Environmental Settings of the Cambrian Explosion
The objective of this work is to examine the development of natural environments alongside the evolution of life throughout the Cambrian explosion. This work will focus on beginnings of life, their natural environments and their evolution in changing environments from the beginning to the end of the Cambrian explosion. The key theme of this essay is the history of life during the Cambrian explosion. This work will attempt to deduce patterns of evolution alongside the development of natural environments.
The Cambrian explosion has the appearance of being a dramatic and sudden appearance of various complex animals approximately 540 million years ago however; it is know that this was not the origin of complex life forms. (Morris, 1985) There are two views on the Cambrian explosion. The first view is that it was indeed a true explosion and that an organism, yet unidentified, was the inventor of biomineralization which resulted in rapid radiation of shelled creatures over a period of ten million years. The other view is that there was a diversification of the phyla earlier and that these developed evolving with biomineralized shells at approximately the same time which is attributed to mineral availability at appropriate levels, rising O2 levels or being triggered by predation appearing. The focus of this study is the problem that is noted in the rapid and mass evolution of animal forms which occurred during the Cambrian explosion.
I. The Burgess Shale – 505 MYA – British Columbia, Canada
The Burgess shale is a Cambrian location in British Columbia which claims fossils of preserved soft-bodied specimens and as well contains species in large number generally not found preserved in the fossil record. This site was discovered in 1907 by Walcott and the fossils were reanalyzed in the 1970s by Whittington and a group that utilized more modern type equipment. After years of analysis they disagreed with Walcott, who claims that the fossils were Metazoan phyla and classes and stated that they represented Metazoan phyla of a novel type. The reconstruction of these fossils resulted in strange creatures with unusual patterns of body. The contemporary understanding of evolution is stated by Fenchel (2002) to explain directional evolution as the replacement of alleles in species populations driven by natural selection.” (2002) Fenchel states that there is “an overwhelming amount of theoretical and experimental documentation as well as data deriving from paleontology and natural history” that provides evidence that this “description is fundamentally correct.” (2002) it has been suggested by some in their writings that the Cambrian explosion resulted from a rise in the oxygen in the atmosphere
The base of the Cambrian Period as well as the base of the Cambrian system of classification is located in Newfoundland and is stated to be “where a particular trace fossil, known as Trichophycus pedum” (the Cambrian Explosion, nd) first appeared. There are stated to be only a few fossil animals dating back 543 million years ago such as these and the Ediacaran assemblage has for the most part disappeared with only a few very small smelly organisms or shelly fauna appearing until the Cambrian Explosion. Bowring et al. (1993) states of the Cambrian Explosion: “This explosion is perhaps the most striking single event documented by the fossil record. In the strict sense, the explosion refers to a geologically abrupt appearance of fossils representing all except two of the living [animal] phyla that had durable (easily fossilizable) skeletons. One of those two phyla is the Porifera (sponges), which was present in the fossil record at an earlier time. The other is the Bryozoa, a phylum that contains some soft-bodied groups and may well have been present but not yet skeletonized. A number of enigmatic organisms of obscure relationships also appear during the explosion, enriching the early Cambrian fauna. Precision dating indicates that the explosion began at 530 Ma (million years ago) and ended before 520 Ma.” (Bowring, et al., 1993)
Documented in the Cambrian fossil record is a radiation of skeletal morphologies of a very real and dramatic nature representing the skeletonization of many lineages that were previously soft-bodied as well as an accelerated diversification within lineages that were already skeletonized. Resulting is stated to be a “disparity – as measured by the number of major animal types – was at least as great as the present.” (Gon, 2005)
II. Maotianshan shales (Chengjiang), 525 MAY, Yunnan Province, China
The Chengjiang biota was discovered by Hou Xianguang in 1984. These are soft-bodied animals which have been preserved in fine mudstones with four species recorded including: (1) Eoredlichia intermedia; (2) Kuanyangia pustulosa; (3) Yunnanocephalus yunnanesis; and (4) Wutingaspis tingi. (Gon, 2005)
III. Emu Bay shale, 525 MYA, South Australia
The Emu Ban Shale formation is located on Kangaroo Island in South Australia and is inclusive of faunal elements including “Anomalocaris, Tuzoia, Isoxys, Xandarella, and Primicaris.” This site also features wonderful specimens of trilobites including: (1) Redlichia takooensis; (2) Emuella polymera, (3) Balcoracania dailyi, and (4) Estaingia (=Hsuaspis) bilobata. (Gon, 2007) the Emu Bay shale is found in shallow water deposition which indicates that preservation of soft tissue “occurred in a range of environmental settings during the Cambrian.” (Gon, 2007) Gon writes that the Emu Bay Shale was at first thought to be late in the Cambrian Explosion however calibration reveal that the “occurrence of R. takooensis and species of Hsuaspis matches the Tsanglangpuian in the Chinese sequence and contemporary South Australian faunas correlate with the Botomina of Siberia. (Gon, 2007) Therefore, Gon states that the Emu Bay Shale age lay between the “Middle Cambrian Burgess Shale, and the upper Atdabanian Chengjian of China.” (Gon, 2007)
IV. Sirius Passet, 518 MYA, Greenland
The work of James W. Valentine (2002) entitled: “Prelude to the Cambrian Explosion” relates that the Sirius Passet fossil located in Greenland dates “from or immediately after the explosion interval” and has “yielded fossils that were preserved under exceptional circumstances that many details of their soft-bodied anatomy can be observed. Many of these softbodied forms belong to phyla that lack durable skeletons altogether and would not be known from those early times except for the unusually preserved fossil assemblages.” (Valentine, 2002) Valentine states that it can be “said with confidence…that the biological factors necessary to produce the Cambrian explosion were evolved during late Neoproterozoic and earliest Cambrian times, an interval which forms a Prelude to that remarkable event.” (2002) Valentine states that the Early Cambrian fossil metazoan “…provides the best indication as to what must have been accomplished during the evolutionary Prelude to the Cambrian explosion. The base of the Cambrian is marked by the appearance of larger penetrating burrows.” (2002) the boundary is stated to be drawn “at the earliest appearance of the trace fossil Trepnichnus pedum in the Chapel Island Formation, Burin Peninsula, Newfoundland.” (Valentine, 2002) Valentine relates that T. pedum is “an arcuate horizontal burrow from which branches rise to probe toward the surface. The earliest Cambrian stage, the Manyiaian or Nemakit-Daldyn, began approximately 443 mya and lasted until the explosion, a period that is at least 13 Ma long and may be as long as 23 Ma” according to Grotzinger et al. (1995) and Landing et al. (1998). Valentine additionally relates that lower Cambrian trace fossils “are generally larger than those of the Neoproterozoic.” (2002) Valentine states that there are in existence “…a number of ecologically-based hypotheses that speak to the taxonomic richness of the Cambrian explosion…” which are of the nature that may “imply conditions during the prelude…” (2002)
The work of PV Sukumaran (2004) entitled: “Cambrian Explosion of Life: The ‘Big Bang’ in Metazoan Evolution” Recent advances in genetics and molecular biology have shed new light on genetic controls of body plan development in metazoan phyla. New fossil discoveries and molecular techniques have also brought in controversies: do the metazoans have deep evolutionary roots in the Precambrian or do the paleontological data confirm a major evolutionary mile- post in the early Cambrian? Besides, molecular evidence shows that regulatory genes that control development of morphology in animals are fairly similar in all phyla, but give rise to very disparate body plans. These advances are providing new lines of evidence to look into the origin of metazoans and thereby into the mystery of the Cambrian explosion.” (Sukumaran, 2004) According to Sukumaran (2004) the ‘neutral theory of molecular evolution “postulated by Motoo Kimura” holds that the majority of the nucleotide substitutions in genes arising out of mutation are selectively neutral or of little functional consequence to the organisms.” However, it is related that there was a slight modification of this theory later on for accommodation of the “observation that most molecular evolutions involves slightly deleterious substitutions rather than strictly neutral ones.” (Sukumaran, 2004) Sukumaran states that molecular data is comprised of “long sequences of nucleotide bases in the nucleic acids” forming the genetic text in DNA. Sections of nucleotide sequences which “code for a particular protein are called genes” which are nucleotide sequences yet the product of their protein are amino acid sequences.” (Sukumaran, 2004) Variations in protein results from gene mutations and that example stated is that while many species of animals have the protein haemoglobin present with the function of carrying oxygen from the lungs to the other body parts that the ‘amino acid sequence of haemoglobin is not the same in all species.” (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 least 80% of skeletal design elements recognized among living and extinct marine metazoans were exploited However, the species diversity produced by the Cambrian radiation, judged at the familial and generic levels, was much below that of succeeding periods.” (2002) Valentine notes the work of Knoll (1996) who stated of the early Cambrian metazoan radiation that it was “…accompanied by a marked diversification of organic-walled microorganisms. Early Cambrian protists include prasinophyte and dasyclad green algae, benthic foraminifers, and a diversity of acritarchs. Prokaryotes are represented by the carbonaceous and calcified remains of cyanobacteria, as well as by biogeochemical signatures, both isotopic and molecular.” (cited in Valentine, 2002) Valentine states that it is problematic in considering how as stated by Thomas et al. (2000) that”…a span of 40 million years embraces the appearance of the first small, simple shells that may have been secreted by metazoans and the subsequent exuberant diversity of Chengjiang and the Burgess Shale. This is not so short a time for an evolutionary ‘explosion.’ However, the proliferation of animals with well-differentiated hard parts characteristic of specific metazoan phyla was largely restricted to the last 15 million years of this interval.” (as cited in Valentine, 2002)
Whatever the reason, and modern genetics has effectively shed light on the mystery of the Cambrian explosion, this period of time in the development and evolution of animals that are complex in nature was one that witnessed a great diversification as well as great disparity in the evolutional development of complex animals which the earth had not witnessed prior to the Cambrian explosion and which the earth has not witnessed since the gigantic leap of animal development during the Cambrian explosion.
Fenchel, Tom (2002) the Origin and Early Evolution of Life. Oxford University Press 2002.
Wray et al., Molecular evidence for deep Precambrian divergence among metazoan phyla, Science, Vol. 274, pp. 568-573, 1996
Gon, S.M. III (2005) Trilobites of Chengjiang, China. 27 Apr 2005. Online available at http://www.trilobites.info/Chengjiang.htm
Gon, S.M. III (2007) Trilobites of the Emu Bay Shale, Australia 7 July 2007. Online available at http://www.trilobites.info/Emu.htm.
Knoll and SB Caroll, Early animal evolution: Emerging views from comparative biology and geology, Science, Vol. 284, pp.2129-2137, 1999.
Morris, Henry M. 1985. Scientific Creationism. Green Forest, AR: Master Books, pp. 80-81.
Watchtower Bible and Tract Society. 1985. Life — How Did it Get Here? Brooklyn, NY, pp. 60-62.
Sukumaran, PV (2004) Cambrian Explosion of Life: the Big Bang in Metazoan Evolution. Resonance 2004 September. Available online at http://www.geneseo.edu/~Bosch/Sukumaran.pdf
The Cambrian Explosion (nd) the Paleontology Page. Online available at http://www.peripatus.gen.nz/Paleontology/CamExp.html
Valentine, James W. (2002) Prelude to the Cambrian Explosions. Annu. Rev. Earth Planet. Sci. 2002. 30:285-306
The Environmental Settings of the Cambrian Explosion
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