|
ScienceWeek
EVOLUTION: ON THE CAMBRIAN EXPLOSION
The following points are made by L.S. Jermiin et al (Science 2005 310:1910):
1) The origin and evolution of animals have remained hotly debated issues ever since Darwin drew attention to the relative paucity of fossils from the Precambrian, which ended 543 million years ago (Mya) [1]. On the one hand, a growing collection of exquisitely preserved fossils of soft-bodied animals from the Cambrian has highlighted the existence of Cambrian representatives of most of the living animal phyla [2]. This has given rise to the "Cambrian Explosion" hypothesis [3] that most animal phyla arose ~543 Mya within a short period. On the other hand, studies of increasingly large molecular data sets suggest that many of the same phyla arose in the Precambrian [4], leading to considerable and occasionally intemperate debate on the timing of divergence of animal phyla. New work [5] presents a new aspect to this controversy.
2) Motivated by the desire to resolve the early evolution of animals, while accounting for insufficient data from some taxa, Rokas et al[5] used an alignment of 12,060 amino acids, encoded by 50 genes, to infer the phylogeny of 16 representatives from nine animal phyla. Eleven of these were from Porifera, Cnidaria, Platyhelminthes, Mollusca, Annelida, and Priapulida. These six phyla have not been adequately represented in recent molecular studies of early animal evolution [4]. Given the aligned data, Rokas et al[5] inferred a phylogeny with several distinct speciation events that were consistently supported by the data, but with a conspicuous polytomy (multiple concurrent divergence events) involving the protostome phyla, and another involving the bilaterate, cnidarian, and poriferan lineages. Recognizing that a polytomy may be due to rapid speciation, or to poor or insufficient data, Rokas et al[5] then explored whether convergence, "rogue" sequences, compositional heterogeneity, missing or inadequate data, or mutational saturation could have affected the phylogenetic estimate, and found that none of these were likely. In the absence of other explanations, they concluded that their data support rapid speciation during the early evolution of animals.
4) Is the conclusion drawn by Rokas et al. sound? At first glance, it appears so, but can their conclusion stand up to closer scrutiny? Accepting that the genes analyzed by the authors evolved without gene duplication and that the amino acids are aligned correctly, most phylogenetic methods assume that the evolutionary dynamics of the 12,060 amino acid sites are independently and identically distributed, and that they evolved under the same stationary, reversible, and homogeneous conditions. The assumptions arise from the need to render phylogenetic methods tractable and easy to use, and they are unlikely to be realistic.
References (abridged):
1. C. Darwin, The Origin of Species by Means of Natural Selection. (John Murray, London, ed. 6, 1888), p. 313
2. D. E. G. Briggs, R. A. Fortey, Paleobiology 32, S94 (2005)
3. S. J. Gould, Wonderful Life: The Burgess Shale and the Nature of History (Hutchinson Radius, London, 1989)
4. E. J. Douzery, E. A. Snell, E. Bapteste, F. Delsuc, H. Philippe, Proc. Natl. Acad. Sci. U.S.A. 101, 15386 (2004)
5. A. Rokas, D. Krüger, S. B. Carroll, Science 310, 1933 (2005)
Science http://www.sciencemag.org
--------------------------------
Related Material:
VISION AND THE CAMBRIAN EXPLOSION
The following points are made by Andrew Parker (citation below):
1) Consider dividing geological time into two parts -- pre-vision and post-vision. The boundary separating these parts stands at 543 million years ago. Considering vision as the most powerful stimulus on Earth, the way the world functions today is the same way it functioned ten million years ago, 100 million years ago and 537 million years ago, after the Cambrian explosion. Similarly, the world was without vision 544 million years ago just as it was 600 million years ago. In the interval of life's history of these two parts, a light switch was turned on. For the second half it remained on, although during the first half it was always off.
2) We know that vision places major restrictions on the external forms of animals today, but before the Cambrian it could not have played such a role because eyes did not exist. Consequently light did not exist as a major stimulus in the behavioral system of animals. By vision I mean the ability to produce visual images, which can be achieved only by animals with eyes. Light is used to determine the direction of sun-light in numerous forms of simple animals. Testament to this are the algae found in the snow at the Burgess quarries in Canada, with their red eyespots but lack of vision. But these have nothing to do with vision. Indeed, some plants even possess simple light perceptors that regulate the shift from vegetative growth to floral development. But this form of light detection is not vision. Vision is the capacity to perceive and classify objects using light, or seeing.
3) The Precambrian was a time where only soft-bodied representatives of the multicelled animal phyla existed. Effectively light as a major stimulus is, or rather visual appearances are, removed from the Precambrian environment because the animals of that time did not possess eyes. Presumably Precambrian animals possessed chemical, sound and/or touch receptors. They may also have possessed simple light perceptors, like the algae in the Canadian snow, but nothing that could form an image. Light could be considered a very minor selection pressure in the Precambrian. It could not have had a direct effect on the evolution of multicelled animals (it could have had an indirect effect in that animals which fed on photosynthetic algae would have been restricted to sunlit zones).
4) Competition and predation would not have been major selective pressures in the Precambrian, but they were taking a foothold. The Ediacaran animals of the Precambrian were gradually developing brains. They were developing ways to pick up environmental cues, or news items, and process that information. They were also evolving the ability to chew, and were gradually developing a rudimentary form of rigidness in their limbs. Precambrian trace fossils or footprints suggest that legs could support bodies off the ground. But as in dark caves today, evolution in general would have been slow in the Precambrian, and may well have continued at a gradual pace had it not been for a single but monumental event.
5) This was an event that, in terms of body parts, would have seemed like any other evolutionary innovation, of which there have been many. But this event was different -- it changed the world forever on a scale not since witnessed. At the end of the Precambrian, while most phyla were evolving gradually, a serious transformation was taking place in the soft-bodied trilobites. A light sensitive patch was becoming more sophisticated. It was dividing into separate units. The nerves servicing each unit were becoming more numerous, and so too were the brain cells they serviced. These nerve and brain cells were either multiplying or being borrowed from the wiring and processing system of another sense. Then the outer covering of each unit began to swell and take on focusing properties. One day all this reached a crescendo -- a compound eye had formed.
Adapted from: Andrew Parker: In the Blink of an Eye. Perseus Publishing 2003, p.268. More information at: http://www.amazon.com/exec/obidos/ASIN/0738206075/scienceweek
--------------------------------
Notes by ScienceWeek:
The geological period known as the Cambrian is the time frame from about 505 million years ago to 545 million years ago. Its most outstanding aspect is the rather sudden appearance of numerous invertebrate fossils, so numerous that some have termed it an explosion of evolutionary processes. Many of the life forms that existed during the Cambrian are long extinct, but their fossils are numerous, and through their fossils the various Cambrian species have been the subject of much study by paleobiologists.
The Cambrian explosion of life forms has been a long-standing puzzle for paleobiologists, and at present there is apparently no single generally accepted explanation. Among the ideas proposed have been, 1) that the explosion of new forms resulted from a sudden increase in atmospheric oxygen; 2) that the explosion is only apparent, and the Precambrian, the period previous to the Cambrian, lacks fossils because of heat and pressure associated with important geological changes; 3) that living forms evolved mostly in freshwater areas, and are therefore absent in Precambrian sediments, which are primarily marine; 4) that changes in the shape and extent of shorelines produced by continental drift dramatically transformed climate and environment; 5) that the previous evolution of DNA recombination and regulatory genes culminated in and sparked the diversity and anatomical complexity manifested in the explosion; 6) that an exponential increase of species could become significant only after attaining a threshold value at the start of the Cambrian; and, 7) that once multicellular organisms appeared, the intrinsic possibilities for variation increased enormously with a resultant explosion of evolved forms.
Unfortunately, there is no evidence to suggest a selection of one of these proposals, although some of them are less convincing than others. And of course the truth may be that more than one factor was involved. No matter the origin, the Cambrian explosion is apparently accepted by most paleobiologists as a real discontinuity, a period that saw the sudden emergence of dozens of new orders and phyla, including sponges, annelids, crustaceans, hemichordates, brachiopods, and mollusks.
--------------------------------
Related Material:
A MOLECULAR DATING TEST OF THE CAMBRIAN EXPLOSION HYPOTHESIS
Notes by ScienceWeek:
The early history of the *Metazoa, whether the Metazoa originated as part of a *Cambrian "explosion" or with an extended *Precambrian "phylogenetic fuse", remains controversial in evolutionary biology. The Cambrian explosion hypothesis -- that the phyla and even classes of the animal kingdom originated in a rapid evolutionary radiation at the base of the Cambrian at 545 to 560 million years ago -- rests on the sudden appearance of a diverse range of animals in the fossil record. Although recent discoveries of *Ediacaran metazoans have extended the record of sponges and bilateral animals to 570 million years ago, the biological affinities of many Ediacaran organisms remains controversial, and the earliest paleontological evidence of metazoan life is no more than 600 million years ago.
However, the absence of earlier metazoan fossils could have been caused by systematic biases in preservation that left the Precambrian history of recognized phyla unrecorded in fossils. Molecular studies have the potential to shed light on the origin of the animal phyla by providing independent estimates of the divergence times, but molecular studies have been criticized for failing to account adequately for variations in rate of evolution.
The following points are made by L. Bromham et al now (Proc. Natl. Acad. Sci. 1998 95:12386):
1) The authors present a study involving a method of dating divergence times from molecular data (both *mitochondrial DNA and nuclear DNA) which addresses the criticisms of earlier studies, and which the authors state provides more realistic but wider confidence intervals.
2) The authors report their data are not compatible with the Cambrian explosion hypothesis as an explanation for the origin of metazoan phyla, and they suggest their data provide additional support for an extended period of Precambrian metazoan diversification. The authors conclude: "Although we cannot provide precise estimates of the origin of metazoan phyla, we can use our results to confidently reject the Cambrian explosion hypothesis, which rests on a literal interpretation of the fossil record and assumes that special evolutionary phenomena, capable of producing profound differentiation in a short period, operated in the Cambrian but not before or since. By contrast, the Precambrian phylogenetic fuse hypothesis assumes no more than we already know to be reasonable: that lineages can diverge gradually over time and that the fossil record contains gaps that can greatly reduce the chances of finding fossils for certain periods or particular types of organisms.
Proc. Nat. Acad. Sci. http://www.pnas.org
--------------------------------
Notes by ScienceWeek:
Metazoa: In general, the term "metazoa" refers to all multicellular animals. Among important distinguishing characteristics of metazoa are cell differentiation and intercellular communication. For certain multicellular colonial entities such as sponges, some biologists prefer the term "parazoa".
Ediacaran metazoans: The term "Ediacaran" refers to an assemblage (until recently the oldest) of soft-bodied marine animals, the assemblage first discovered in the Ediacara Hills in Australia.
mitochondrial DNA: Mitochondrial DNA (sometimes denoted as mtDNA), found in the mitochondria of all eukaryotes, is believed to evolve in parallel with nuclear DNA, but also believed to be inherited primarily in the maternal lineage in animals.
ScienceWeek http://scienceweek.com
|