Cambrian explosion: Difference between revisions
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The '''Cambrian explosion''' refers to the geologically sudden appearance of complex multi-cellular macroscopic [[organisms]] between roughly 542 and 530 [[million]] years ago ([[mya (unit)|mya]]). This period marks a sharp transition in the [[fossil record]] with the appearance of the earliest members of many [[phyla]] of [[metazoan]]s (multicelluar animals). The "explosive" appearance of this [[adaptive radiation]] results both from rapid evolutionary change and the limits of previous technology to appreciate [[microfossil]]s which formed the foundation of the fossil record before this time. |
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== Fossils == |
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The '''Cambrian Explosion''' refers to the geologically sudden appearance of a number of new complex [[Metazoa|macroorganisms]] between 543 and 530 million years ago ([[mya (unit)|mya]]). The apparently explosive radiation from obscure beginnings was partly an artifact of disregarding [[microfossil]]s, which were scarcely detectable with 19th-century technology, and concentrating solely on the hard-shelled ''macrofossils'' that defined the [[Phylum|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 [[skeleton|skeletal]] body features, whereas the traces of the hard-to-analyze "[[small shelly fauna]]" of Cambrian beginnings were ignored. |
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This period of [[evolution]] is source to some of the most unusual [[fossil]]s ever recovered. A single formation, the [[Burgess shale]], has provided some of the best insights into this period of dramatic evolutionary change and experimentation that laid the foundation for most major modern animal body plans. Also appearing at this time are a wide variety of enigmatic and exotic configurations that appear to be unrelated to any modern animals. |
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With time, advanced [[microscopy]] has gradually revealed the range of earlier [[microfossil]]s. 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. |
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Before the explosion, the fossil record is dominated by single-[[Cell (biology)|celled]] organisms with only the rare soft-bodied [[Ediacaran fauna]] and certain microfossils showing that multi-cellular life forms had arisen roughly 30 million years earlier (Xiao et al. 1998). |
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More recent microfossil finds have showed "Pre-Cambrian" life consisting of more than [[Protista|single- celled organism]]s or simple [[diploblastic]] fauna (two-layers of cells in mats and sheets, allowing every cell to be in contact with its watery mineral-rich environment). In 1994, [[triploblastic]] animals (organisms with more than two layers, and who therefore rely on internal organs and systems for their cells' supplies of food and waste disposal), were discovered preserved as phosphatized embryos in rocks from southern China [Xiao et al. 1998]. These fossils were estimated to be 570 million years of age and thus were even older than the [[Ediacaran]] fauna found in strata about 10 million years younger. |
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With the Cambrian explosion came the evolution of [[shell]]s and other hard body parts. As shells are more easily preserved in sediment than soft body parts, this makes life forms of this and subsequent periods much easier to study in the [[fossil record]] than their [[Precambrian]] counterparts. This also contributes to the perception of an abrupt change in the fossil record. |
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Thus, more recent research has reached a new consensus, agreeing that this [[adaptive radiation]] of [[animal]] [[Phylum_(biology)|phyla]] started about 30 million years earlier, around 570 mya, before the official beginning of the [[Cambrian]] [[geologic period]]. |
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== |
== Causes of the Cambrian Explosion == |
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The extinctions connected with the [[Varangian glaciation]] that preceded this radiation of newer phyla (popularized as '[[Snowball Earth]]'), along with subsequent [[greenhouse warming]] of the Earth, is theorized to have provided the [[evolution|evolutionary impetus]]. It is thought that the development of [[sexual reproduction]] increased the rate of evolutionary change. |
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The Cambrian explosion may have been precipitated by several enviormental changes occuring 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. |
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Evidence for earlier [[multicellular]] animal forms, which may have been the precursors of this radiation, date from 600 mya. Notable among these are [[trace fossil]]s in the form of imprints of [[animal]]s and their activities, such as [[burrow]]s in mud, produced by animals that [[paleontologist]]s call the [[Ediacarian|Ediacaran]] [[Fauna (animals)|fauna]]. These [[organism]]s were soft-bodied and are found with various strange body forms. The so-called [[small shelly fauna]] of the ensuing [[Tommotian]] period included ''[[Cloudinia]]'' and its kin. |
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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 maintainence of such colonies also assisted in the formation of the first [[triploblastic]] animals (organisms with more than two layers of cells) 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 to impetus for rapid evolution to fill many new environments. |
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The original and most widely-publicized source of fossils from the actual radiation period is the [[Burgess shale|Burgess Shale]] in [[British Columbia]]. Some Burgess Shale organisms display strikingly unusual body plans that are not easily connected with any phyla known since. |
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== Diversification == |
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A popular account of the paleontological analysis of the Burgess Shale is given in ''[[Wonderful Life (book)|Wonderful Life]]'' by [[Stephen Jay Gould]]. |
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Of the 20 metazoan phyla with extensive fossil records, at least 11 first appeared in the Cambrian. Of the remainder, 1 is known to Precambrian and the other 8 first appear more recently (Collins 1994). An additional 12 soft-bodied phyla have poorly defined fossil records, but it is speculated that a significant number of these may also be Cambrian in origin. |
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The Cambrian Explosion has recently been a controversial topic regarding the history and [[evolution]] of life, with the idea posited that the [[Burgess shale|Burgess Shale]] preserved such a wide variety of life and that the "Cambrian Explosion" was actually a slower radiation of animal forms than previously thought. The idea of an "explosion" of life in the Cambrian period is still being debated. |
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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). |
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The debate centers in part around an earlier notion that all phyla in existence today (and all others now extinct) except one were first found in this period. If one were to compare the timeline of life (from 4bya) to a single 24-hour day, it is as though all phyla sprang into being between 8:45pm and 8:49pm. |
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According to more recent research, only some phyla appear in the Cambrian explosion. On talkorigins.org, in response to the Creationist claim of sudden appearance, Mark Isaak [Isaak 2004] gives the following summary: |
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:Only some phyla appear in the Cambrian explosion. In particular, all plants post-date the Cambrian, and flowering plants, by far the dominant form of land life today, only appeared about 140 Mya [Brown 1999]. |
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:Even among animals, not all types appear in the Cambrian. Cnidarians, sponges, and probably other phyla appeared before the Cambrian. Molecular evidence shows that at least six animal phyla are Precambrian [Wang et al. 1999]. Bryozoans appear first in the Ordovician. Many other soft-bodied phyla don't appear in the fossil record until much later. Although many new animal forms appeared during the Cambrian, not all did. According to one reference [Collins 1994], 11 of 32 metazoan phyla appear during the Cambrian, one appears Precambrian, 8 after the Cambrian, and 12 have no fossil record. |
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:And that just considers phyla. Almost none of the animal groups that people think of as groups, such as mammals, reptiles, birds, insects, and spiders, appeared in the Cambrian. The fish that appeared in the Cambrian were unlike any fish alive today. |
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The sheer variety of forms found in the Burgess shale and other sites, has made some skeptical that single period of ~10-15 million years could have been long enough to give rise to such diversity. 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. |
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==References== |
==References== |
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Collins, Allen G., 1994. Metazoa: Fossil record. http://www.ucmp.berkeley.edu/phyla/metazoafr.html . |
Collins, Allen G., 1994. Metazoa: Fossil record. http://www.ucmp.berkeley.edu/phyla/metazoafr.html . |
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Isaak, Mark, 2004. "CC300: Cambrian Explosion." http://www.talkorigins.org/indexcc/CC/CC300.html. Copyright © 2004. |
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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: 163-71. |
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: 163-71. |
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Revision as of 02:09, 15 December 2005
The Cambrian explosion refers to the geologically sudden appearance of complex multi-cellular macroscopic organisms between roughly 542 and 530 million years ago (mya). This period marks a sharp transition in the fossil record with the appearance of the earliest members of many phyla of metazoans (multicelluar animals). 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.
Fossils
This period of evolution is source to some of the most unusual fossils ever recovered. A single formation, the Burgess shale, has provided some of the best insights into this period of dramatic evolutionary change and experimentation that laid the foundation for most major modern animal body plans. Also appearing at this time are a wide variety of enigmatic and exotic configurations that appear to be unrelated to any modern animals.
Before the explosion, the fossil record is dominated by single-celled organisms with only the rare soft-bodied Ediacaran fauna and certain microfossils showing that multi-cellular life forms had arisen roughly 30 million years earlier (Xiao et al. 1998).
With the Cambrian explosion came the evolution of shells and other hard body parts. As shells are more easily preserved in sediment than soft body parts, this makes life forms of this and subsequent periods much easier to study in the fossil record than their Precambrian counterparts. This also contributes to the perception of an abrupt change in the fossil record.
Causes of the Cambrian Explosion
The Cambrian explosion may have been precipitated by several enviormental changes occuring 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 maintainence of such colonies also assisted in the formation of the first triploblastic animals (organisms with more than two layers of cells) 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 to impetus for rapid evolution to fill many new environments.
Diversification
Of the 20 metazoan phyla with extensive fossil records, at least 11 first appeared in the Cambrian. Of the remainder, 1 is known to Precambrian and the other 8 first appear more recently (Collins 1994). An additional 12 soft-bodied phyla have poorly defined fossil records, but it is speculated that a significant number of these may also 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 sheer variety of forms found in the Burgess shale and other sites, has made some skeptical that single period of ~10-15 million years could have been long enough to give rise to such diversity. 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.
References
Collins, Allen G., 1994. Metazoa: Fossil record. http://www.ucmp.berkeley.edu/phyla/metazoafr.html .
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: 163-71.
Xiao, Zhang, and Knoll, "Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite," 1998, Nature, vol. 391, pp. 553-58.