The Mystery of the Cambrian Explosion

An Enduring Evolutionary Enigma

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The geologically sudden appearance of every major kind, or phylum, of life around 541 million years ago—an event known as the “Cambrian Explosion”—was the most significant moment in the history of biology since the origin of life itself.¹ This explosive episode witnessing the emergence of the primary body plans of all multicellular forms of complex life (both plant and animal) serves as the scientific dividing line between the geologic “age of visible life” (the Phanerozoic Eon), and all the preceding geologic ages (collectively referred to as the Pre-Cambrian).

When this sudden burst of biological complexity was first discovered in the fossil record by geologists such as William Buckland and Adam Sedgewick, it defied the geological Uniformitarian view of Charles Lyell and Thomas Henry Huxley and posed a major challenge to the evolutionary gradualism of Charles Darwin. Although many have attempted to solve the paleontological puzzle passed on to succeeding generations as “Darwin’s Dilemma,” the Cambrian Explosion still remains a mystery without any clear scientific resolution.

Shaking the Bedrock of Uniformitarianism

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In his famous 1788 Theory of the Earth, uniformitarian geologist James Hutton argued that the forces that shape the Earth today have been in operation for endless ages with “no vestige of a beginning and no prospect of an end.” Hutton’s view contended that the ancient Greek philosopher Aristotle, whose ideas had long been revered in Europe, was right about the Cosmos and the Earth in holding “that the universe, and with it the Earth and human life, are not just extremely ancient but literally eternal, without any created beginning or final end.”² For Hutton there was no age of reptiles before the dawn of humankind, and there was certainly not a beginning to complex forms of life.

In the 1800s geologist Charles Lyell—the so-called “vanquisher of the reactionary forces of Religion”—followed in Hutton’s uniformitarian footsteps.³ Lyell dismissed the idea of mass extinction events and any other apparently sudden “revolutions” in the deep past.⁴ Like Hutton before him, Lyell asserted that all events and forces of Earth’s geological past were essentially the same (or uniform). Thus any idea of special surges of creation or unique historical catastrophes was dismissed as scientifically null and void.

William Buckland—Lyell’s geology professor—disagreed.⁵ Pioneering geologist, palaeontologist, first researcher to describe dinosaurs, first President of the British Association for the Advancement of Science, and Oxford University’s most prominent professor and scientist in the first half of the nineteenth century, Buckland was also an Anglican Priest and theologian who believed in the Biblical affirmation that God created the Earth. According to Buckland, the geological sciences supported the truth of Scripture in that there was a time before humans had walked the earth, before complex forms of life, and even before the planet Earth itself existed.

Buckland’s interpretation of the fossil record radically differed from that of his predecessor Hutton and his student Lyell. Rather than an endless cycle of repeating processes going back to a beginningless time, Buckland argued that Earth had a true history with unique ages progressing through time as unique groupings of animals came and went.

As Buckland explains: “The myriads of petrified Remains which are disclosed by the researches of Geology all tend to prove that our Planet has been occupied in times preceding the Creation of the Human Race, by extinct species of Animals and Vegetables, made up, like living Organic Bodies, of 'Clusters of Contrivances,' which demonstrate the exercise of stupendous Intelligence and Power. They further show that these extinct forms of Organic Life were so closely allied, by Unity in the principles of their construction, to Classes, Orders, and Families, which make up the existing Animal and Vegetable Kingdoms, that they not only afford an argument of surpassing force, against the doctrines of the Atheist and Polytheist; but supply a chain of connected evidence, amounting to demonstration, of the continuous Being, and of many of the highest Attributes of the One Living and True God.”⁶

Buckland also maintained that there was a beginning of complex life and that this could be discerned in the fossil record. He noted the sudden appearance of diverse skeletal remains in the lowest known fossiliferous rocks of the Cambrian (which we now consider the beginning of the Cambrian explosion) and he examined the geological evidence of the Cambrian explosion to demonstrate God's design and wisdom in creation. Contrary to the uniformitarian interpretation of Hutton and Lyell which permitted no distinct or unique epochs in the history of the creation of life, Buckland asserted that “we have found abundant proofs, both of the Beginning and the End of several successive systems of animal and vegetable life; each compelling us to refer its origin to the direct agency of Creative Interference.”

Thus, declared Buckland, “We conceive it undeniable, that we see, in the transition from an Earth peopled by one set of animals to the same Earth swarming with entirely new forms of organic life, a distinct manifestation of creative power transcending the operation of known laws of nature: and, it appears to us, that Geology has thus lighted a new lamp along the path of Natural Theology.”

Adam Sedgwick, Buckland’s paleontological and geological counterpart at Cambridge—who was likewise a devout theologian and Anglican priest—named the “Cambrian” period (after the Roman name for Wales) and also discerned in the rock strata of the Cambrian formations a true “vestige of a beginning” for life on earth, notwithstanding Lyell’s rejection of any such possibility.⁷ Sedgwick found that the fossil record petered out and disappeared altogether in the even more ancient formations before the Cambrian. For him, as for Buckland, the very beginning of the Palaeozoic era “had an obvious bearing on the fundamental problem of the origin of life itself.”⁸

In the mid-nineteenth century, it became clear that the lowest and oldest of the Palaeozoic systems, the Cambrian, which Sedgwick had based on rocks in Wales, was characterized by abundant fossils that were distinct from those in the overlying (and therefore later) strata. For a time Sedgwick, Buckland and others called the formations in which these Cambrian fossils were found “Primordial”, because their fossils “were the earliest unambiguous signs of life of any kind.” However, these primordial forms of life were not at all primitive—but rather they were highly diverse, large, and exceedingly complex, with sophisticated eyes, appendages, and bodily constructions that one might find in modern animals.

Even if they were only “a modest foretaste of the much greater diversity still to come,” the multifarious and multifaceted forms of life that inhabited the primordial Cambrian seas were clearly not found in the more ancient layers beneath the Cambrian. Since no life was found in these lower “basement” or “primary” rock layers (now known as Archean) they were labelled the Azoic (“no life”) Era.⁹

A Paleontological Puzzle Leading to Darwin’s Dilemma

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The existence of astonishingly complex life at the very beginning of the fossil record—with not a trace of any lifeform at all before it—posed a serious problem for Charles Darwin’s evolutionary theorizing, which posited extremely slow and gradual change beginning with the smallest and simplest imaginable forms of life. If Darwin’s view of evolution was right then the creatures of the early Cambrian period were not what one would expect. In order for all of the complex and highly diverse Cambrian organisms to have evolved slowly and gradually from “some one primordial form,” as Darwin had supposed, the Precambrian history of life would have had to be almost unimaginably lengthy: extending at least as far back before the Cambrian as the total time that had elapsed since the start of that period.

But if that were so, one would expect to see at least some sort of fossil record for the first half of the total history of life. Yet, at the time of Darwin, the first half of the history of life—before the Cambrian—was apparently (and mysteriously) without any clear fossil record at all. The absence of a fossil record prior to the Cambrian Period famously troubled Darwin, who considered the sudden appearance of complex animals to be “undoubtedly of the gravest nature.” Darwin wrote that if his theory of evolution by natural selection were true then “it is indisputable that before the lowest [Cambrian] stratum was deposited long periods [must have] elapsed, as long as, or probably far longer than, the whole interval from the [Cambrian] age to the present day; and that during these vast periods the world [must have] swarmed with living creatures.”¹⁰

Darwin also conjectured “that the ancestral vertebrate, an animal with an adult phenotype resembling the common embryological Bauplan [or body plan] of all modern vertebrates, must have lived long before the dawn of Cambrian times.”¹¹ Yet, the Pre-Cambrian fossil record that had been discovered in Darwin’s day had not “swarmed with living creatures” and there was no evidence for an animal with a vertebrate body plan long before the Cambrian. Darwin said that he could give “no satisfactory answer” as to why older fossiliferous deposits had not been found. As far as he could see, there were only two possible explanations. The first was that there was no fossil record because there had been no life to be recorded.

In this first case, the diverse Cambrian organisms evolved from “some one primordial form” relatively quickly in a decidedly un-Darwinian kind of evolution. Darwin found this first option highly problematic. The second possible explanation was that all the ancestors of the diverse Cambrian animals had been “soft-bodied,” and therefore were unlikely to be preserved as fossils. In this second scenario, all animals that left fossils would have had to acquire “hard parts” (such as shells that could easily be fossilized) at about the same time. Darwin was not content with the second explanation either, and at the close of the 19^th century the origin and early evolution of life before the Cambrian explosion remained deeply mysterious.

The Fact of the Cambrian Explosion, an Enduring Evolutionary Enigma

“Darwin’s dilemma,” regarding the “missing Precambrian history of life” has stood out as “one of the greatest unsolved mysteries in natural science”¹² and has continued to puzzle scientists for over a hundred and fifty years. Even while the hunt for Precambrian fossils has continued steadily since the 1850s¹³, and even in the midst of the discovery of a record of Precambrian fossils that now extends back over three billion years, Darwin's dilemma regarding the origin and early evolution early complex life persists, “because incontrovertible fossil evidence for animals remains largely, or some might say completely, absent from [Precambrian] rocks.”¹⁴

Indeed, “all fossil records still point to an explosion of animal life near the beginning of the Cambrian.”¹⁵ Beyond that, many more fossils bearing witness to a Cambrian explosion of life have been found. From these recent discoveries it is now clear that every known animal phylum, and several extinct phyla, made their first appearance in the fossil record during the first 20 million years of the Cambrian Period.¹⁶ With the Cambrian explosion all known “animal phyla with very distinct body plans arrive on the scene in a geological blink of an eye, with little or no warning of what is to come in rocks that predate this interval of time.”¹⁷ The Cambrian explosion thus “marks the beginning of macroevolution, evolution at the level of taxa, and even Bauplaene (body plans).”¹⁸ The current scientific consensus is that Cambrian explosion is an empirical fact, that “the Cambrian explosion is real,” and that “its consequences set in motion a sea-change in evolutionary history.”¹⁹

While the evidence for the reality of the Cambrian explosion has steadily accumulated over the past century and a half, the nature of this evidence has led to an increased skepticism regarding Darwinian natural selection as a key cause in this event. Indeed, the more complete the fossil record and the molecular evidence have become, the more scientists have questioned whether Darwinian evolution played any central role in the rapid diversification of life at the beginning of the Cambrian.

For instance, in the late 20^th and early 21^st century Harvard Biologist Stephen J. Gould pointed out that the fossil record of the early Cambrian contains very little to support the uniformitarianism and gradualism affirmed by Darwin. Instead, the evidence from paleontological history—and especially the Cambrian fossils—shows that species persisted for millions of years with very little change, then suddenly transformed or speciated in a relatively brief geological window of time.

With regard to our current scientific evidence, says Gould, “the explosion of multicellular life now seems as abrupt as ever—even more so since the argument now rests on copious documentation of Precambrian life, rather than a paucity of evidence that could be attributed to imperfections of the geological record.”²⁰ Even more recently researchers have pointed out that “the eruptive character of the Cambrian diversification obviously excludes,” the Darwinian evolutionary mechanisms of “gene mutations, gene recombination, and drift as its possible causes.”²¹

With regard to the nature of evolution immediately before the Cambrian period, the scientific consensus of microbiologists has followed Carl Woese, who argued in the early 21st century that the first 3.5 billion years of life’s evolution was non-Darwinian. According to Woese’s understanding, evolution during life’s earliest—and longest—phase was dominated by the communal lateral exchange of genes (or Horizontal Gene Transfer). Darwinian natural selection (and vertical gene transfer) did not become a factor in evolution until more complex life forms emerged. According to this understanding Darwinian evolution only emerged after—and as a consequence of—the Cambrian explosion.

While many theories have been proposed over the last 150 years to explain “the biological big bang,” and what gave rise to it, “today, we are still grappling with the question, but no closer to understanding the nature and causes of the Cambrian explosion.” Despite the accumulation of an immense and highly detailed fossil record, our advances in understanding molecular clocks, our insights into developmental genetics, our knowledge regarding the influence of extrinsic factors such as ocean chemistry and atmospheric oxygen, and our numerous attempts to make sense of the causal basis of the Cambrian explosion, it still remains as one of the deepest mysteries in life’s history. “Just like in Darwin’s time, it continues to be one of the greatest enigmas of modern biology.”

1. J. Paterson, G. Edgecombe, Michael S. Y. Lee, “Trilobite evolutionary rates constrain the duration of the Cambrian explosion” (February 2019) Proceedings of the National Academy of Sciences 116 (10).

2. Martin Rudwick, Earth’s Deep History: How It Was Discovered and Why It Matters (The University of Chicago Press, 2014) 27.

3. Rudwick, Earth’s Deep History, 163.

4. Rudwick, Earth’s Deep History, 164.

5. Simon Conway Morris, The Crucible of Creation: The Burgess Shale and the Rise of Animals (Oxford University Press, 1998) 141.

6. William Buckland, Geology and Mineralogy considered with reference to Natural Theology (London: W. Pickering, 1836).

7. Martin Rudwick, Worlds Before Adam: The Reconstruction of Geohistory in the Age of Reform, (University of Chicago Press, 2008) 448.

8. Rudwick, Earth's Deep History, 224.

9. Rudwick, Earth’s Deep History, 224.

10. Charles Darwin, On the Origin of Species by Means of Natural Selection, (John Murray: London, 1859), 287.

11. Stephen J. Gould, The Structure of Evolutionary Theory (Harvard University Press 2002), 154; Darwin, 338.

12. William Schopf, “Solution to Darwin’s dilemma: Discovery of the missing Precambrian record of life,” Proceedings of the National Academy of Sciences 97:13 (June 20, 2000) 6947–6953; Schopf says Darwin regarded “he lack of a rich fossil record predating the rise of shelly invertebrates that marks the beginning of the Cambrian Period of geologic time”… “to be the most vexing problem facing his theory of evolution… an inexplicable absence that could be truly urged as a valid argument against his all-embracing synthesis.”

13. Douglas H. Erwin and. James W. Valentine, The Cambrian Explosion: The Construction of Animal Biodiversity (Roberts & Co, 2013).

14. J.A. Cunningham, A.G. Liu, S. Bengtson, P.C. Donoghue, “The origin of animals: Can molecular clocks and the fossil record be reconciled?” Bioessays 39:1 (January 2017) 1-12.

15. Xingliang Zhang and Degan Shu, “Current understanding on the Cambrian Explosion: questions and answers,” PalZ (Paläontologische Zeitschrift) 95 (2021), 643.

16. This includes both Bryozoans and Chordates; See Z. Zhang, “Fossil evidence unveils an early Cambrian origin for Bryozoa,” Nature 599:7884 (2021) 251-255.

17. Kevin J. Peterson, Michael R. Dietrich and Mark A. McPeek, “MicroRNAs and metazoan macroevolution: insights into canalization, complexity, and the Cambrian explosion,” BioEssays, 31:7 (2009) 736-747.

18. Nelson Çabej, Epigenetic Mechanisms of the Cambrian Explosion (Academic Press, 2019), ix.

19. Simon Conway Morris, “The Cambrian “explosion”: slow-fuse or megatonnage?” Proceedings of the National Academy of Sciences 97, (2000) 4429; Conway Morris continues, “Although the pattern of evolution is clearer, the underlying processes still remain surprisingly elusive.”

20. Gould, The Structure of Evolutionary Theory, 154.

21. Çabej, Epigenetic Mechanisms of the Cambrian Explosion, x.