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The Cambrian Explosion and the Origins of Diversity

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The Cambrian Explosion and the Origins of Diversity Focus: Illuminating Science credit: http://rst.gsfc.nasa.gov/Sect20/A12c.html The Cambrian Explosion and the Origins of Diversity here is a grandeur in this view environmental change or fundamental By Anirudh Penumaka Tof life.” Darwin famously wrote PRGLÀFDWLRQWRWKHRUJDQLVPVWKHP- these words about his theory of evo- selves which suddenly open the door lution, and indeed there is something for many new forms of life to arise. astounding about the idea that change $VSHFLÀFSHULRGRI LQFUHDVHGHYROX- “During the Cambrian is the only constant of life, mirroring tion, commonly called the Cambrian yet also transcending the change we “explosion,” occurred approximately ‘explosion,’ life on Earth FDQVHHLQWKHÁHHWLQJOHQJWKRIKXPDQ 560 million years ago (2). Life on Earth underwent a massive lifetimes (1). Over billions of years, underwent a massive surge that created and as the result of a calculus we are all of the major phyla of organisms that surge that created all of only beginning to untangle, life evolved exist even today. Life, which had been from simple, single celled forms into primarily unicellular, quickly became the major phyla of or- complex organisms, some of which multicellular, and organisms evolved ganisms that exist even are capable of emotion, thought, and much more complex morphologies than strikingly, consciousness. But this slow had previously existed (2). today” march of gradual change by means of The Cambrian explosion witnessed natural selection may not be that-a the appearance of diverse phyla includ- slow march-after all. Evolution may ing those characterized by mineralized instead proceed through periods of long skeletons such as Arthropoda, Echino- stagnation, interspersed with periods dermata, Mollusca, and Brachiopoda, RILQFUHGLEOHDQGUDSLGGLYHUVLÀFDWLRQ in addition to phyla made up of soft to generate the vastly different organ- bodied species such as Annelida, isms that inhabit the Earth today. Dur- Onycophora, and Priapulida (3). In his ing these periods of fast evolutionary book Wonderful Life, Stephen Gould progress, there is often a linked major discusses two contrasting theories 44 Harvard Science Review fall 2011 Focus: Illuminating Science about the Cambrian explosion. 7KHÀUVWWKHDUWLIDFWWKHRU\ KROGVWKDWWKHUHZHUHGHÀQLWH Precambrian ancestors to phyla accorded a Cambrian origin; however, by coincidence these ancestors were not preserved in the fossil record, leading us to errantly conclude that there was a period of rapid divergence of life (4). Although, it has been suggested that the Cambrian H[SORVLRQPD\DUWLÀFLDOO\DSSHDU in the fossil record because of changes in the process or rate of fossilization during this time, this objection does not represent the predominant opinion. The major events of the Cambrian explosion are thought to have occurred over a period of just 20 million years, a very small amount of time in Figure 1. Artist redering of Cambrian period species. evolutionary terms (2). Funda- many of their essential features (6). complex forms is closely tied to the oxy- mental theories explaining the Cambrian Approximately 300 million years ago, genation event. Saltzman et al. discuss explosion fall into the broad categories a period of higher oxygen content in the fact that during the Cambrian and of changing environmental conditions the atmosphere was associated with an Ordovician periods, there were separate DQGIDUUHDFKLQJJHQHWLFPRGLÀFDWLRQV increase in the size of insects (8). Higher instances of “plankton and animal di- An increase in atmospheric oxygen amounts of oxygen meant larger body YHUVLÀFDWLRQµWKDWLVUHIHUUHGWRDVWKH concentration appears to be a major sizes could be achieved without con- Great Ordovician Biodiversification trend that long preceded the Cambrian straints due to the unavailability of oxy- Event (7). Authors of this study sug- explosion. Oxygen first appeared in gen to support tissues, and also allowed gest that the oxygen levels rose in a later VLJQLÀFDQWDPRXQWVRQ(DUWKWR for hardened skeletons to develop. A part of the Cambrian era, which also 3 billion years ago, when various mi- group led by Guy Narbonne published had a profound impact on the chemical croorganisms released oxygen into the a study in 2007 which asserts that gla- UHDFWLRQVGHÀQLQJWKHFRPSRVLWLRQRI atmosphere. Cyanobacteria greatly con- ciers covered much of the Earth more the oceans and the diversity of marine tributed to the release of free oxygen in than 580 million years ago, and thereby organisms (7). the atmosphere through their photo- prevented the increase in oxygen levels One of the major problems with synthetic processes (5). A second major until they began to melt. Narbonne sug- assessing the importance of the oxygen oxygenation event, which occurred gests that the melting of these glaciers hypothesis is that it is not possible to approximately a billion years ago, was a before the Cambrian released a large accurately predict how much oxygen strong driving force that supported the amount of nutrients into the oceans (9). the primitive organisms requiring development of complex organisms The increase in nutrient content allowed oxygen actually used. The divergence during the Cambrian era (6). During small organisms, called phytoplankton, of prokaryotes and eukaryotes occurred the past 700 million years, atmospheric to proliferate; these organisms released about 1.5 billion years ago, and oxygen oxygen levels have remained relatively oxygen into the atmosphere to raise the may have been important in that very stable at the 21% that is observed to- atmospheric oxygen concentration (10). early major transition (6). Indeed, there day (7). The link between rising oxygen A group called the Ediacara biota, which has been evidence that free oxygen OHYHOVDQGFRPSOH[LW\LVFRQÀUPHGE\D are fossils of soft bodied and complex is a driving force for cells to evolve a number of events. For instance, oxygen organisms, initially arose during the compartmentalized format. Fluck et al. levels rose from 10% to 21% about period when glaciers were melting, sug- made the important observation that 425 million years ago, and at this same gesting that the evolution of these more the DNA molecules must reside in time, vertebrates evolved and attained credit: http://rst.gsfc.nasa.gov/Sect20/A12c.html fall 2011 Harvard Science Review 45 Focus: Illuminating Science WKHSURWHFWHGFRQÀQHVRIWKHQXFOHXV therefore, the compartmentalized on oxygen (12). The ability to utilize away from the high energy reactions cells required the presence of higher R[\JHQZDVWUHPHQGRXVO\EHQHÀFLDOWR and oxidative stress that occur in the levels of oxygen before they evolved. organisms because respiration using ox- mitochondria (11). A consequence of Just as transmembrane proteins were ygen can yield four times the amount of the compartmentalized architecture important for the development of energy as corresponding non-oxygenic of eukaryotic cells is that there must compartmentalization, these proteins pathways. Although oxygen can result in be major differences in the reduction- also play an important role in the PRUHHIÀFLHQWSURFHVVHVWKHVLGHHIIHFW oxidation potentials in the different communication between cells in of releasing radical oxygen elements had compartments of the cell. A rapid multicellular organisms (10). Therefore, to be addressed (13). Selection eventu- increase in the levels of oxygen may the higher levels of atmospheric oxygen ally remodeled older systems to protect have placed selection pressure in favor were required for eukaryotes, and against the effects of oxygen radicals, of compartmentalization precisely eventually multicellular organisms, to and these systems passed very early in because of the protection offered evolve. the history of life to “all three domains by compartmentalization against the Furthermore, the complexity of the of life” (12). effects of a reactive species such prevailing organisms appears to be Along with environmental changes, as oxygen (11). If oxygen was such positively correlated with the gradual genetic changes in the form of homeo- a powerful driving force for the appearance of an aerobic respiration tic (Hox) genes may also have been transition to eukaryotes, then it is also mechanism (5). Increased levels of at- responsible for the sudden expansion reasonable to hypothesize that further mospheric free oxygen have to be linked of body forms. Homeotic genes are increases in oxygen levels may have to increased “network complexity” responsible for the basic segment mor- LQÁXHQFHGDQHYHQJUHDWHUSXVKWRZDUG that cannot be achieved with anaerobic phology of many organisms, so the a multicellular level of complexity. processes (12). In fact, many scientists expansion of this set of genes meant Acquisti et al. have suggested that cells claim that oxygen not only facilitated de- that a common tool box of body archi- need oxygen in order to “maintain novel velopment, but also created an entirely WHFWXUHFRXOGEHPRGLÀHGWRSURGXFHD communication-related transmembrane new set of processes that were entirely broad range of species (14). All of the proteins.” A major piece of evidence oxygen dependent. The increased levels phyla during the Cambrian period had for this theory comes from the fact that of oxygen just prior to the Cambrian era clusters of Hox genes that contained transmembrane proteins tend to contain may have led to natural selection acting between four and thirteen individual higher oxygen levels than do other on the pathways involved in anaerobic genes (15). Hox genes are found only in proteins
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