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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

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26 GSA Today | December 2020