John Collyer's 17th 'fact seldom taught' was the claim that "[at] the base of the fossil record there is evidence of many highly complex creatures, but no evidence of an evolutionary sequence." Collyer is referring to the Cambrian explosion, and was parroting the standard special creationist misconception that at the base of the Cambrian, modern phyla appear suddenly, without any evidence of earlier forms in the older strata.
This is incorrect. In the Ediacaran era (immediately prior to the Cambrian) we have evidence of bilaterian life, including arthropods and molluscs. There are also the enigmatic Ediacaran biota, which are definitely multicellular, but may even be a failed attempt at a new form of life. Furthermore, not all phyla appeared at the beginning of the Cambrian – some took several million years to appear. In fact, the so-called “explosion” is more like a slow fuse, as it took place not in a geological instant, but over several million years.
Special creationists often allege that the transitions between these early phyla are absent, and claim that this is a problem for evolution. The intelligent design apologist Steven Meyer claims:
To say that the fauna of the Cambrian period appeared in a geologically sudden manner also implies the absence of clear transitional intermediates connecting the complex Cambrian animals with those simpler living forms found in lower strata. Indeed, in almost all cases, the body plans and structures present in Cambrian period animals have no clear morphological antecedents in earlier strata. [1]
Meyer is of course wrong [2]. The Cambrian Explosion, poses no major problems for evolutionary biology but once again represents the depth of special creationist ignorance of the subject they criticise.
Complex life ancestral to modern phyla predated the Cambrian
The period immediately before the Cambrian is known as the Ediacaran and extended from 635 million years ago to 542 million years ago. Most of the Ediacaran life forms are unusual in their appearance, being mainly tubular and frond-like, and first appeared around 580 million years ago, largely vanishing at the end of the Ediacaran 542 million years ago. [3] No consensus exists as to their place in the tree of life. One hypothesis is that they represent a unique phylum now extinct, unrelated to extant metazoans, while other hypotheses try to fit them as ancestral forms of modern phyla. Irrespective of where they fit in evolutionary history, the existence of the Ediacaran biota alone refutes the creationist assertion that complex life was largely absent prior to the Cambrian.
Apart from the unique Ediacaran biota, complex metazoan life has been found in the Ediacaran period, and these are usually interpreted as related to modern phyla:
- Vernanimalcula guizhouena: small (< 180 micrometres) organisms believed to be the earliest bilaterians, that is, animals with bilateral symmetry. They have been found [4] in the Doushantuo Formation in southwest China, and are dated to 580-600 million years ago, at least 40 million years before the Ediacaran-Cambrian transition. This interpretation has been challenged by some [5], but if they represent genuine bilaterian fossils [6], they would provide evidence to support the idea that all the modern phyla did not emerge at the start of the Cambrian.
- Kimberella: this is generally accepted as a bilaterian, and is dated at around 558 million years ago, which is about 15 million years before the beginning of the Cambrian. It is generally assumed to be related to the molluscs. [7]
- Spriggina: currently considered to be an arthropod, Spriggina [8] bears a passing resemblance to trilobites, and may well be related to them. It has been dated to around 550 million years ago.
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| All specimens are curated at the Paleontological Institute (PIN), Russian Academy of Sciences, Moscow. All are preserved on undersides of sandstone channel casts in member 9 unless otherwise noted. All scale bars are 1 cm. a, Large, well preserved specimen, with deep central depression. Note folding of soft tissues away from shell edge, which has buckled (arrow) (PIN 3993/4003). b, Specimen partially folded over, demonstrating deformation of soft parts and bilateral symmetry of organism (PIN 3993/4026). c, Largest specimen known; note lateral displacement of shell, with crenellations extending out from under shell edge at right (PIN 3993/4004) (d). Specimen from thinly interbedded argillite and siltstone, member 1 of local section; arrow indicates position of anterior bulge (PIN 3993/4001). e, One of two specimens showing series of pits (spicules?) along proximal ridge (PIN 3993/4036). f, Small but well preserved specimen showing primary features (PIN 3993/4033). g, Specimen showing rotation and displacement of shell from the main axis (PIN 3993/4027). h, Poorly preserved specimen from talus, probably from sandstones of member 10 of local section; note continuation of zones around posterior (pointed) end, as well as possible buccal mass indicated by arrow (PIN 3993/4009). - Nature 388, 868-871(28 August 1997) |
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| a, Dorsal view. c, Crenellated zone; l, lobe; s, striae; dr, distal ridge; pr, proximal ridge; m, medial depression; a, anterior knoll. b, View of living organism, with folds of 'crenellated zone' extended beyond margin of shell; the folds would usually have been retracted under the shell at the time of burial. Nature 388,868-871(28 August 1997) |
Spriggina floudensi
Model of Spriggina floundersi.
Image: Benjamin Healley
Source: Museum Victoria
The "Cambrian Explosion" actually had a very long 20 million year fuse.
The length of the Cambrian explosion is anything but short. Keith Miller, a palaeontologist at Kansas State University who is also an evangelical Christian notes that if one defines the Cambrian explosion as the length of time from the start of the Cambrian period to the earliest appearance of diverse invertebrate communities in the fossil record, the length of time is around 20 million years. [9] That is a lot of time in which the "instantaneous appearance" of major phyla can occur. On that point, Miller notes:
However, not all living animal phyla with a fossil record first appear within this time window. The colonial skeleton-bearing bryozoans, for example, are not known from the fossil record until the end of the Cambrian around 491 million years ago. More significantly, several living invertebrate phyla have a fossil record that extends into the late Neoproterozoic before the Cambrian. Sponges have been recognized as early as 580 million years, cnidarians (the group includes jellyfish and anemones) are present among the Ediacaran animals at around 555 million years, and the stem groups...for some other phyla were also likely part of the Ediacaran communities.
Defining the Cambrian “explosion” is not as straightforward as it might seem. Although there was clearly a major burst of evolutionary innovation and diversification in the first 20 million years or so of the Cambrian, this was preceded by an extended period of about 40 million years during which metazoans (multicellular animals) arose and attained critical levels of anatomical complexity. The Ediacaran saw the appearance of organisms with the fundamental features that would characterize the later Cambrian organisms (such as three tissue layers, and bilaterally symmetric bodies with a mouth and anus), as well as the first representatives of modern phyla. The base of the Cambrian is not marked by a sharp dramatic appearance of living phyla without Precambrian roots. It is a subjectively defined point in a continuum. The Cambrian “explosion” appears to have had a “long fuse.” - Emphasis mine
The Cambrian Explosion much touted by creationists, namely the sudden appearance of all major phyla instantaneously at the base of the Cambrian without any evolutionary relationship between each other, does not exist. Evolutionary biologist Nick Matzke, in a comprehensive trashing of Darwin's Doubt, ID apologist Steven Meyer's book on the Cambrian explosion notes that:
...even on the most conservative interpretation of the fossil record which relegates all of the classic Ediacaran fossils to the stem below the bilaterian common ancestor, or to cnidarians, or to even more remote positions, we still have this sequence observed in the fossil record:
1. Before 700 mya, maybe well before: Single-celled eukaryotes (acritarchs)
2. Earlier Ediacaran: Multicellular animal eukaryotes, but simple, sponge-grade organisms3. Later Ediacaran: Multicellular animal eukaryotes with more complexity, i.e. cnidarian-grade organisms4. Very late Ediacaran: Simple slug-grade/worm-grade organisms (at least their tracks and burrows) – the first ones only making surface tracks and lacking burrowing ability. Making tracks suggests that the organisms have at least a front end and a back end, a mouth, anus, and gut connecting them. These are almost certainly bilaterians.5. Very late Ediacaran: The very first biomineralized “skeletons”, e.g. Cloudina, basically a worm secreting a tube, as well as the first evidence of predatory boring. Cloudina gets no mention at all in Meyer’s book.6. At the beginning of the Cambrian, we start to see more complex burrowing – e.g., vertical burrowing through sediment, clearly indicating worm-grade organization and an internal fluid skeleton, i.e. a coelom. The burrows gradually increase in complexity over 10 my.7. Small shelly fauna: The shells, which started very small and very simple, gradually diversify and get more complex, radiating especially in the Tommotian. By the end of the Tommotion, some of the “small shellies” can be identified as parts of larger, “classic” Cambrian animals. The Tommotian is an utterly key period for any serious discussion of the Cambrian Explosion. Unfortunately, the word “Tommotian”, or any equivalent terminology (the detailed stratigraphy of the Cambrian is still being worked out, see Erwin & Valentine 2013 for a review), does not even appear in the book! The Small Shelly Fauna (SSF) gets just one (one!) mention in the book, buried in endnote 27 of Chapter 4, a whole chapter devoted to debunking the idea that the Ediacaran fauna is “ancestral” to bilaterians. (See discussion of the concept of “ancestral” below, which Meyer makes a complete hash of; however, I would tend to agree that the evidence is not good that the classic Ediacarans are within the bilaterian crown, as much because of the late date of #4-6, above, as anything.)8. The earliest identifiable representatives of Cambrian “phyla” don’t occur until millions of years after the small shelly fauna have been diversifying, and they tend to be taxa on the stem below the crown of living phyla, rather than placeable within the crown. Trilobites are an exception, but what is often missed is that deposits like the Chenjiang have dozens and dozens of trilobite-like and arthropod-like organisms that fall cladistically outside of these respective clades. These are transitional forms! How can this fact not be highlighted!?!9. In general, the earliest Cambrian relatives of the living phyla tend to be a lot more wormlike or sluglike than most modern representatives of the living phyla. Of course, many of the living phyla are basically still worms, and the more complex living phyla (e.g. molluscs, chordates) have early-diverging representatives or relatives that are rather more wormlike than the better-known representatives with more complex bodyplans. Even the earliest “fish” – actually either stem-group craniates, stem-group cephalochordates, or stem-group chordates – are basically filter-feeding worms that happen to swim. They don’t have jaws, scales, limbs, a bone skeleton, or anything else that most readers would associate with the word “fish”.All of this is pretty good evidence for the basic idea that the Cambrian “Explosion” is really the radiation of simple bilaterian worms into more complex worms, and that this took something like 30 million years just to get to the most primitive forms that are clearly related to one or another living crown “phyla”, and occurred in many stages, instead of all at once.
Far from a sudden appearance of life without any ancestors, as special creationists such as Collyer allege, this is more like what actually took place at the Cambrian:
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Given this, one would hope that evolution denialists in our community would stop repeating myths about the Cambrian adaptive radiation.
References
1. Meyer, S.C., M. Ross, P. Nelson, & P. Chien. 2003. "The Cambrian explosion: biology's big bang" in J. A. Campbell & S. C. Meyer, eds., Darwinism, Design and Public Education (2003, Michigan State University Press) p 326
2. Nick Matzke (see ref 10) along with Donald Prothero are two of the more prominent scientists to have eviscerated ID apologist and philosopher Steven Meyer's book Darwin's Doubt. Prothero, a respected vertebrate palaeontologist notes in his review the depths of Meyer's intellectual dishonesty: "Even more damning, Meyer completely ignores the existence of the first two stages of the Cambrian (nowhere are they even mentioned in the book, or the index) and talks about the Atdabanian stage as if it were the entire Cambrian all by itself. His misleading figures...imply that there were no modern phyla in existence until the trilobites diversified in the Atdabanian. Sorry, but that’s a flat-out lie. Even a casual glance at any modern diagram of life’s diversification demonstrates that probable arthropods, cnidarians, and echinoderms are present in the Ediacara fauna, mollusks and sponges are well documented from the Nemakit-Daldynian Stage, and brachiopods and archaeocyathids appear in the Tommotian Stage—all millions of years before Meyer’s incorrectly defined “Cambrian explosion” in the Atdabanian." (Emphasis in the original).
3. Conway Morris S (1993). "Ediacaran-like fossils in Cambrian Burgess Shale–type faunas of North America". Palaeontology 36 (0031–0239): 593–635.
4. Chen J et al “Small Bilaterian Fossils from 40-55 million years before the Cambrian” Science (2004) 305:218-22.
5. Bengtson, S.; Budd, G. “Comment on “small bilaterian fossils from 40 to 55 million years before the Cambrian”. Science (2004) 306: 1291a.
6. Chen, Jun Yuan, Paola Oliveri, Eric Davidson and David J. Bottjer. “Response to Comment on "Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian"” Science (2004) 306: 1291
7. Fedonkin, M. A.; Waggoner, B. M. (1997). "The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism". Nature 388 (6645): 868–871
8. Glaessner, Martin F. (1958). “New Fossils from the Base of the Cambrian in South Australia" Transactions of the Royal Society of South Australia 81: 185–188
9. Miller K “The Cambrian ‘Explosion,’ Transitional Forms, and the Tree of Life" BioLogos Blog Dec 3 2010
10. Matzke N "Meyer's Hopeless Monster, Part II" Panda's Thumb June 19 2013
