The Cambrian Explosion: What Happened, and Does It Point to Design?

About half a billion years ago, in the early Cambrian period, the fossil record changes character. Rocks that held mostly microbial mats, puzzling soft-bodied organisms and simple tracks begin to fill with animals that have shells, eyes, legs and guts: trilobites, early relatives of molluscs and worms, and the first members of our own phylum, the chordates. Paleontologists call this the Cambrian explosion.

Darwin himself named the event as a difficulty for his theory, and since 2013 it has been the centrepiece of a well-known argument for intelligent design, Stephen Meyer's Darwin's Doubt. Popular apologetics often presents the explosion as an instant in which every animal body plan appeared "fully formed" with no ancestors. Popular skepticism sometimes answers that there was no explosion at all. Neither summary is accurate.

This article sets out what the fossils and dating show, what Meyer argues and what his expert critics reply, and says plainly where the evidence helps the design argument and where it weakens it.

In brief

The Cambrian period began about 538.8 million years ago. Over roughly the next twenty to thirty million years, most of the major animal groups that leave good fossils make their first appearance, with famous windows such as the Chengjiang biota of China (no older than about 518 million years) and the Burgess Shale of Canada (about 505 million years). The event is real, and mainstream paleontologists affirm it. But it was neither instantaneous nor without precursors: animals already lived in the preceding Ediacaran period, small shelly fossils and burrows come before the classic faunas, and many Cambrian animals are "stem groups" rather than fully modern forms. Molecular clocks suggest the animal lineages split well before the Cambrian, by an uncertain margin. No single explanation of the causes commands agreement. Stephen Meyer argues that the burst of new biological information is best explained by an intelligent cause. Expert reviewers, notably the paleontologist Charles Marshall in Science, judged that he exaggerated the event's suddenness and misdescribed how body plans evolve; Meyer's colleagues have replied on timing and with newer genomic studies. The design inference is a live argument, not a proven conclusion.

Thesis: The Cambrian explosion is a real and still incompletely explained episode in the history of life, but it is longer, more gradual at its edges and better supplied with forerunners than popular design arguments claim, so it fits the Orthodox confession of God as Maker of all things without serving as a knock-down proof of design.

What happened, and when

The dates

The base of the Cambrian is defined at Fortune Head in Newfoundland by the first appearance of a distinctive burrow trace fossil (Treptichnus pedum; the International Commission on Stratigraphy's page uses its older name, Trichophycus pedum). The Commission's current chart dates that boundary to 538.8 million years ago, a figure revised in 2022 from the older and still widely quoted 541 or 542.¹ {{very-strong: The Cambrian began about 538.8 million years ago.}}

The first few million years of the Cambrian are dominated by tiny, often hard-to-classify "small shelly fossils." Trilobites, the best-known Cambrian animals, appear only about 521 million years ago.² The richest early window, the Chengjiang biota of Yunnan, famous for its preserved soft tissues, was shown by zircon dating published in 2018 to be no older than about 518 million years, and the authors concluded that the explosion was "a protracted process (>22 myr)."³ The Burgess Shale in British Columbia, found by Charles Walcott in 1909, is about 505 million years old, and the Royal Ontario Museum describes it as recording "the tail end of the event."⁴

How long did it take?

A 2019 review in Nature Ecology & Evolution put the main rise in animal diversity and abundance "between ~540 and 520 million years ago," and treated it as one of several successive radiations from the late Ediacaran into the early Palaeozoic.⁵ Nick Matzke, a longtime critic of intelligent design, has argued that it took "something like 30 million years" just to reach the first animals clearly related to living phyla.⁶ Claims that every body plan appeared within five or ten million years with nothing before it do not fit current dating, though Meyer and some specialists describe a shorter pulse inside this sequence (see below).

First, many Cambrian fossils are what paleontologists call stem groups: extinct animals with some but not all of the features of a living group. The Royal Ontario Museum's example is Opabinia, which has arthropod-like jointed claws but sits outside the arthropod crown group.⁷ Such intermediate animals are what common descent expects, and one reason specialists resist the phrase "fully formed." Second, none of this makes the explosion an illusion. The same museum concludes that "it is safe to call this event an 'explosion'."⁸ {{strong: The Cambrian explosion was a real biological event, not merely an artefact of preservation.}}

What came before

Darwin's difficulty

In chapter 9 of On the Origin of Species (1859), Darwin noted that the oldest fossil-bearing rocks he knew, the "lowest Silurian," already held trilobites. He wrote: "To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer." A little later he added: "The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained."⁹ He suggested that the record was very incomplete and that earlier animal life would be found.

The Ediacaran world

Part of Darwin's prediction has come true. The Ediacaran period, from about 635 million years ago to the start of the Cambrian, contains the first large, many-celled, soft-bodied organisms. Some, like the frond-shaped rangeomorphs, remain hard to classify.¹⁰ Others are now accepted as animals: in 2018 Ilya Bobrovskiy's team extracted molecular fossils from Dickinsonia and found that it "solely produced cholesteroids, a hallmark of animals," making it one of the oldest confirmed animals in the rock record.¹¹ Late Ediacaran rocks also preserve simple horizontal trails on the sea floor, and the earliest mineralised tubes, such as Cloudina, appear before the Cambrian boundary.¹²

The other part of Darwin's prediction has not come true in the way he hoped. No long Precambrian series of clear ancestors for each Cambrian group has been found. The Ediacaran animals make the explosion "appear less abrupt," in the Royal Ontario Museum's words, but most of them are not obvious grandparents of trilobites or chordates.⁸

The molecular clocks

Molecular clocks estimate from living animals' DNA when their lineages separated. A 2011 synthesis in Science by Douglas Erwin and colleagues concluded that "the major animal clades diverged many tens of millions of years before their first appearance in the fossil record," with their developmental "toolkits" in place during the Cryogenian period (about 720 to 635 million years ago).¹³ On this view the Cambrian records the ecological success of animals more than their first origin.

Molecular clocks have real limits. A 2015 study by Mario dos Reis and colleagues found that crown animals probably originated in the Cryogenian, but that uncertain fossil calibrations and clock violations rule out precise dates, so that "attempts to build evolutionary narratives of early animal evolution based on molecular clock timescales appear to be premature."¹⁴ {{debated: How long before the Cambrian the animal lineages separated remains uncertain.}}

Why then? The scientific proposals

Scientists have offered three broad families of explanation, often in combination.

• Environment. Animals need oxygen, and rising oxygen in the oceans is a long-standing candidate. The Royal Ontario Museum notes, though, that "there does not seem to be much variation in oxygen levels across the Ediacaran-Cambrian boundary," and that the last worldwide glaciation "seems to have ended around 635 million years ago," long before the explosion.¹⁵
• Ecology. Predators and prey drive each other into "arms races." A 2013 study led by Erik Sperling, with Andrew Knoll, noted that such ecological triggers explain the pattern but not the timing, while oxygen does the reverse; using modern low-oxygen zones, it found that low oxygen limits carnivory, linking the two.¹⁶
• Development. Animal bodies are built by gene regulatory networks, systems of genes that switch one another on and off. In 2006 Eric Davidson and Douglas Erwin proposed that the deepest "kernels" of these networks were established early and then became highly resistant to change, which would help explain why body plans appeared early and then persisted.¹⁷

How new body plans arise is debated within mainstream biology itself, which is part of why the Cambrian attracts design theorists.

The design argument: Darwin's Doubt

Stephen Meyer, who holds a Cambridge doctorate in the history and philosophy of science and directs the Discovery Institute's Center for Science and Culture, published Darwin's Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design with HarperOne in 2013. In an interview that year he described the explosion as "an important event in the history of life where nearly all of the major animal body plans appear abruptly in the fossil record without any apparent evolutionary precursors," and said that later discoveries "have only made the Cambrian explosion problem worse" for Darwin's theory.¹⁸

Marshall summarised the book's central claim as being that "the origin of new animal body plans requires vast amounts of novel genetic information."¹⁹ Meyer argues that undirected mutation and selection cannot generate so much functional information in the time available, and that the features of the explosion are "better explained by intelligent design than purely undirected evolutionary processes," in the words of the interviewer's summary. He also argues that shared features between animal groups need not prove common ancestry: "That different organisms share some of the same parts could easily reflect common design rather than common descent."¹⁸ (The wider information argument is set out in The Design Inference and DNA as Information.)

What the scientific reviewers said

The most important review came from Charles Marshall, a paleontologist at the University of California, Berkeley and a specialist in early animal evolution, writing in Science in September 2013. His points, stated in his own words:¹⁹

• On suddenness: Meyer "completely omits mention of the Early Cambrian small shelly fossils" and misreads molecular phylogenetics, both of which "cause him to exaggerate the apparent suddenness of the Cambrian explosion."
• On mechanism: current understanding of how bodies develop "indicates that new phyla were not made by new genes but largely emerged through the rewiring of the gene regulatory networks (GRNs) of already existing genes." Meyer's objection that tampering with such networks is usually lethal, Marshall argued, applies to today's networks, which carry half a billion years of later evolution, not to the less encumbered networks of the early Cambrian.
• On method: he called the approach a "(sophisticated) 'god of the gaps'" argument, and charged the book with scholarship that was "highly selective" where it mattered most.

Marshall had "dared hope" the book would expose real problems in paleontologists' thinking. Nick Matzke's review at The Panda's Thumb pressed similar points from phylogenetics, calling the many Chengjiang arthropod-like animals that fall outside the living groups "transitional forms."⁶

How design theorists replied

Meyer and his colleagues have answered their critics at length. On timing, Casey Luskin of the Discovery Institute replied in 2013 that "Meyer does discuss the small shelly fossils on page 425 of Darwin's Doubt," and that Meyer had not exaggerated the event: the book gives the explosion about ten million years and places its "main pulse" of new phyla and classes in a span of no more than about six million years, following a 1993 dating study by Samuel Bowring and colleagues. He also argued that the explosion proper begins after the first small shelly fossils.²⁰ Specialists, too, speak of a rapid pulse: the 2011 review by Erwin and colleagues says diverse bilaterian groups "emerged apparently within a few million years during the early Cambrian."¹³ The disagreement is partly over what is measured: a short burst of first appearances, or the whole sequence from the first small shelly fossils to Chengjiang (more than twenty million years).

The strongest later reply concerns Marshall's claim that new genes did not drive the explosion. In 2021 Luskin pointed to two genomic studies.²⁰ Jordi Paps and Peter Holland reconstructed the gene content of the first animal and reported, "contrary to the prevailing view," an "unprecedented increase in the extent of genomic novelty during the origin of metazoans."²¹ Peter Heger and colleagues identified 157 genes that arose at the origin of bilaterian animals, including the whole Nodal signalling pathway, and wrote that, "contradicting the current view," these genes are robustly associated with key bilaterian features.²² Luskin's conclusion: "Marshall's proposal does not eliminate the need to explain the origin of genetic information."

This is fair: new genes appear to have mattered more than Marshall's 2013 summary allowed. Two limits apply. Both studies concern genes that arose at the origin of animals or of bilaterians, which on the molecular clocks may lie well before the Cambrian fossils, so they do not show the information appearing within the Cambrian window. And both sets of authors explain their findings within evolutionary biology; neither suggests a designer. The studies strengthen Meyer's premise that animal origins involved real genetic novelty, without establishing his conclusion about its cause.

An honest assessment

Where the design argument is weakened by the evidence:

• Meyer's geological "blink of an eye" fits, at most, a pulse of first appearances inside a sequence of more than twenty million years, itself part of a longer series of radiations that began in the Ediacaran.
• "Without any apparent evolutionary precursors" overstates the case. There are Ediacaran animals, pre-Cambrian burrows and tubes, early small shelly fossils, and many Cambrian stem-group animals that combine features of different living groups.
• The reviews by specialists in early animal evolution that we could find were strongly critical. Reviews do not settle a question, but they should restrain anyone who presents the book as having refuted the specialists.
• Much of the case rests on what current theories have not yet explained, and such gaps have closed before.

Where the design argument retains real force:

• The event is real, and mainstream scientists disagree about its triggers and mechanisms.
• How developmental gene networks and the new genes that animals needed first arose remains an open research question, and recent genomics has increased the novelty to be explained.
• Design theorists present their case as an inference to the best explanation, since intelligence is a known cause of functional information, not only as a gap. Whether that inference is legitimate in natural history is a philosophical question as much as a scientific one, argued in our pieces on design as an inference and in Is Intelligent Design a 'God of the Gaps' Argument?

{{debated: The Cambrian explosion is best explained by an intelligent cause acting in the history of life.}} That is our honest rating of the design inference from the Cambrian alone.

The Orthodox position

The Orthodox Church confesses in the Nicene Creed one God, "the Father Almighty, Maker of Heaven and Earth and of all things visible and invisible."²³ That is dogma. How God brought animals into being, over what span of time and through what secondary causes, is not defined by any council, and Orthodox Christians hold a range of views; we survey them in Can a Christian Accept Evolution?

St Basil the Great, preaching in his Hexaemeron on the command "Let the earth bring forth" grass and plants, said that God's command became "as a natural and permanent law" for the earth, giving it "the power to produce fruit for all ages to come," so that "this first command teaches nature what she has to do in the course of ages."²⁴ Basil also took the fixity of kinds for granted (diseased wheat, he says, is "not a change of kind"), so he should not be enlisted for or against evolution. What he offers is a way of seeing: natural processes unfolding over ages are not rivals to the Creator but the working out of His word.

St Paul says that God's "invisible attributes are clearly seen, being understood by the things that are made" (Romans 1:20, NKJV), and the Psalmist, "O Lord, how manifold are Your works! In wisdom You have made them all" (Psalm 104:24, NKJV; Psalm 103 in the Septuagint). On the Orthodox view, the well-understood processes of creation bear the marks of divine wisdom as much as the puzzling ones, and Christians are found on both sides of the scientific argument. The geneticist Theodosius Dobzhansky, born in the Russian Empire, a practising Christian and one of the architects of modern evolutionary theory, wrote in 1973: "I am a creationist and an evolutionist. Evolution is God's, or Nature's method of creation."²⁵ Design theorists such as Meyer argue, by contrast, that undirected processes cannot account for what we see. An Orthodox reader can weigh both on the evidence without treating either as the faith itself. (See also Has Science Disproved Christianity?, and on the age of the earth, Genesis & the Age of the Earth.)

The strongest objections

"There was no explosion; it is just better preservation"

The skeptic's version holds that animals with hard parts simply fossilise better, so the arrival of shells makes life seem to appear suddenly. The Royal Ontario Museum states the point fairly: "preservation bias alone could create the appearance" of an explosion. But the museum concludes that the early Cambrian was still "a time of major change," with new body plans and ecological niches, and that the word "explosion" is justified.⁸ Soft-bodied windows like Chengjiang show that the change was not only in shells.

"Intelligent design is a god of the gaps"

This is Marshall's main charge, and it has force: if future research explains the origin of animal body plans, an argument built on present ignorance would fall. Design theorists reply that they argue from what intelligence is known to do, not only from what evolution has not yet done. A design inference can be legitimate in principle, but its strength here depends on how complete the naturalistic alternatives are, and those are still being developed.

"The critics are defending a paradigm, not following the evidence"

Some critics' rhetoric has been dismissive. But Marshall's central objections concern data, such as the small shelly fossils and gene regulation, and should be answered on the data. Where Meyer's side has done that, as with the genomic studies, it has scored a real point; where it has not, a charge of prejudice is no substitute.

What we can say confidently

• The Cambrian period began about 538.8 million years ago, and most major animal groups with good fossil records first appear in the following twenty to thirty million years.
• The Cambrian explosion was a real event, affirmed by mainstream paleontologists, not an invention of creationists or merely an artefact of preservation.
• It had forerunners: Ediacaran animals such as Dickinsonia, pre-Cambrian burrows and mineralised tubes, and early Cambrian small shelly fossils.
• Many Cambrian animals are stem groups, not fully modern representatives of today's phyla.
• No single explanation of the explosion's causes commands scientific agreement.
• The Orthodox Church confesses God as Maker of all things and has defined no doctrine on the mechanism or timing of animal origins.

What remains disputed

• How long before the Cambrian the animal lineages diverged, and how far molecular clocks can be trusted for such ancient events.
• Which triggers mattered most: oxygen, ecology, developmental genetics, or a combination.
• How much of the explosion depended on new genes and how much on rewiring existing ones; recent genomic studies suggest more novelty than was assumed in 2013.
• Whether undirected processes can generate the information needed for new body plans in the time available, and whether a design inference from the Cambrian is a legitimate explanation.
• Among Christians, including Orthodox Christians, whether God's creative work is better described through evolutionary processes or through distinct acts within natural history.

Conclusion

The Cambrian explosion is neither the ancestor-less instant of some apologetics nor the non-event of some skepticism. It was a real and relatively rapid flowering of animal life, prepared by an older Ediacaran world, still incompletely explained, and argued over by able scientists of more than one persuasion.

For the Orthodox Christian, nothing in that picture threatens the faith. We do not believe in God because of a gap in paleontology, and would not cease to believe if it closed. We confess the Father Almighty, Maker of all things visible and invisible, and we see His wisdom both in what we understand and in what still astonishes us. The trilobites of Chengjiang and the strange animals of the Burgess Shale belong to the "manifold works" of the Psalm. Christians who think the Cambrian points to design may argue so, provided they tell the whole story, and those who see in it the patient working of secondary causes may say so too.

Notes

1. International Commission on Stratigraphy (ICS), International Chronostratigraphic Chart, v2024/12 (base of Cambrian 538.8 ± 0.6 Ma; base of Ediacaran about 635 Ma; base of Cambrian Stage 3 about 521 Ma); ICS chart change log 2012-2022, entry 2022/02 ("numeric age base Fortunian is 538.8 +/- 0.2 Ma (Linnemann et al., 2019)"); ICS, "GSSP for Fortunian Stage" (Fortune Head, Newfoundland; first occurrence of Trichophycus pedum), stratigraphy.org. ↩
2. Royal Ontario Museum, "Fossil Evidence," The Burgess Shale website, burgess-shale.rom.on.ca (small shelly fossils; trilobites "About 521 million years ago"). ↩
3. Chuan Yang, Xian-Hua Li, Maoyan Zhu, Daniel J. Condon and Junyuan Chen, "Geochronological constraint on the Cambrian Chengjiang biota, South China," Journal of the Geological Society 175, no. 4 (2018): 659-666, doi:10.1144/jgs2017-103 (maximum age 518.03 ± 0.69/0.71 Ma). ↩
4. Royal Ontario Museum, "Uniqueness and Timing" (age about 505 million years; "the tail end of the event") and "Discoveries" (Walcott, 1909), burgess-shale.rom.on.ca. ↩
5. Rachel Wood et al., "Integrated records of environmental change and evolution challenge the Cambrian Explosion," Nature Ecology & Evolution 3 (2019): 528-538, doi:10.1038/s41559-019-0821-6. ↩
6. Nick Matzke, "Meyer's Hopeless Monster, Part II," The Panda's Thumb, 19 June 2013, pandasthumb.org/archives/2013/06/meyers-hopeless-2.html. ↩
7. Royal Ontario Museum, "Stem Group and Crown Group Concepts," burgess-shale.rom.on.ca. ↩
8. Royal Ontario Museum, "Was there an Explosion at all?," burgess-shale.rom.on.ca. ↩
9. Charles Darwin, On the Origin of Species by Means of Natural Selection (London: John Murray, 1859), chapter 9, "On the Imperfection of the Geological Record," section "On the sudden appearance of groups of Allied Species in the lowest known fossiliferous strata." First-edition text, Project Gutenberg eBook 1228. ↩
10. Royal Ontario Museum, "Enigmatic Ediacarans," burgess-shale.rom.on.ca. ↩
11. Ilya Bobrovskiy, Janet M. Hope, Andrey Ivantsov, Benjamin J. Nettersheim, Christian Hallmann and Jochen J. Brocks, "Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals," Science 361 (2018): 1246-1249, doi:10.1126/science.aat7228. ↩
12. Royal Ontario Museum, "Fossil Evidence" (trace fossils; Cloudina as "one of the earliest animals with a mineralized skeleton"), burgess-shale.rom.on.ca. ↩
13. Douglas H. Erwin, Marc Laflamme, Sarah M. Tweedt, Erik A. Sperling, Davide Pisani and Kevin J. Peterson, "The Cambrian conundrum: early divergence and later ecological success in the early history of animals," Science 334 (2011): 1091-1097, doi:10.1126/science.1206375. ↩
14. Mario dos Reis, Yuttapong Thawornwattana, Konstantinos Angelis, Maximilian J. Telford, Philip C. J. Donoghue and Ziheng Yang, "Uncertainty in the timing of origin of animals and the limits of precision in molecular timescales," Current Biology 25 (2015): 2939-2950, doi:10.1016/j.cub.2015.09.066. ↩
15. Royal Ontario Museum, "Triggers of the Cambrian Explosion," burgess-shale.rom.on.ca. ↩
16. Erik A. Sperling, Christina A. Frieder, Akkur V. Raman, Peter R. Girguis, Lisa A. Levin and Andrew H. Knoll, "Oxygen, ecology, and the Cambrian radiation of animals," Proceedings of the National Academy of Sciences 110 (2013): 13446-13451, doi:10.1073/pnas.1312778110. ↩
17. Eric H. Davidson and Douglas H. Erwin, "Gene regulatory networks and the evolution of animal body plans," Science 311 (2006): 796-800, doi:10.1126/science.1113832. ↩
18. "Darwin's Doubt: An Interview with Stephen C. Meyer," Justin Taylor's blog, 30 May 2013 (as archived at goodreads.com/author_blog_posts/4276373). The book: Stephen C. Meyer, Darwin's Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design (New York: HarperOne, 2013), ISBN 978-0-06-207147-7. ↩
19. Charles R. Marshall, "When Prior Belief Trumps Scholarship," review of Darwin's Doubt, Science 341 (20 September 2013): 1344, doi:10.1126/science.1244515. The review is behind a paywall; quotations were checked against the extended excerpt reproduced by Jerry Coyne, "Famous paleobiologist savages Stephen Meyer's ID book," Why Evolution Is True, 20 September 2013. ↩
20. Casey Luskin, "Small Shelly Fossils, and the Length of the Cambrian Explosion," 23 October 2013, scienceandculture.com/2013/10/small_shelly_fo; Luskin, "How 'Sudden' Was the Cambrian Explosion? Nick Matzke Misreads Stephen Meyer and the Paleontological Literature; New Yorker Recycles Misrepresentation," 16 July 2013, scienceandculture.com/2013/07/how_sudden_was_ (quoting Darwin's Doubt, pp. 71-73, and S. A. Bowring et al., "Calibrating Rates of Early Cambrian Evolution," Science 261 (1993): 1293-1298); Luskin, "Did the Origin of Animals Require New Genes?," 6 May 2021, scienceandculture.com/2021/05/did-the-origin-of-animals-require-new-genes. All published by the Discovery Institute (Science and Culture Today, formerly Evolution News). ↩
21. Jordi Paps and Peter W. H. Holland, "Reconstruction of the ancestral metazoan genome reveals an increase in genomic novelty," Nature Communications 9 (2018): 1730, doi:10.1038/s41467-018-04136-5. ↩
22. Peter Heger, Wen Zheng, Anna Rottmann, Kristen A. Panfilio and Thomas Wiehe, "The genetic factors of bilaterian evolution," eLife 9 (2020): e45530, doi:10.7554/eLife.45530. ↩
23. The Symbol of Faith (Nicene-Constantinopolitan Creed), Orthodox Church in America translation, oca.org/orthodoxy/prayers/symbol-of-faith. ↩
24. Basil the Great, Hexaemeron, Homily 5 ("The Germination of the Earth"), sections 1 and 5, trans. Blomfield Jackson, Nicene and Post-Nicene Fathers, 2nd series, vol. 8 (1895), ccel.org/ccel/schaff/npnf208.viii.vi.html. On the fixity of kinds, see 5.5 ("This is not a change of kind"). ↩
25. Theodosius Dobzhansky, "Nothing in Biology Makes Sense except in the Light of Evolution," The American Biology Teacher 35, no. 3 (1973): 125-129, doi:10.2307/4444260; text as reproduced in the PBS "Evolution" online library. ↩

Bibliography

Primary sources

• Darwin, Charles. On the Origin of Species by Means of Natural Selection. London: John Murray, 1859. First edition, Project Gutenberg eBook 1228.
• The Holy Bible, New King James Version. Nashville: Thomas Nelson, 1982.
• International Commission on Stratigraphy. International Chronostratigraphic Chart, v2024/12; chart change log 2012-2022; "GSSP for Fortunian Stage." stratigraphy.org.

Orthodox and patristic sources

• Basil the Great. Hexaemeron. Translated by Blomfield Jackson. Nicene and Post-Nicene Fathers, 2nd series, vol. 8. New York: Christian Literature Company, 1895. ccel.org.
• Orthodox Church in America. "The Symbol of Faith." oca.org.

Other scholarship (including scholars who disagree)

• Bobrovskiy, Ilya, et al. "Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals." Science 361 (2018): 1246-1249.
• Davidson, Eric H., and Douglas H. Erwin. "Gene regulatory networks and the evolution of animal body plans." Science 311 (2006): 796-800.
• Dobzhansky, Theodosius. "Nothing in Biology Makes Sense except in the Light of Evolution." The American Biology Teacher 35 (1973): 125-129.
• dos Reis, Mario, et al. "Uncertainty in the timing of origin of animals and the limits of precision in molecular timescales." Current Biology 25 (2015): 2939-2950.
• Erwin, Douglas H., et al. "The Cambrian conundrum: early divergence and later ecological success in the early history of animals." Science 334 (2011): 1091-1097.
• Heger, Peter, et al. "The genetic factors of bilaterian evolution." eLife 9 (2020): e45530.
• Marshall, Charles R. "When Prior Belief Trumps Scholarship." Science 341 (2013): 1344.
• Matzke, Nick. "Meyer's Hopeless Monster, Part II." The Panda's Thumb, 19 June 2013.
• Paps, Jordi, and Peter W. H. Holland. "Reconstruction of the ancestral metazoan genome reveals an increase in genomic novelty." Nature Communications 9 (2018): 1730.
• Royal Ontario Museum. The Burgess Shale website: pages on the Cambrian explosion, Ediacarans, fossil evidence, triggers, stem and crown groups, and the age and discovery of the Burgess Shale. burgess-shale.rom.on.ca.
• Sperling, Erik A., et al. "Oxygen, ecology, and the Cambrian radiation of animals." Proceedings of the National Academy of Sciences 110 (2013): 13446-13451.
• Wood, Rachel, et al. "Integrated records of environmental change and evolution challenge the Cambrian Explosion." Nature Ecology & Evolution 3 (2019): 528-538.
• Yang, Chuan, et al. "Geochronological constraint on the Cambrian Chengjiang biota, South China." Journal of the Geological Society 175 (2018): 659-666.

Sources from the intelligent design movement

• Meyer, Stephen C. Darwin's Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design. New York: HarperOne, 2013. With "Darwin's Doubt: An Interview with Stephen C. Meyer," Justin Taylor's blog, 30 May 2013.
• Luskin, Casey. "Did the Origin of Animals Require New Genes?" Science and Culture Today, 6 May 2021. With his replies of 16 July 2013 ("How 'Sudden' Was the Cambrian Explosion?") and 23 October 2013 ("Small Shelly Fossils, and the Length of the Cambrian Explosion").