Environmental Settings of the Cambrian Thesis

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" (Sukumaran, 2004) Mutation is what results in the difference and may be utilized as a measure of the time that has elapsed since separation of the species from the common ancestor during evolution. This is a method of "inferring the divergence of time of clades from a common ancestor by means of gene/protein sequencing" and has been termed 'molecular dating'. The process is one in which there is a calibration of time in comparison to the Phanerozoic era fossil data and then expoliation is conducted for providing the estimation time for divergence of phyla. (Sukumaran, 2004; paraphrased) Indeed, if life did evolve as posited in the work of Charles Darwin then "the abrupt appearance of diversified life at the beginning of the Cambrian period was not explainable." (Sukumaran, 2004) However, Sukumaran explains that gradualism is not a central tenet to the idea that there has been an evolution of life forms through the process of natural selection and at the insistence of TH Huxley, Darwin essentially dropped the 'gradual' from his idea of evolution although he did have great hope that science would solve this problem. However, the "mystery remains." (Sukumaran, 2004)

V. Features of the Cambrian Explosion

It is related that the animals arising during the Cambrian explosion "represent unusual evolutionary mileposts." (Sukumaran, 2004) in fact, two-thirds of the phyla of kingdom Animalia is stated to be Cambrian animals and 37 of these are "present day animals, and almost all of them were established by the close of the Cambrian explosion, including all shelled invertebrates like mollusca, echinodermata and arthropoda. Thus, the evolution of major life forms as represented in the fossil record is not a gradual one: there was very little diversification of basic body plan in the Precambrian, but a sudden burst at the early Cambrian." (Sukumaran, 2004) Sukumaran notes that also remarkable in regards to the Cambrian explosion is "the remarkable morphological disparity and evolutionary stasis displayed by Cambrian fauna." (Sukumaran, 2004) it is important according to Sukumaran to differentiate "between diversity and disparity among animals" in that diversity 'refers to small-scale differences that are evident at the species level, whereas large-scale morphological differences among animals evident at taxonomically higher levels than species are generally referred to as disparity." (2004)

While there are about 37 basic body architectures of the Cambrian explosions "each of these body plans exhibit clear morphological differences or disparity from the others." (Sukumaran, 2004) Specifically, these body plans providing a definition for each phylum "do not grade into one another over the course of geological history but maintain their morphological isolation or disparity from all other phyla" and furthermore each of this "also exhibit a remarkable stability or statis during their time on earth, meaning that after their appearance they maintained their characteristic body architecture without any evidences of alteration." (Sukumaran, 2004) Another feature of the Cambrian explosion noted by Sukumaran is the "quantum jump in biological complexity" because when the Cambrian explosion is compared to the "small increase in complexity that occurred between the origin of life ~3.85 Ga ago and the first appearance of multicellular algae (1 Ga ago), the Cambrian explosion is a huge increase in biological complexity." (2004)

VI. Theories Attempting to Explain the Cambrian Explosion Examined

Sukumaran also relates that the missing artifact theory does not hold up to close inspection because "the lower Cambrian sediments near Chengjian, China have preserved soft tissues and several organs such as eyes, stomachs, digestive glands, sensory organs and nerves, besides fossilized embryos..." all of which are observations that "shake the very foundations of the artifact theory." (2004) Sukumaran states that the theory of 'deep divergence' is one that attempts to explains the possibility that the metazoans "had an invisible evolutions history in the Precambrian" which failed to be recorded in the fossil record. There are however, several shortcomings stated to exist in regards to this theory: (1) an extensive period of soft-bodied evolution is questionable from a paleontological point-of-view. Preservation of numerous soft-bodied Cambrian animals as well as Precambrian embryos and microorganisms undermines the deep divergence hypothesis; (2) Subsequent molecular estimates by Ayala and colleagues (1998) are in agreement with paleontological evidence, questioning the deep divergence hypothesis; (3) the proteins that Wray and colleagues have analyzed are not involved in the development of animal body plans and therefore they would not have played any role in the origin of new phyla; (4) Another major problem with the deep divergence hypothesis is that the protein clock does not tick at a constant rate. Unlike radiometric clocks widely used in dating rocks, molecular clocks depend upon both biological and environmental factors. For instance, different genes in different clades evolve at different rates; and (5) Above all the molecular clock is calibrated with Phanerozoic (<550 Ma) fossil record that may not be reliable in dating the origin of the much older (1 to 1.2 Ga old) animal phyla." (Sukumaran, 2004)

Those who support the deep divergence hypothesis place emphasis on the fact that "...molecular data only document the time of divergence of lineages while paleontological data record the divergence of morphological form." (Sukumaran, 2004) However, it is stated in Sukumaran's work that it is not necessary to correlate morphological and genetic changes over the time of evolution. "The morphological expression and therefore preservation of organisms as fossils occurs millions of years after lineages diverge; there may be long gaps between separations of sister groups from a common ancestor and manifestation of their morphological disparity as fossils." (2004)

VII. Cambrian Explosion from the View of Modern Genetics

Sukumaran explores the mystery of the Cambrian explosion from the view of modern genetics and states that unicellular life "...is relatively simple; there is little division of labor and the single cell performs all functions of life. Obviously the genetic information content of unicellular organisms is relatively meager. Multicellular life, on the other hand, requires more genetic information to carry out myriads of cellular functions as their cells are differentiated into different cell types, tissues and organs. But new cell types themselves require specialized proteins, and novel proteins arise from novel gene sequences, that is new genetic information. As the organisms that appeared in the Cambrian explosion had many more novel and specialized cell types than their prokaryotic ancestors, the amount of new genetic information that arose in the Cambrian explosion represents a large increase in biological information. In metazoans, cell differentiation from simple to multiple cell types is controlled by genetic mechanisms. It is now understood that cells differentiate from master cells called stem cells present in embryos (embryonic stem cells). It is this cell differentiation that led to evolution of various body plan architecture in the Cambrian explosion. Now the question is were there genetic controls in early eukaryotic cells for cell differentiation. Many different classes of regulatory genes are present in metazoans that are responsible for their body architecture. A set of such regulatory genes called Hox genes occur in clusters in DNA segments of metazoans. These genes are best known for their roles in determining developmental patterns in model biological systems such as the fruit fly Drosophila and the nematode worm Caenorhabditis. It appears that Hox genes are widely and probably universally distributed in metazoan phyla. Molecular data confirm that these regulatory genes were already in place before the Cambrian explosion." (Sukumaran, 2004)

Sukumaran states that there were several factors that contributed in a collective manner to the Cambrian explosion: (1) the role played by regulatory genes in that gene sequencing has constructed new metazoan phylogenies which give rise to the suggestion that "much of the basic gene regulatory machinery required for their body plan development was in place significantly before the Cambrian explosion; (2) the Cambrian explosion of animals happened immediately after the late Proterozoic snowball earth event" and is has been shown that extreme climatic events result in a catastrophic effect on the biosphere. All forms of eukaryotic life would have perished under the climatic stress of a global ice cover. The congenial habitats created on the earth by the disappearance of global ice cover are believed to have facilitated the explosive radiation of multicellular life in the early Cambrian; (3) the increase in molecular oxygen is likely to have been crucial. (Sukumaran, 2004) Sukumuran questions whether the Cambrian explosion was in reality an evolutionary event or if indeed this period actually represents a "mineralization event." (2004) Valentine states in regards to the disparity in life forms that the structural elements "...deployed in the skeletons of Burgess Shale animals (Middle Cambrian) incorporate 146 of 182 character pairs defined in this morphospace. Within 15 million years of the appearance of crown groups of phyla with substantial hard parts, at…

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