Cambrian explosion

The Cambrian explosion refers to the geologically sudden appearance of complex multi-cellular macroscopic organisms between roughly 542 and 530 million years ago. This period marks a sharp transition in the fossil record with the appearance of the earliest members of many phyla of metazoans (multicellular animals) including the first known vertebrate (Myllokunmingia). The "explosive" appearance of this adaptive radiation results both from rapid evolutionary change and the limits of previous technology to appreciate microfossils which formed the foundation of the fossil record before this time.

From the modern point of view, the apparently explosive radiation from obscure beginnings was partly an artefact of disregarding microfossils, which were scarcely detectable with 19th-century technology, and concentrating solely on the hard-shelled macrofossils that defined the phyla well established by 19th-century biologists, all of which were multiple-celled metazoa. Apparently abruptly, many kinds of fossils appearing in the Burgess Shale were seen showing obvious skeletal body features, whereas the traces of the hard-to-analyze "small shelly fauna" of Cambrian beginnings were ignored.

With time, advanced microscopy has gradually revealed the range of earlier microfossils. Prior to the discovery in 1909 of the Burgess Shale—incompletely published at the time and largely forced into existing categories as "precursors"— no fossilizations of early soft-bodied organisms had been published, and the vast reach of undiscovered earlier life was consigned to an enormous space of time— the "Pre-Cambrian" of old-fashioned schoolbooks.

More recent microfossil finds have shown "Pre-Cambrian" life consisting of more than single-celled organisms or simple diploblastic fauna. In 1994, phosphatized triploblastic embryos were discovered in rocks from southern China (Xiao et al. 1998). Evidence for Ediacaran triploblasts was available long before this discovery.

Causes of the Cambrian explosion

There is no universally accepted cause, and the matter is the subject of ongoing debate within the scientific community. A wide range of biological and geological factors have been proposed as possible triggers for the explosion. These range from ecological competition, hox genes and the breakup of Rodinia. Recently scientists have suggested major climatic changes, including a near-global glaciation, may have played a role.

The Cambrian explosion may have been precipitated by several environmental changes occurring in and just before this period. First the Varangian glaciation gave rise to a Snowball Earth in which all, or nearly all, of the oceans are covered entirely with ice. This was followed by a deglaciation and rapid global warming just before the beginning of the explosion itself. In modern Arctic environments, single-celled organisms often form mats on the underside of ice sheets in order to maximize their exposure to sunlight. It is possible that adaptations useful to the maintenance of such colonies also assisted in the formation of the first triploblastic animals estimated to be 570 million years of age (Xiao et al. 1998). In addition, the Snowball Earth environment would have given rise to relatively few ecological niches, so the subsequent deglaciation and global warming may have provided an impetus for rapid evolution to fill many new environments.

Rising levels of photosynthetic atmospheric oxygen produced by mats of photosynthetic bacteria and algae during the Ediacaran may have played a role in the emergence of large metazoans that required oxygen for respiration. Recent work has linked this increase in atmospheric oxygen to changes in global soil weathering patterns following the Cryogenian and the possible emergence of a primitive terrestrial biota (Kennedy et al. 2006).

Yet another thoery describing this occurence is simply that the evolution of the eye caused intense diversification because predators could better locate prey, and vice versa. (read In the Blink of an Eye)

And, the final theory is that the massive and sudden influx of animal life was the work of God.

Diversification

Of the 20 metazoan phyla with extensive fossil records, at least 11 first appeared in the Cambrian. Of the remainder, one is known from Precambrian and the other eight from the Phanerozoic eon (Collins 1994). An additional 12 soft-bodied phyla have poorly defined fossil records, many of which are conjectured to be Cambrian in origin.

Though this period is definitely of special significance in terms of rapid diversification and the emergence of new forms, some of that significance is likely to be overstated by the focus on macroscopic forms in the ways phyla are observed and defined. Molecular evidence suggests that at least six animal phyla had established themselves as distinct evolutionary paths during the Precambrian (Wang et al. 1999).

The large variety of fossil life forms found in the Burgess shale, the much younger Maotianshan shales of China, and more than three dozen other Cambrian sites on every continent except Antarctica leads many researchers to be skeptical that 10 to 15 million years is insufficiently long to explain the great diversity exhibited in the Cambrian fossil record. An emerging view is that the Cambrian explosion is the macroscopic conclusion to a prolonged period of evolution begun ~30 million years earlier with the innovation of multi-cellular organisms.

The Cambrian "explosion" continues to fascinate the non-specialist. Stephen Jay Gould's Wonderful Life (1989) provides the best general account for the layman of the paleontological analysis of the Burgess Shale; the book was the 1991 winner of The Aventis Prizes for Science Books.

References

• Collins, Allen G. "Metazoa: Fossil record". Retrieved Dec. 14, 2005.
• Kennedy, M., M. Droser, L. Mayer., D. Pevear, and D. Mrofka (2006). "Clay and Atmospheric Oxygen". Science. 311 (5766): 1341. doi:10.1126/science.311.5766.1341c.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Wang, D. Y.-C., S. Kumar and S. B. Hedges (1999). "Divergence time estimates for the early history of animal phyla and the origin of plants, animals and fungi". Proceedings of the Royal Society of London, Series B, Biological Sciences. 266 (1415): 163–71. doi:10.1098/rspb.1999.0617.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Xiao, S., Y. Zhang, and A. Knoll (1998). "Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite". Nature. 391: 553–58. doi:10.1038/35318.{{cite journal}}: CS1 maint: multiple names: authors list (link)

External links

• The Cambrian "explosion" of metazoans and molecular biology: would Darwin be satisfied?
• On embryos and ancestors by Stephen Jay Gould
• The Cambrian "explosion": Slow-fuse or megatonnage?