Friday, 21 June 2013

Cambrian Explosion- A Missing Link?

Missing link? What do you mean 'missing'? The alleged dilemma of Darwin's doubt is no more and yet another creationist fallacy has been placed back on the shelf of incredulity. Indeed the Cambrian 'explosion' seemed like the 'poofing' of all animal phyla into being at once, in an instant and too rapid for evolution to pose an intelligible explanation. Yet as we sequence genomes and excavate fossils, we unlock the vaults of life's palaeontological history and uncover an phylogeny of forms that fall into a nested hierarchy. Darwin proposed that the Precambrian hosted a spawning range of organisms and the time between the genesis of the earth to the threshold of the Cambrian must have been enormous to account for such diversity. Central to understanding what happened to ignite the Cambrian is the notion of deep time, earth is around 4.6 BYA (billion years old), which in turn is divided into two major supereons, the Precambrian (4.6 BYA to 550 MYA) and the Phanerozoic (550 MYA-onwards). The older and former of the two, is divided into the Archean (4.6 to 2.5 BYA) and the Proterozoic (2.5 BYA to end of Precambrian). The former of the two, the Phanerozoic, really encompasses much of the recent history of the earth and is split into the Paleozoic, Mesozoic and the Cenozoic (now). Furthermore, we know how ancient the antiquity of life on earth is, the slight anomaly between 13C to 12C allows us to push the clock back to 3.75 BYA where the first photosynthetic organisms reigned, while 3.5 BYA we see the emergence of stomolobites and bacteria and 1.4 BYA we see the first eukaryotic cells appear. The supposed incompleteness of the Precambrian fossil record has been overthrown by recent developments, the Edicardian fauna of Australia date back to about 590 MYA and comprised of soft-bodied life forms; while the hard bodied Tommotian fauna of Russia date back to the beginning of the Cambrian. But the infamous Burgess Shale seem to emerge immediately after the inception of the Cambrian era and are most likely related to modern snails, worms and clams; however like many Cambrian era lineages, they underwent extinction much like the trilobites at the end of the Paleozoic as well as the dinosaurs and ammonites at the end of the Mesozoic. The main reasons of the apparent fossil rarity and 'incompleteness' include the relatively geologically active era of the Hadean and the successive eras, which would have lead to the destruction of Precambrian fossils and rocks. Secondly, the characteristically soft-bodied species of the Cambrian fossilise rather poorly relative to Post-Cambrian life forms with shells and bones, this results in the recent discovery of turbellians; giving the illusion that they have no precursors. But such an 'explosion' was by no means as explosive as many would envisage, it was geologically instantaneous (taking place across a span of 15 or so million years). Among the explanations of what went on during the Cambrian include the ecological notion of adaptive radiation, whereby if a species occupies a niche or habitat which exposes it to little or no competition, it develops phenotypic novelties to exploit the environment. Gould proposed in Wonderful Life, that a 'cooling off period' of stabilisation punctuates the relatively rapid period of evolutionary change, which may explain why the Burgess Shale did not make it to today. Moreover, raised levels of atmospheric oxygen may have served as 'rocket fuel' to primitive life while the 'Snowball earth' scenario posits that new niche creation may be viable. But among the best explanations of the Cambrian explosion is the arsenal of the Hox family of genes that evolved from the paraHox cluster; recent studies suggest the origin of the bilaterian genetic morphology may extend as far back as the Edicardian, giving enough time for successive gene duplications to produce an adaptive radiation and hence a Cambrian explosion. Boom!

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