Frank Bloggs, Earth and Ocean Sciences, UBC

1. WHAT CAUSED THE CAMBRIAN EXPLOSION? Frank Bloggs, Earth and Ocean Sciences, UBC The Cambrian Explosion is the name given to the rapid diversification of life that occurred at the base of the Cambrian around 540 million years ago. Although many soft bodied forms existed just prior to this event, creatures with hard parts and skeletons suddenly appear at this time (Ausich and Lane 1992). The question that remains to be answered is why this diversification occur then and not earlier. Reconstruction of Anomalocaris, an enigmatic arthropod from the Burgess Shale Formation in BC Taken from Briggs et al. 1994. This and other forms are part of the vast explosion of new forms that developed from the Cambrian Explosion. The Cambrian diversification of animal life.Purves et al. 2003 WAS IT PREDATION? Several authors including Simon Conway Morris (1998) have suggested that predation caused an ‘arms race’ that increased the pace of evolution: As creatures developed protective shells predators had to adapt to overcome these new defensive structures. Late Precambrian fossils of Cloudina have tiny holes bored into them suggesting active predation in metazoans (creatures consisting of more than 1 cell) was initiated just prior to the Cambrian (Bengtson and Zhao, 1992). OXYGEN EXPLANATIONS In order for creatures to grow large and complex oxygen must be able to diffuse to all tissues. This requires a certain concentration of oxygen in the oceans. Logan et al. (1995) suggest that the evolution of the metazoan guts and facel pellets permitted rapid burial of waste material that would have otherwise used up oxygen as the waste products decayed in the water column. IS OCEAN CHEMISTRY THE KEY? The chemistry of the Earth’s oceans has changed many times. A significant event is recorded just before the base of the Cambrian (Narbonne et al. 2004). It is possible that this change may have permitted creatures to develop hard part skeletons at this time. However, direct evidence of any relationship has yet to be proved (Morris 2000). CONCLUSIONS It is likely that there is more than one specific reason for the Cambrian explosion. Biotic components and abiotic components probably had a role to play. In addition, other large scale factors such as paleo-plate configuration and perhaps unknown astronomical factors may have had influence on the sudden radiation of new life forms. Additional data from new Early Cambrian sections in China may help throw new light this important event. REBOUND FROM EXTINCTION Other Authors such as Kerr (2002) believe that the apparent Cambrian explosion is merely the radiation of new forms into niches left unoccupied by creatures following a mass extinction in the Precambrian. REFERENCES Ausich W. I. and Lane, G. N. 1999. Life of the Past, fourth edition. Prentis Hall, 321 pp. ISBN 0-13-896069-0. Briggs, E. G., Erwin. D. H. and Collier, F. J. 1994. The fossils of the Burgess Shale.Smithsonian, 238 pp. ISBN 1-56098-364-7. Kerr, R. A. 2002. Evolution: A Trigger for the Cambrian Explosion? Science, 298: 1547-1547. Logan G.A., Hayes J.M., Hieshima G.B. and Summons R.E. 1995. Terminal Proterozoic reorganisation of biogeochemical cycles. Nature, 376: 53-56. Morris, C. M. 1998 The Crucible of Creation. Oxford Univ. Press, 276 pp. ISBN 0192862022. Morris, C. M. 2000. The Cambrian "explosion": Slow-fuse or megatonnage? PNAS, 97(9): 4426-4429 Narbonne, G. M., Kaufman, A. J. and Knoll, A. H. 1994. Integrated chemostratigraphy and biostratigraphy of the Windermere Supergroup, northwestern Canada: Implications for Neoproterozoic correlations and the early evolution of animals. Geological Society of America Bulletin, 106: 1281-1292 Purves, W. K., Sadava, D., Orians, G. H. and Heller, C. H. 2003, Life: The Science of Biology, fourth edition. Freeman, 1121 pp. ISBN 0-7167-9856-5. Runnegar, B. 1992. Evolution of the earliest animals in Schopf J. W. [editor], Major Events in the History of Life.  Jones & Bartlett, 208 pp. ISBN 0867202688 Trichophycus Pedum trace fossils at Fortune Head Newfoundland. These earliest Cambrian traces demonstrate that metazoans were actively interacting with the environment at the time of the Cambrian explosion. Cloudina fossils demonstrating boringsBengston and Zhao, 1992 Precambrian Ediacaran Creatures. Did their extinction permit the Cambrian Explosion? Image from Morris 2000.