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Focus: Illuminating Science credit: http://rst.gsfc.nasa.gov/Sect20/A12c.html The Explosion and the Origins of Diversity

here is a grandeur in this view environmental change or fundamental By Anirudh Penumaka Tof .” 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 FDQVHHLQWKHÁHHWLQJOHQJWKRIKXPDQ 560 million 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 witnessed may not be that-a the appearance of diverse phyla includ- slow march-after all. may ing those characterized by mineralized instead proceed through periods of long skeletons such as Arthropoda, Echino- stagnation, interspersed with periods dermata, , 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 (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 ancestors to phyla accorded a Cambrian origin; however, by coincidence these ancestors were not preserved in the 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 content in the fact that during the Cambrian and of changing environmental conditions the atmosphere was associated with an periods, there were separate DQGIDUUHDFKLQJJHQHWLFPRGLÀFDWLRQV increase in the size of (8). Higher instances of “ and 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 , of prokaryotes and 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 (7). The link between rising oxygen A group called the Ediacara biota, which has been evidence that free oxygen OHYHOVDQGFRPSOH[LW\LVFRQÀUPHGE\D are 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, 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 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 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 (10). Communication between respiration to preserve some parts of the Animalia, and most likely the compartments of the cells requires these pathways and also to forge new arose through repetitive “tandem gene specialized transmembrane ; pathways that were entirely dependent GXSOLFDWLRQµ  0RUHVSHFLÀFDOO\RU- ganisms that had segmented body forms had greater numbers of Hox genes. The phyla that emerged during the Cambrian era had “very similar sets of Hox genes,” suggesting that Hox genes did not be- come duplicated during this period (16). The major function of Hox genes is to lay the foundation for the production of more specialized cell types through further downstream signaling cascades, and to place the resulting tissues in the proper orientation. In evolution, acquir- ing a new function does not necessarily mean the development of genes that are entirely novel, because previously existing genes can simply be co-opted, RUGXSOLFDWHGDQGPRGLÀHGWRSURGXFH a very different gene with a different function (14). Hox genes are unique to metazoans, and the families Figure 2. Summary of animal life since the end of the Precambrian period. that arose during this period of time are credit: http://rst.gsfc.nasa.gov/Sect20/A12c.html

46 Harvard Science Review ‡ fall 2011 Focus: Illuminating Science shared among many types of organisms. new body patterns observed during iridium in the geological record led to It is very possible that the Cambrian ex- the Cambrian explosion. However, re- the theory of a cataclysmic asteroid im- plosion occurred as a result of a critical searchers also make the important point pact to explain the demise of mass of accumulated genetic changes that even ancient species, such as the 65 million years ago. It appears that that were suddenly utilized to lead to the protosome- ancestor, had understanding of the Cambrian explo- rapid divergence of organisms into far a set of Hox genes. Therefore, simply sion is at a similar , where there are more complex organisms (14). Even be- possessing these genes was not enough many lines of evidence, conjecture, and fore the Cambrian explosion, the signal- “to trigger a metazoan radiation” (17). theory, but, unlike the single explanation ing pathways for basic metazoan body To produce the radiation, there would of an asteroid impact, the Cambrian plans may already have been established. also have to have been alterations to the explosion may be an emergent property Gould and others point to the same line way the Hox genes were put together, as of the many simultaneous changes tak- of reasoning. In other words, rather well as in the regulation of Hox gene ex- ing place at the time. It is often said that than evolving a separate addition or pression and their downstream signaling species alive today are only a small frac- new gene for each new body pattern or pathways (17). The rapid evolution of tion of the species that once lived, and body segment, the pattern of regulatory unique morphologies during the Cam- there is perhaps no greater testament networks and Hox genes was already in brian era probably involves a number to the power of evolution that even SODFHDQGVPDOOHUPRGLÀFDWLRQVLQWKH of explanations, yet it is also relevant with such destruction, still bursts regulatory genes and control elements to explore why this period of evolution- with diversity and prolific creativity. were used to effect changes to the body ary creativity did not persist for longer. Understanding the Cambrian explosion plans (17). Ohno highlights the fact that, Erwin et al. suggest an explanation for retains great importance because the with a liberal assumption of a steady why the biological innovation during basis of many of our archetypal beliefs rate of 1 base pair per billion, the Cambrian period eventually came and sense of awe about the beauty of it would take 10 million years to achieve to a stop, rather than continue to create living things was conceived during this a 1% change in the genetic material. ever more diverse body forms (3). An time. In our attempt to search back Considering that the Cambrian explo- important point is that the creation of through the tree of life to divine those sion occurred in only 20 – 50 million new forms of organisms created a new VWLOOP\VWHULRXVHYHQWVWKDWLQÁXHQFHG years, this enormous event could not be landscape of predatory, competitive, life, we also search for our own origins explained simply by ordinary and indirect interactions that cannot be and our own evolution. LQVSHFLÀFJHQHV  ,QVWHDG2KQR easily characterized, but that profoundly suggests that the organisms belonging LQÁXHQFHGIXWXUHHYROXWLRQDU\HYHQWV —Anirudh Penumaka ‘12 is a Chemical to even diverse phyla formed during The authors suggest that available eco- and Physical concentrator in Adams the Cambrian explosion could have ORJLFDOQLFKHVPD\KDYHEHFRPHÀOOHG House. “nearly the identical genome” and that meaning that even when new forms de- the immense variety was actually due to veloped, they could not become estab- “differential usage of the identical set of lished because the already References 1. Darwin, C. Annotated by Costa, J.T. 2009. The Annotated Origin. A genes” (15). For instance, under this sce- reached their ecological limits. A more Facsimile of the First Edition of . Harvard University Press: Cambridge, Massachusetts. 2. Marshall, C.R. 2006. Explaining the Cambrian “Explosion” of . nario, and vertebrates would subtle explanation is that the complexity Annu. Rev. Earth Planet. Sci. 34:355 – 384. 3. Erwin, D., Valentine, J., Jablonski, D. 1997. The Origin of Animal Body have a common ancestor approximately and interdependency of pathways and Plans. American Scientist; March-April issue 2007. 4. Gould, S.J. 1989. Wondeful life: the and the nature of 560 million years ago. Therefore, the regulatory mechanisms had evolved to a history. W.W. Norton and Company; New York, USA. 5. Falkowski, P.G. 2006.Tracing Oxygen’s Imprint on Earth’s Metabolic common ancestor would have had time SRLQWZKHUHPDMRUPRGLÀFDWLRQVFRXOG Evolution. Science 331(5768): 1724 – 1725. 6. Baudouin-Cornu, P. and Thomas, D. 2007. Evolutionary biology: Oxygen to evolve regulatory pathways that pro- no longer take place without being mal- at life’s boundaries. Nature 445(7123): 35 - 36. 7. Saltzman, M.R et al. 2011. Pulse of atmospheric oxygen during the late Cambrian. Proc. Natl. Acad. Sci. USA: 3876-3881. YLGHGRSSRUWXQLWLHVIRUPRGLÀFDWLRQV adaptive in a larger sense (3). 8. Koch, L. G. and Britton, S.L. 2009. Aerobic metabolism underlies com- plexity and capacity. American Journal of Respiratory Cell and Molecular to these pathways for generating “major The two major prevailing theories re- Biology 40: 507-510. &DQÀHOG'(/DWH1HRSURWHUR]RLF'HHS2FHDQ2[\JHQDWLRQDQG morphological innovations” (15). garding the Cambrian explosion are not the Rise of Animal Life. Science 315, 5808: 92-95. 10. Acquisti C, Kleffe J & Collins S. 2007. Oxygen content of transmem- Because there were so many new mutually exclusive; in fact, it may have brane proteins over macroevolutionary time scales. Nature 445: 47–52. 11. Fluck, M., Webster, K.A., Graham, J., Giomi, F., Gerlach, F., Schmitz, types of morphological patterns during been the case that both rising oxygen A. 2007. Coping with cyclic oxygen availability: evolutionary aspects. Integr. Comp. Biol. 47 (4): 524 – 531. 12. Raymond, J. and Segre, D. 2006. The Effect of Oxygen on Biochemi- the Cambrian and it seems unlikely that levels and fundamental genetic changes cal Networks and the Evolution of Complex Life. Science 311 (5768): 1764-1767. this could occur simply through an in- nurtured an environment that set the 13. Thannickal, V.J. 2009. Oxygen in the Evolution of Complex Life and the Price We Pay. American Journal of Respiratory Cell and Molecular stance of greatly accelerated duplication stage for the Cambrian explosion to oc- Biology 40: 507-510. 14. Morris, S.C. 2000. The Cambrian “explosion”: Slow-fuse or megaton- of Hox genes initiated by some genetic cur. Sixty years ago, scientists could not nage? Nature 445: 35-36. 15. Ohno, S. 1996. The notion of the Cambrian pananimalia genome. Proc. change. Distinct phyla can have different say with any certainty what caused the Nati. Acad. Sci. USA 93: 8475-8478. 16. Knoll, A.H. and Carroll, S.B. 1999. Early Animal Evolution: Emerg- ing Views from Comparative Biology and . Science 284(5423): collections of Hox genes, which were dinosaurs to become extinct. Ingenious 2129 – 2137. 17. Valentine, J.W. et al. 1999. Fossils, molecules and : new PRGLÀHGRYHUWLPHWRSURGXFHPDQ\ techniques involving observations of perspectives on the Cambrian Explosion. Development 126: 851 – 859.

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