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It's Time to Defuse the Cambrian “Explosion”

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It's Time to Defuse the Cambrian “Explosion” It’s Time to Defuse the Cambrian “Explosion” Jacob Beasecker, Zach Chamberlin, Nicole Lane, Katie Reynolds, Jack Stack, Kailey Wahrer, Abigail Wolff, Dept. of Earth & Environmental Sciences, Michigan State University, East Lansing, Michigan 48824, USA; Jo Devilbiss, Corey Wahr, Dept. of Integrative Biology, Michigan State University, East Lansing, Michigan 48824, USA; Dan Durbin, Hannah Garneau, Dept. of Fisheries & Wildlife, Michigan State University, East Lansing, Michigan 48824, USA; and Danita Brandt, Dept. of Earth & Environmental Sciences, Michigan State University, East Lansing, Michigan 48824, USA Volcanoes may erupt explosively. Mete- (BSCS, 1961). Three years later, the phrase, metazoan body plans) show that biological oroids may explode on entering the atmo- “Cambrian evolutionary explosion,” with the innovation was not limited to the Cambrian sphere. A microwaved grape may explode middle, qualifying adjective “evolutionary,” but proceeded apace as life expanded from (Conover, 2019). However, a growing body of to distinguish it from physical or chemical the marine environment into new terrestrial research suggests that biodiversity at the dawn processes, was used in a paper describing ecospace (Deline et al., 2018); new fossil dis- of the Cambrian Period did not explode. Data, the evolution of oxygen in Earth’s early coveries point to evolutionary continuity amassed in the century and a half since atmosphere (Berkner and Marshall, 1964). of biomineralizing animals across the Charles Darwin (1859) agonized that the Ultimately, the binomial form prevailed, Ediacaran–Cambrian transition (Cai et al., apparent absence of Precambrian lifeforms referring to the biosphere, and the “Cambrian 2019); integration of biostratigraphical and was the weakest link in his theory of evolution explosion” has propagated ever after without geochemical records indicates that biological by natural selection, support the view that bio- explicit authorship attribution. transitions of the late Proterozoic and early logical diversity at the beginning of the Eminent Precambrian geologist and paleo- Phanerozoic were a series of successive radi- Cambrian Period did not burst violently, deto- biologist Preston Cloud was an early critic ations that built upon each other (Wood et al., nate, shatter, or blow up. In this contribution, of the adjective “explosive” to describe the 2019). In sum, the processes and the time we trace the origin of the phrase “Cambrian Cambrian biodiversification. Cloud noted scale over which these processes acted were explosion,” give reasons for moving away that the time scale involved could have been more complex than implied by a phrase that from using it, and offer an alternative for millions of years, hardly “explosive” in the signals a single event. describing intervals of significant increase in widely understood use of the word (a point But perhaps the most compelling reason the diversity of life. reiterated by Marshall, 2006). Cloud also to reassess the use of the word “explosion” The bibliographic pedigree of the phrase remarked, presumably facetiously, that such to describe biodiversification during the “Cambrian explosion” is uncertain; its origin episodes probably were not accompanied by Cambrian, separate from linguistic lineage is not clearly established in peer-reviewed lit- a loud noise (Cloud, 1948). and disciplinary developments, is its appro- erature. By the early twentieth century, the The images conjured by “Cambrian explo- priation by followers of non-scientific expla- abrupt appearance of abundant (macro-) fos- sion” are vivid and Internet-ready; a Google nations for life’s origin. Authors of anti-evolu- sils in the Cambrian was canon in historical search on “Cambrian explosion memes” tion tracts were among the earliest adopters of geology textbooks (Schuchert and Dunbar, returned more than 300K results. However, the phrase (Ridenour, 1967). Misuse of the 1933). The earliest use of the adjective “explo- the concept implied by the word “explosion” concept of an early, explosive episode of evo- sive,” with reference to an evolutionary rate, does not do justice to advances in our lution continues today (exchanged life disci- was likely George Gaylord Simpson’s “explo- understanding since the Modern Synthesis pleship, http://exchangedlife.com/); in this sive evolution” to describe a general pattern of (Huxley’s 1942 coinage describing the merger arena, the Cambrian explosion is commonly rapid diversification early in the history of of natural selection with Mendelian genetics) styled as falsifying evolutionary theory and a lineage (Simpson, 1944). Mid-twentieth- was modern. A few examples: molecular phy- flummoxing “evolutionists,” neither of which century contemporaries echoed use of this logenetics (Suárez-Díaz and Anya-Muíoz, accusations are accurate, correct, or true. phrase in characterizing a general evolution- 2008) makes possible construction of hypoth- “Diversification” and “radiation” may not ary pattern (Henbest, 1952; Colbert, 1953). eses for evolutionary development during the have the visceral appeal of “explosion,” but Use of the phrase “explosive evolution” to “prelude” to the Cambrian (Valentine, 2002); both alternatives are suitable, fitting, apt, describe rapid diversification during the early the ability to resolve biosignatures and proper, and applicable (Marshall, 2006; Cambrian morphed into “The Cambrian Proterozoic biogeochemical cycles (Rothman Sperling and Stockey, 2018) without carrying Explosion” under obscure circumstances. The et al., 2003) pushed the appearance of com- the implication of catastrophic rate or other- earliest published occurrence known to us is a plex biological processes deeper into the pre- worldly mechanism. Certainly, biodiversifi- section heading in an early version of an Cambrian past; measures of morphological cation at the beginning of the Cambrian was experimental high school biology curriculum disparity (that is, the variety of different unique (Erwin et al., 1987)—all those new GSA Today, v. 30, https://doi.org/10.1130/GSATG460GW.1. CC-BY-NC. 26 GSA Today | December 2020 body plans—but no evolutionary rules were Cai, Y., Xiao, S., Li, G., and Hua, H., 2019, Diverse conflict between science and scripture: Ventura, broken, nor is there mystery or discipline- biomineralizing animals in the terminal Ediacaran California, Gospel Light Publications, 168 p. dividing controversy, as is claimed by anti- Period herald the Cambrian explosion: Geology, Rothman, D.H., Hayes, J.M., and Summons, R.E., v. 47, no. 4, p. 380–384, https://doi.org/10.1130/ 2003, Dynamics of the Neoproterozoic carbon cycle: science concerns who seize on the term G45949.1. Proceedings of the National Academy of Sciences of “explosion.” Cloud, P.E., 1948, Some problems and patterns of the United States of America, v. 100, p. 8124–8129, After the Cambrian, the next major expan- evolution exemplified by fossil invertebrates: https://doi.org/10.1073/pnas.0832439100. sion in biodiversity occurred during the mid- Evolution; International Journal of Organic Evo- Schuchert, C., and Dunbar, C.O., 1933, A Textbook dle Ordovician, a chapter in the history of lution, v. 2, p. 322–350, https://doi.org/10.1111/ of Geology—Part II—Historical Geology, third j.1558-5646.1948.tb02750.x. edition: New York, John Wiley and Sons, 551 p. life referred to as the Great Ordovician Colbert, E.H., 1953, Explosive evolution: Evolution; Simpson, G.G., 1944, Tempo and Mode in Evolution: Biodiversification Event or GOBE (Webby International Journal of Organic Evolution, v. 7, New York, Columbia University Press, 237 p. et al., 2004; Harper et al., 2015; Servais and p. 89–90, https://doi.org/10.1111/j.1558-5646.1953 Servais, T., and Harper, D.A.T., 2018, The great Or- Harper, 2018; Stigall et al., 2019). The term .tb00063.x. dovician biodiversification event (GOBE): Defi- “event” may be as problematic as “explo- Conover, E., 2019, Microwaved grapes make fireballs, nition, concept and duration: Lethaia, v. 51, and scientists now know why: Science News, p. 151–164, https://doi.org/10.1111/let.12259. sion” in its implication of a short time period. https://www.sciencenews.org/article/grapes-spark Sperling, E.A., and Stockey, R.G., 2018, The temporal We note that the word “event” in GOBE is -microwave-plasma (accessed March 2019). and environmental context of early animal evolu- redundant, as “biodiversification” is itself an Darwin, C., 1859, On the Origin of Species by Means tion: Integrative and Comparative Biology, v. 58, event. (Similar pleonastic phrases encoun- of Natural Selection: London, John Murray. p. 605–622, https://doi.org/10.1093/icb/icy088. tered in other venues include “sales event” Deline, B., Greenwood, J.M., Clark, J.W., Puttick, Stigall, A.L., Edwards, C.T., Freeman, R.L., and M.N., Peterson, K.J., and Donoghue, P.C.J., 2018, Rasmussen, C.M.Ø., 2019, Coordinated biotic and and “birth event.”) We suggest, as an alterna- Evolution of metazoan morphological disparity: abiotic change during the Great Ordovician Biodi- tive to “Cambrian explosion,” the Great Proceedings of the National Academy of Sciences versification Event: Darriwilian assembly of early Cambrian Biodiversification (GCB), a con- of the United States of America, v. 115, p. E8909– Paleozoic building blocks: Palaeogeography, Pal- struction parallel to that for the Ordovician E8918, https://doi.org/10.1073/pnas.1810575115. aeoclimatology, Palaeoecology, v. 530, p. 249– episode, absent the redundant and problem- Erwin, D.H., Valentine,
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