The Debate on Cambrian Explosion - Research Paper Example

?Cambrian Explosion The term Cambrian refers to period in the earth’s history roughly estimated as being between 543 and 490 million years ago. The name Cambrian derives from the discovery of the first fossils from this era by British geologist Adam Sedgwick in a part of Wales called the Cambrian Hills. (Parker, 2003, p. 7) This is an extremely important period in the development of life on earth, because many scientists believe that it begins with a relatively short period of some 5 million years, between 543 and 538 million years ago, which is known as “the Cambrian explosion”. Parker defines it thus: “The Cambrian explosion is the evolutionary episode in which all animal phyla attained complex external forms.” (Parker, 2003, p. 9) The phylum is the body plan of an animal, and in the period before 543 million years ago, called the Precambrian era, there were only three distinct animal phyla, whereas at 538 million years ago, there were 38, and this phenomenal increase is what gives rise to the term “Cambrian explosion.” Some aspects of this theory are, however, disputed, and there are studies which support a much more gradual development of the 38 animal phyla over a much longer time, reaching back long before the so-called explosion period from 543 million years ago. This paper examines the key arguments in favour and against the phenomenon known as the Cambrian explosion concludes with some reflection on what the implications of the Cambrian explosion debate are for modern science. Most scholars agree that there is a step-change in the fossil record at around 543 million years ago. There are some problems, however, in assuming that this surviving record showing a sudden increase in phyla variation gives us a true and complete picture of what actually happened. As scientists discover and analyse further samples, and as new technologies emerge which allow different kinds of analysis, the picture can change dramatically. Above all, it must always be remembered that the preservation of specimens is largely a matter of chance, and there is always the possibility that key pieces of evidence have not survived, or have not yet been discovered: “While fossils provide the only tangible evidence for the origin of animals, some key early metazoan fossils are rare and many lack informative characters, making the derivation of a rigorous phylogenetic hypothesis based solely on fossil data difficult.” (Cartwright and Collins, 2007) Referring to new technologies such as comparative genomics , Cartwright and Collins conclude that “currently available genomic data supports the origin of a complex genome predating the Cambrian radiation, with the ancestral genome possessing the molecular toolkit necessary for an ‘explosion’ of body plans and complex traits.” (Cartwright and Collins, 2007) This position modifies the theory of the Cambrian explosion only slightly, since it theorizes a large number of evolutionary changes before 543 million years ago, at the molecular level, which then had the effect of causing a rapid divergence of organisms in the Cambrian explosion period. Factors external to the animals then prompt the adaptations that we see in the physical record. Some of the more popular science books propose single factors as “triggers” for the Cambrian explosion, such as changes in the way that light affected the earth which in turn prompte the evolution of sight, which had the effect of increasing pressure on organisms to avoid being seen and eaten by other organisms (Parker, 2003). The importance of the Burgess shale deposits in British Columbia, Canada is recognized as a crucial piece of evidence and has been labelled as the indicator of a “Big Bang” in animal evolution. (Carroll, 2005, p. 138) The diversity of forms is explained in terms of adaptation to changes in the environment: “Genes in the tool kit are important actors in this picture, but the tool kit itself represents only possibilities, not destiny. The drama of the Cambrian was driven by ecology on a global scale.” (Carrol, 2005 p. 138). In the nineteenth century it was already evident to Darwin that the apparent rapidity of the Cambrian radiation as a potential argument against his carefully thought out theory of evolution. “In On the Origin of the Species … he argued that, should his theory prove true, the Cambrian radiation must actually be the taphonomic culmination of a long period of Precambrian evolution hidden from the fossil record and thus mistaken by palaeontologists for evolutionary quiescence.” (Lieberman, 1999) Some twentieth century studies agree with Darwin but not because they are attached to a theory of gradual evolution as such. Since the 1970s there has been some work which casts doubt on the theory of the Cambrian explosion for example in the detection of evidence that particular forces were at work on a very large scale in the environment at that time which may have influenced early life forms. Stanley highlights the role of “ near saturation of Precambrian algal systems in the absence of cropping by herbivors” (Stanly, 1975) which was a limiting factor on early life forms. There was, following this line of argument, a potential for earlier development of the eurkaryotic condition but it was delayed until conditions in the environment became more favorable. Lieberman, on the other hand, points to the large number of “tectonic events” during the late Neoproterozoic and Early Cambrian periods, such as extensive cratonic fragmentation and concludes that “This emphasizes the importance of physical earth history in generating evolutionary patterns” (Lieberman, 1999) These studies broadly support the hypothesis that there was a sudden evolutionary explosion which was indeed concentrated in this narrow time frame. Other recent studies postulate the existence in this period of intermediate life forms which were soft bodied like larvae, and which have not survived in the fossil record. (Davidson, Peterson and Cameron, 1995) Comparisons with on modern marine life, where there is a greater amount of phylum diversity, has led to some consideration of the distinction between animals which develop from a prototype of the adult body form, and animals which have larval body plans which are very different from the adult body form. The argument for the existence of such larvae in the Precambrian era rests on explaining the evolutionary advantages which they would have over single celled organisms: “locomotion, feeding, dispersal ability and site selection” and the demonstrable tendency of larval forms to evolve more quickly and converge more upon one another .(Sly, Snoke and Raff, 2003) Following this reasoning, the larval stage is the key in the Cambric radiation period, but also, most significantly, in the period immediately before: “the evolution of planktonic larvae did have an impact on diversity of metazoans by allowing more diversity in developmental modes, exploitation of new resources and increasing dispersal into new environments. As a consequence, the introduction of complex life histories would have accelerated the Cambrian radiation and helped produce more complex marine ecosystems.” (Sly, Snoke and Raff, 2003). This theory argues for an earlier development of the larval forms, followed by an accelerated radiation, but not explosion, in the early Cambrian period. The molecular clock method, has produced quite a number of conflicting studies, many of which have indicated much earlier origins for animal phyla in the period 800 to 1200 million years ago. (Blair and Hedges, 2004) In a meta-analysis of this topic, Blair and Hedges conclude that faulty methodology has resulted in recent studies in favour of the Cambrian explosion and concluded : “we have analysed larger sequence data sets with all available time estimation methods, including Bayesian, and found deep Precambrian divergences among animal phyla, consistent with clock studies over the past 3 decades.” (Blair and Hedges, 2004) This study supports a more leisurely time scale for phylum diversification suggests that the term “Cambrian radiation” more accurately represents what happened than “Cambrian explosion.” In conclusion, then, it seems that scientists from various specialized fields agree that the so-called Cambrian explosion is a feature of the fossil record, but there is by no means any agreement on whether this represents reality, or is due to the way fossils are preserved and discovered. Furthermore, there is still disagreement on how far back in time the changes from single celled to multi celled and more complex organisms are likely to have taken place. It appears that the genetic potential is present very much earlier, and that there are likely to have been forms which have not survived in the fossil record. It seems, then, that modern science can accept the term “Cambrian radiation” based on the evidence that survices, but that the more rapid descriptors like “explosion” and “big bang” may be an overstatement of the facts. References Blair, J.E. and Blair Hedges, S. (2004) Molecular Clocks Do Not Support the Cambrian Explosion. Molecular Biology and Evolution 22 (3), 387-390. Carroll, S.B. (2005) Endless Forms Most Beautiful: The New Science of Evo Devo. New York: W.W. Norton. Cartwright, P. and Collins, A. (2007) Fossils and phylogenies: integrating multiple lines of evidence to investigate the origin of early major metazoan lineages. Integrative and Comparative Biology 47 (5), 744-751. Davidson, E.H., Peterson, K.J. and Cameron. R.A. (1995) Origin of bilaterian body plans: evolution of developmental regulatory mechanisms. Science 270 (5240), 1319-1325. Lieberman, B.S. (1999) Testing the Darwinian Legacy of the Cambrian Radiation Using Trilobite Phylogeny and Biogeography. Journal of Paleontology 73 (2), 176-181. Parker, A. (2003) In the Blink of an Eye: How VISION sparked the big bang of evolution. Cambridge, MA: Basic Books. Sly, B., Snoke, M.S. and Raff, R.A. (2003) Who came first – larvae or adults? Origins of bilaterian metazoan larvae. International Journal of Developmental Biology 47, 623-632. Stanley, S.M. (1976) Ideas on the timing of metazoan diversification. Paleobiology 2. 209-219. Read More