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Untangling “Professor Dave’s” Confusion about the Cambrian Explosion

Photo: Fossil of Opabinia regalis, a Cambrian animal, by Jstuby at English Wikipedia, Public domain, via Wikimedia Commons.

In a series at Evolution News, I am currently reviewing the attack (Farina 2022) on Stephen Meyer and Darwin’s Doubt by popular science YouTuber Dave Farina (aka Professor Dave). You will find my first post in the series here. We have seen the absurdly low quality of this individual’s video. But there is much more. I have added timecodes in square brackets for easier reference.

[TC 7:44] If all is well with the fossil record as evidence for Darwinian gradualism, as Mr. Farina claims, then why does he have to raise his next point, invoking the “obvious limitations to the fossil record” due to rare fossilization and sampling bias? He says that Dr. Meyer’s rejection of the “artifact hypothesis” fails to acknowledge the validity of these limitations. Instead, he says, Meyer lied about the Doushantuo Formation (more about that later). However, it is Farina, who is distorting the truth, as Meyer in Darwin’s Doubt discusses at length the limitations of the fossil record and explores in detail why these limitations cannot explain away the absence of the assumed ancestors of the Cambrian phyla in the Ediacaran strata (Meyer 2013a: 56–62). My article on the demise of the artifact hypothesis should settle this issue beyond reasonable doubt (Bechly 2020d), because numerous Ediacaran localities of the Burgess-Shale-type could and would have preserved even small and soft-bodied assumed ancestors of the Cambrian phyla but there are none. The same was acknowledged by Scheffer (2009), who said “it could be that earlier rocks were not as good for preserving fossils. However, very well-preserved fossils do exist from earlier periods, and it is now generally accepted that the Cambrian explosion was real.” The artifact hypothesis was also rejected by the leading experts Erwin & Valentine (2013) in their book on the Cambrian Explosion (see Luskin 2013a for quotes).

By the way: if Farina or anybody else should respond that such Evolution News articles don’t count because they are not peer-reviewed scientific papers, you just have to mention that all those articles are heavily based on such peer-reviewed mainstream studies and only report and comment on them. You will find all the references there.

[TC 8:50] Farina disputes the claim that “Animals appear for the first time in the Cambrian explosion,” which Meyer made in a popular video. Farina pedantically says this is objectively wrong because Meyer himself mentioned sponges and Kimberella as a probable simple animal from the Ediacaran. Therefore, Farina considers the Cambrian only as a diversification of animal life not its origin. This is either false or misleading. It is false if it refers to the core problem of the Cambrian Explosion, which is the abrupt origin of most bilaterian animal phyla or body plans. It is misleading if it refers to the origin of animals in the sense of Metazoa because (as Farina correctly noted) Meyer never disputes the existence of Precambrian metazoans such as sponges and cnidarians, and even acknowledges Kimberella as a possible bilaterian, though there are good reasons to doubt that Kimberella was a mollusk or a bilaterian (see further on for a critical discussion of Kimberella).

Dating the Cambrian Explosion

[TC 9:22] Farina claims that the dating of the Cambrian Explosion as 530-520 mya is wrong too. He cites Erwin et al. (2011) as establishing:

  • Earliest skeletal fossils in the latest Ediacaran
  • Plates, spines, shells in the Fortunian 541-530 mya
  • First occurrences of metazoan phyla in the latest Ediacaran 555 mya with a dramatic rise of 25 mya in the first stages of the Cambrian.

[TC 10:48] Farina therefore thinks that the Cambrian Explosion was at least 25 my (not 10 my) long and Meyer should have known this because Erwin’s paper appeared before Meyer’s book. Well, I actually agree that the total length of the Cambrian Explosion, as documented by the fossil record, may have been about 25 million years. But this point is immaterial because many experts also agree that the main pulse was much shorter and precisely in the range given by Meyer (see the numerous studies quoted in review articles by Luskin 2013 and CSC 2019). Here are just two prominent examples:

  • Bowring et al. (1993) concluded that the “period of exponential increase of diversification lasted only 5 to 6 m.y. … it is unlikely to have exceeded 10 m.y.”
  • In the standard textbook on the Cambrian Explosion, the renowned experts Erwin & Valentine (2013: p. 5) (yes, that’s the same Douglas Erwin who was lead author of the study quoted by Farina) dated the main diversification to a “geologically brief interval between about 530 to 520 Ma.” That’s 10 million years, exactly as stated by Meyer. Even the older Erwin et al. (2011) paper quoted by Farina found 13 bilaterian animal phyla originating in the Lower Cambrian Stage 3 (see his Supplementary Information), which lasted about 7 million years and includes the famous Sirius Passet and Chengjiang biota.

Furthermore, the fact that the first fossils found of different Cambrian animal phyla are somewhat spread out in time does not at all contradict a much narrower event for their origin, because the so-called Signor-Lipps effect has to be taken into account. This effect refers to the fact that the oldest and youngest discovered fossils often fail to represent the actual first and last occurrences of these taxa in the history of life. Finally, it does not make a significant difference for the problem of the Cambrian Explosion whether it lasted 5 million years or 25 million years, because even the latter range would be orders of magnitude too short to accommodate the origin and spreading of the required genetic changes, based on standard population genetics (see further on for this waiting time problem).

Two Phases of the Cambrian Explosion

[TC 11.07] Farina cites more recent research by Zhuralev & Wood (2018), who suggested two phases of the Cambrian Explosion with two separate radiations, one of stem lineages 542-513 mya, and one of crown lineages from 513-485 my (connecting with the onset of the Great Ordovician Biodiversification Event, aka GOBE). This is totally irrelevant, though, as the problem of Cambrian Explosion is the origin of the body plans in the first radiation and not their diversification in the second one. That there were many other later radiations is not under dispute, but the narrow windows of time for most of these radiations show that the Cambrian Explosion is just one of many examples of abrupt origins that contradict gradualist Darwinian expectations (Bechly & Meyer 2017Bechly 2021e). It makes the problem worse not better.

[TC 11:41] Farina misrepresents the claim for the suddenness of the Cambrian Explosion. He says the claim is that “an enormous number of species showed up in an unimaginable short period of time.” Nope, that is not the problem at all. The problem is the abrupt origin of complex body plans of bilaterian animal phyla, without any fossil evidence for their gradual evolution in the preceding layers. He calls it dishonesty to exploit the apparent suddenness suggested by the word “Explosion” but fails to mention that this suddenness is emphasized in tons of technical papers on the Cambrian Explosion, which was named “Explosion” and “Evolution’s Big Bang” for a reason. Again, see Luskin (2013b) for an extensive review of all the mainstream technical papers that use words like “sudden” or “abrupt” to describe the Cambrian Explosion and the origin of the phyla and their respective body plans. 

[TC 12:00] Farina says that in actuality the two phases of the Cambrian Explosion took 70 million years. Well, except for the authors of the mentioned study and their research group, hardly any respected expert on the Cambrian Explosion ever claimed that it spanned 70 million years, mainly because the term “Cambrian Explosion” only refers to the first radiation. Another recent study seeks to identify three phases of the Cambrian Explosion but estimates its maximal duration to be about 40 million years (Zhang & Shu 2021). However, these authors qualify this maximum estimate with the following admission: “Most body plans emerged within [my emphasis] this time window. The birth of each body plan might be even in a much shorter [my emphasis] time interval.” They also ask if the Cambrian Explosion poses a challenge to evolution, but with hand-waving dismiss this dangerous possibility, simply maintaining that millions of years is plenty of time for evolutionary change.

A Long Time, Biologically Speaking?

[TC 12:10] Like the latter authors, Farina also claims that the Cambrian Explosion was a long time biologically for the diversification to take place. Really? Such bold claims, whether made by scientists or by YouTubers, are not science but just an unsubstantiated opinion. If real hard science like the math of population genetics is used to address this question, the Darwinian house of cards collapses (see further on for this waiting time problem). Also, the available time was certainly not as long as claimed. How did Zhang & Shu (2021) arrive at their maximum estimate of 40 million years? Similar to Daley et al. (2018), they simply took the documented absence of metazoan communities at 560 mya as their starting point in the Ediacaran and the full development of the metazoan communities at 521 mya in the Lower Cambrian as their end point. However, the Cambrian Stage 1 begins at 541 million years and there is no fossil record of uncontroversial metazoan communities prior to this, as explicitly granted by Daley et al. and others. Zhang & Shu simply added 20 million years of the terminal Ediacaran to the Cambrian Explosion. They did so without any evidence whatsoever. Casey Luskin (2013b) wrote an excellent article about how mainstream experts support Meyer’s dating and description of the suddenness of the Cambrian Explosion, which totally debunks Farina’s claims (also see CSC 2019). 

A Word from the Experts

I don’t have much to add to Luskin’s (2013b) article, apart from a few more references from some true experts:

  • Scheffer (2009: 169-170): “The collapse of the Ediacaran fauna is followed by the spectacular radiation of novel life-forms known as the Cambrian explosion. All of the main body plans that we know now evolved in as little as about 10 million years. It might have been thought that this apparent explosion of diversity might be an artifact. For instance, it could be that earlier rocks were not as good for preserving fossils. However, very well preserved fossils do exist from earlier periods, and it is now generally accepted that the Cambrian explosion was real.”
  • Erwin & Valentine (2013: 5-6): “[A] great variety and abundance of animal fossils appear in deposits dating from a geologically brief interval between about 530 to 520 Ma, early in the Cambrian period. During this time, nearly all the major living animal groups (phyla) that have skeletons first appeared as fossils (at least one appeared earlier). Surprisingly, a number of those localities have yielded fossils that preserve details of complex organs at the tissue level, such as eyes, guts, and appendages. In addition, several groups that were entirely soft-bodied and thus could be preserved only under unusual circumstances also first appear in those faunas. Because many of those fossils represent complex groups such as vertebrates (the subgroup of the phylum Chordata to which humans belong) and arthropods, it seems likely that all or nearly all the major phylum-level groups of living animals, including many small softbodied groups that we do not actually find as fossils, had appeared by the end of the early Cambrian. This geologically abrupt and spectacular record of early animal life is called the Cambrian explosion. …Taken at face value, the geologically abrupt appearance of Cambrian faunas with exceptional preservation suggested the possibility that they represented a singular burst of evolution, but the processes and mechanisms were elusive. Although there is truth to some of the objections, they have not diminished the magnitude or importance of the explosion. … Several lines of evidence are consistent with the reality of the Cambrian explosion.”
  • Lee et al. (2013): “The near-simultaneous appearance of most modern animal body plans (phyla) ∼530 million years ago during the Cambrian explosion is strong evidence for a brief interval of rapid phenotypic and genetic innovation … The abrupt appearance of most modern animal body plans (often ranked as phyla and classes) over half a billion years ago is one of the most important evolutionary events after the origin of life.”
  • Briggs (2015): “We now know that the sudden appearance of fossils in the Cambrian (541–485 million years ago) is real and not an artefact of an imperfect fossil record … all the major animal groups evolved in a relatively short time during the Cambrian explosion.”
  • Buatois et al. (2016): “The majority of body plans were established during the Cambrian Explosion (CE), whereas the significant taxonomic increases during the Great Ordovician Biodiversification Event (GOBE) were manifest at lower taxonomic levels.”
  • Davis (2019): “The Cambrian explosion was far shorter than we thought.”
  • Cabey (2020): “Nevertheless, now, 150 years after The Origin, when an incomparably larger stock of animal fossils has been collected, Darwin’s gap remains, the abrupt appearance of Cambrian fossils is a reality, and we are still wondering about the forces and mechanisms that drove it. Despite the fact that, from time to time, a small number of students have questioned the reality of the Cambrian explosion on the same ground as Darwin, today’s consensus is that the Cambrian explosion is a scientific fact (Linnemann et al., 2019) and ‘The Cambrian explosion is real and its consequences set in motion a sea-change in evolutionary history’ (Conway Morris, 2000Nichols et al., 2006).”
  • Heger et al. (2020): “The Cambrian explosion was a unique animal radiation ~540 million years ago that produced the full range of body plans across bilaterians. The genetic mechanisms underlying these events are unknown.”

How short was the Cambrian Explosion? Recent evidence suggests that it may have been extremely short (Bechly 2021d). A study by Linnemann et al. (2018) demonstrated that the window of time between the latest appearance date (LAD) of the alien Ediacaran biota and the first appearance date (FAD) of the complex Cambrian biota was only 410,000 years. Again, we must take the Signor-Lipps effect into account, but you get an idea about the biological and geological abruptness of this event.

Furthermore, Farina’s claim that the Cambrian Explosion was a long time, biologically speaking, is nonsense. See the recent study by Daley et al. (2018) that I have discussed in two previous articles (Bechly 2018aBechly 2021d). The bottom line is that this study showed that only 13 million years were available for the transition from simple stem-metazoans to the fully developed and highly complex body plan of crown group arthropods, with exoskeleton, articulated legs, gut system, nervous system, and highly efficient compound eyes. This time span equals about the average longevity (5-10 my) of just two successive marine invertebrate species (Levinton 2001: 384, table 7.2). Of course, this does not mean that more overlapping speciation events could not have happened during this time, but the comparison gives a good feeling for how short this time is in biological terms. This feeling can be corroborated with hard science and math: as correctly emphasized by Meyer in his book (Meyer 2013a), the standard mathematical tools of population genetics show that such a window of time is orders of magnitude too short to accommodate the waiting times for the origin and fixation of the required genetic changes (see further).

[TC 12:26] Farina claims that Meyer is dishonest for saying that “many animal phyla show up for the first time in the Cambrian.” This is beyond ridiculous as exactly this statement can be found all over the technical literature about the Cambrian Explosion. It is the single most uncontroversial fact about the Cambrian Explosion that is affirmed by virtually all experts. Even Dawkins himself says, “It is as though they were just planted there, without any evolutionary history.” (The Blind Watchmaker, 1986, pg. 229-230.) An invertebrate biology textbook states (Barnes et al. 2001: 9–10): “Most of the animal phyla that are represented in the fossil record first appear, ‘fully formed,’ in the Cambrian some 550 million years ago…The fossil record is therefore of no help with respect to the origin and early diversification of the various animal phyla.”

Apparently based on then-grad student Nick Matzke’s review of Darwin’s Doubt (Matzke 2013), Farina parrots the silly quibble about the arbitrary definition of the Linnean rank of a phylum. Yes, cladists are right about this critique but it completely misses the point.

[TC 12:55] In support of this irrelevant claim, Farina quotes Budd & Jensen (2000), who said that characterizing phyla by particular types of “body plan” seems to be based on an artifact of classification. That’s fine, but the issue of the body plans remains no matter what you call them. The issue is not the artifactual arbitrariness of Linnean categorical ranks, which may be happily granted. The abrupt origin of the very different body plans of Cambrian animal phyla does not disappear only because you dispense with the phylum category in a modern phylogenetic classification. The fundamental differences among the distinct body plans was the reason these groups of animals were categorized as different phyla in the first place. You can change the names however you want, but the biological facts persist.

The mentioned paper by Budd & Jensen (2000) also redefined “a body plan [a]s that set of features plesiomorphically shared by extant taxa in a monophyletic clade.” Well, I’m sorry to say so, but the authors seem to be quite confused about cladistics, which Mr. Farina is of course not competent to recognize. Their definition is not only nonsensical but objectively erroneous, as it would imply that, for example, the body plan of vertebrates does not include vertebrae because those are synapomorphically shared and not symplesiomorphically shared by all vertebrates.

[TC 14:15] Therefore, it does not come as a big surprise that Farina incorrectly defines the term “plesiomorphic” as “traits that are shared by all the members of a group but are not unique to that group.” Nope, that is not what the term means. Plesiomorphic is just the technical cladistic term for a “primitive” or unmodified character state compared to the changed or derived state that is called apomorphic. Plesiomorphic and apomorphic only denote the polarity of a character and do not imply any statements of similarity between organisms. Only the terms “symplesiomorphy” and “synapomorphy” imply a shared character state and a homology hypothesis. Symplesiomorphic simply means a shared primitive homologous character state, such as multicellularity shared by ants and lions. But Farina’s definition would not even correctly define symplesiomorphy, because his definition would likewise apply for homoplasies (non-homologies) that are not uniquely shared, such as the wings of birds and insects. Farina doesn’t have a grasp of the basics of cladistic terminology. Maybe he should study my online “Glossary of Phylogenetic Systematics” to learn something before he tries to teach others.

Next, “‘Lying on the Internet’? Debunking Dave Farina on Stephen Meyer.”