Volcanoes may erupt explosively. Meteoroids may explode on entering the atmosphere. A
microwaved grape may explode (Conover, 2019). However, a growing body of research suggests
that biodiversity at the dawn of the Cambrian Period did not explode. Data, amassed
in the century and a half since Charles Darwin (1859) agonized that the apparent absence of
Precambrian lifeforms was the weakest link in his theory of evolution by natural selection,
support the view that biological diversity at the beginning of the Cambrian Period did not
burst violently, detonate, shatter, or blow up. In this contribution, we trace the origin of
the phrase “Cambrian explosion,” give reasons for moving away from using it, and offer an
alternative for describing intervals of significant increase in the diversity of life.
The bibliographic pedigree of the phrase “Cambrian explosion” is uncertain; its origin is not
clearly established in peer-reviewed literature. By the early twentieth century, the abrupt
appearance of abundant (macro-) fossils in the Cambrian was canon in historical geology
textbooks (Schuchert and Dunbar, 1933). The earliest use of the adjective “explosive,” with
reference to an evolutionary rate, was likely George Gaylord Simpson’s “explosive evolution”
to describe a general pattern of rapid diversification early in the history of a lineage
(Simpson, 1944). Mid-twentieth-century contemporaries echoed use of this phrase in
characterizing a general evolutionary pattern (Henbest, 1952; Colbert, 1953).
Use of the phrase “explosive evolution” to describe rapid diversification during the early
Cambrian morphed into “The Cambrian Explosion” under obscure circumstances. The earliest
published occurrence known to us is a section heading in an early version of an experimental
high school biology curriculum (BSCS, 1961). Three years later, the phrase, “Cambrian
evolutionary explosion,” with the middle, qualifying adjective “evolutionary,” to
distinguish it from physical or chemical processes, was used in a paper describing the
evolution of oxygen in Earth’s early atmosphere (Berkner and Marshall, 1964). Ultimately,
the binomial form prevailed, referring to the biosphere, and the “Cambrian explosion” has
propagated ever after without explicit authorship attribution.
Eminent Precambrian geologist and paleobiologist Preston Cloud was an early critic of the
adjective “explosive” to describe the Cambrian biodiversification. Cloud noted that the time
scale involved could have been millions of years, hardly “explosive” in the widely
understood use of the word (a point reiterated by Marshall, 2006). Cloud also remarked,
presumably facetiously, that such episodes probably were not accompanied by a loud noise
The images conjured by “Cambrian explosion” are vivid and Internet-ready; a Google search on
“Cambrian explosion memes” returned more than 300K results. However, the concept implied by
the word “explosion” does not do justice to advances in our understanding since the Modern
Synthesis (Huxley’s 1942 coinage describing the merger of natural selection with Mendelian
genetics) was modern. A few examples: molecular phylogenetics (Suárez-Díaz and Anya-Muíoz,
2008) makes possible construction of hypotheses for evolutionary development during the
“prelude” to the Cambrian (Valentine, 2002); the ability to resolve biosignatures and
Proterozoic biogeochemical cycles (Rothman et al., 2003) pushed the appearance of complex
biological processes deeper into the pre-Cambrian past; measures of morphological disparity
(that is, the variety of different metazoan body plans) show that biological innovation was
not limited to the Cambrian but proceeded apace as life expanded from the marine environment
into new terrestrial ecospace (Deline et al., 2018); new fossil discoveries point to
evolutionary continuity of biomineralizing animals across the Ediacaran–Cambrian transition
(Cai et al., 2019); integration of biostratigraphical and geochemical records indicates that
biological transitions of the late Proterozoic and early Phanerozoic were a series of
successive radiations that built upon each other (Wood et al., 2019). In sum, the processes
and the time scale over which these processes acted were more complex than implied by a
phrase that signals a single event.
But perhaps the most compelling reason to reassess the use of the word “explosion” to
describe biodiversification during the Cambrian, separate from linguistic lineage and
disciplinary developments, is its appropriation by followers of non-scientific explanations
for life’s origin. Authors of anti-evolution tracts were among the earliest adopters of the
phrase (Ridenour, 1967). Misuse of the concept of an early, explosive episode of evolution
continues today (exchanged life discipleship, http://exchangedlife.com/); in this arena, the
Cambrian explosion is commonly styled as falsifying evolutionary theory and flummoxing
“evolutionists,” neither of which accusations are accurate, correct, or true.
“Diversification” and “radiation” may not have the visceral appeal of “explosion,” but both
alternatives are suitable, fitting, apt, proper, and applicable (Marshall, 2006; Sperling
and Stockey, 2018) without carrying the implication of catastrophic rate or otherworldly
mechanism. Certainly, biodiversification at the beginning of the Cambrian was unique (Erwin
et al., 1987)—all those new body plans—but no evolutionary rules were broken, nor
is there mystery or discipline-dividing controversy, as is claimed by anti-science concerns
who seize on the term “explosion.”
After the Cambrian, the next major expansion in biodiversity occurred during the middle
Ordovician, a chapter in the history of life referred to as the Great Ordovician
Biodiversification Event or GOBE (Webby et al., 2004; Harper et al., 2015; Servais and
Harper, 2018; Stigall et al., 2019). The term “event” may be as problematic as “explosion”
in its implication of a short time period. We note that the word “event” in GOBE is
redundant, as “biodiversification” is itself an event. (Similar pleonastic phrases
encountered in other venues include “sales event” and “birth event.”) We suggest, as an
alternative to “Cambrian explosion,” the Great Cambrian Biodiversification (GCB), a
construction parallel to that for the Ordovician episode, absent the redundant and
problematic “event” suffix. Because the phrase “mass extinction” is applied to multiple
biodiversity crises through time, even though each event is unique in the organisms affected
and the contributing causes, so might “great biodiversification” become a less volatile
descriptor for intervals of notable increase in life’s diversity.
We submit that, for scientific, semantic, and societal reasons, it is time to lay the term
“Cambrian explosion” to rest for any use other than historical reference.
- Berkner, L.V., and Marshall, L.C., 1964, The history of oxygenic concentration in the
Earth’s atmosphere: Discussions of the Faraday Society, v. 37, p. 122–141,
- Biological Sciences Curriculum Study (BSCS), 1961, High School Biology, Green Version,
revised edition: Boulder, University of Colorado, 265 p.
- Cai, Y., Xiao, S., Li, G., and Hua, H., 2019, Diverse biomineralizing animals in the
terminal Ediacaran Period herald the Cambrian explosion: Geology, v. 47,
no. 4, p. 380–384, https://doi.org/10.1130/G45949.1.
- Cloud, P.E., 1948, Some problems and patterns of evolution exemplified by fossil
invertebrates: Evolution; International Journal of Organic Evolution, v. 2,
p. 322–350, https://doi.org/10.1111/j.1558-5646.1948.tb02750.x.
- Colbert, E.H., 1953, Explosive evolution: Evolution; International Journal of Organic
Evolution, v. 7, p. 89–90, https://doi.org/10.1111/j.1558-5646.1953.tb00063.x.
- Conover, E., 2019, Microwaved grapes make fireballs, and scientists now know why:
Science News, https://www.sciencenews.org/article/grapes-spark-microwave-plasma
(accessed March 2019).
- Darwin, C., 1859, On the Origin of Species by Means of Natural Selection: London, John
- Deline, B., Greenwood, J.M., Clark, J.W., Puttick, M.N., Peterson, K.J., and Donoghue,
P.C.J., 2018, Evolution of metazoan morphological disparity: Proceedings of the National
Academy of Sciences of the United States of America, v. 115, p. E8909–E8918,
- Erwin, D.H., Valentine, J.W., and Sepkoski, J.J., Jr., 1987, A comparative study of
diversification events: The early Paleozoic versus the Mesozoic: Evolution:
International Journal of Organic Evolution, v. 41, p. 1177–1186,
- Harper, D.A.T., Zhan, R-B, and Jin Jisuo, 2015, The great Ordovician biodiversification
event: Reviewing two decades of research on diversity’s big bang illustrated by mainly
brachiopod data: Palaeoworld, v. 24, p. 75–85,
- Henbest, L.G., 1952, Significance of evolutionary explosions for diastrophic division of
Earth history: Introduction to the symposium: Journal of Paleontology, v. 26,
- Huxley, J., 1942, The Modern Synthesis: London, George Allen & Unwin, Ltd., 645 p.
- Marshall, C.R., 2006, Explaining the Cambrian “explosion” of animals: Annual Review of
Earth and Planetary Sciences, v. 34, p. 355–384,
- Ridenour, F., 1967, Who says? A discussion of basic questions on the Christian faith:
Including the existence of God, the trustworthiness of the Bible, the conflict between
science and scripture: Ventura, California, Gospel Light Publications, 168 p.
- Rothman, D.H., Hayes, J.M., and Summons, R.E., 2003, Dynamics of the Neoproterozoic
carbon cycle: Proceedings of the National Academy of Sciences of the United States of
America, v. 100, p. 8124–8129, https://doi.org/10.1073/pnas.0832439100.
- Schuchert, C., and Dunbar, C.O., 1933, A Textbook of Geology—Part II—Historical Geology,
third edition: New York, John Wiley and Sons, 551 p.
- Simpson, G.G., 1944, Tempo and Mode in Evolution: New York, Columbia University Press,
- Servais, T., and Harper, D.A.T., 2018, The great Ordovician biodiversification event
(GOBE): Definition, concept and duration: Lethaia, v. 51, p. 151–164,
- Sperling, E.A., and Stockey, R.G., 2018, The temporal and environmental context of early
animal evolution: Integrative and Comparative Biology, v. 58, p. 605–622,
- Stigall, A.L., Edwards, C.T., Freeman, R.L., and Rasmussen, C.M.Ø., 2019, Coordinated
biotic and abiotic change during the Great Ordovician Biodiversification Event:
Darriwilian assembly of early Paleozoic building blocks: Palaeogeography,
Palaeoclimatology, Palaeoecology, v. 530, p. 249–270,
- Suárez-Díaz, E., and Anya-Muíoz, V.H., 2008, History, objectivity, and the construction
of molecular phylogenies: Studies in History and Philosophy of Biomedical Sciences,
v. 39, p. 451–468, https://doi.org/10.1016/j.shpsc.2008.09.002.
- Valentine, J.W., 2002, Prelude to the Cambrian Explosion: Annual Reviews of Earth and
Planetary Sciences, v. 30, p. 285–306,
- Webby, B.D., Paris, F., Droser, M.L., and Percival, I.G., eds., 2004, The Great
Ordovician Biodiversification Event: New York, Columbia University Press, 496 p.,
- Wood, R., Liu, A.G., Bowyer, F., Wilby, P.R., Dunn, F.S., Kenchington, C.G., Hoyal
Cuthill, J.F., Mitchell, E.G., and Penny, A., 2019, Integrated records of environmental
change and evolution challenge the Cambrian Explosion: Nature Ecology & Evolution,
v. 3, p. 528–538, https://doi.org/10.1038/s41559-019-0821-6.
Manuscript received 11 Apr. 2020.
Revised manuscript received 14 Sept. 2020.
Manuscript accepted 17 Sept. 2020.
Posted 2 Oct. 2020.
© 2020, The Geological Society of America. CC-BY-NC.