Gehling, 2008).…”

Equinodermos del Cámbrico de España: situación actual de las investigaciones y perspectivas futuras

“…La «explosión del Cámbrico» se considera el evento más importante de la historia de la vida, pero sus causas y consecuencias son temas muy controvertidos entre los paleontólogos (Conway Morris, 2006). En unos pocos millones de años numerosos grupos de metazoos aparecieron en los mares, y entre ellos se encontraban los equinodermos.…”