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  • Writer's pictureJon Peters

The Cambrian "Explosion" - In Context

Updated: Feb 28



"Thus we have seen that the "Cambrian explosion" is a myth. It is better described as the Cambrian slow fuse. It takes from 600 to 520 million years ago before the typical Cambrian fauna of large shelly organisms (especially trilobites) finally develops."

Donald Prothero, Part II. Evolution? The Fossils Say Yes! From: Evolution - what the fossils say and why it matters. 2nd ed. 2017.



The Anti-evolutionist Claim: Complex forms appear suddenly in the Cambrian layer with few or no precursors. This is not predicted by evolution and fits best with a creation event. For most Young Earth Creationists, it represents the basal layer of a Global Flood that occurred about 4,000 years ago. Answers In Genesis gives an exact year: “Using the Bible, well-documented historical events, and some math, we find that the Flood began approximately 4,359 years ago in the year 1656 AM or 2348 BC. Some may look for an exact date (i.e., month and day), but we are not given that sort of precision in Scripture.”(3).  For Progressive Creationism (Reasons To Believe, an Old Earth Creationism group) it is interpreted as an early creation event millions of years ago to be followed by many cycles of extinctions and new creation events through thousands of feet of sedimentary rock. In addition, anti-evolutionists note many of the Cambrian fossils are unique and do not persist above the Cambrian. 


A senior member of the Intelligent Design Discovery Institute, Stephen Meyer, has practically made a career of promoting the Cambrian Explosion as supposedly disproving evolution. Since creationists believe the only alternative to evolution is some type of creationism, many creationists don’t feel the need to propose, discuss, or test a specific and falsifiable alternative to evolution. For Meyer and others, it’s a one and done paleontological rug pulled out from evolution if the Cambrian indicates flora and fauna suddenly appearing. But does complex life suddenly appear in the Cambrian with no evidence of precursors?



Introduction


The geological record around the Cambrian is noted below. The Cambrian Period is the first and oldest layer of the Paleozoic Era as one ascends towards our time. Logically enough, the Era before the Paleozoic is called the Precambrian and is composed especially for this discussion of a Period immediately before the Cambrian called the Ediacaran. 


Used by permission. Fair use educational purpose



Persons exposed to anti-evolutionist material about the Cambrian may get the impression that life and certainly complex life just appeared suddenly in the Cambrian. That is not the case. The Cambrian Period spans a period of about 538 mya to 495 mya but fossils are known to science that show life going back to 3.5 billion years in the form of cyanobacteria from the Archaea rocks of western Australia. These are microfossils (1).  So life did not appear in the Cambrian. The oldest rocks on the planet are 3.8 billion years old; the earth is 4.6 billion years old. Life did not wait too long to get started but for about 2 billion years life on earth consisted only of single celled organisms, called prokaryotes. “There are hundreds of microfossil sites around the world. Everywhere we look in rocks between 3.5 billion years old and about 1.75 billion years old, we see nothing more complicated than prokaryotes and stromatolites. The first fossil cells that are large enough to have been eukaryotes do not appear until 1.75 billion years ago, and multicellular life does not appear until 600 million years ago (2)”. We know we can even trace our eukaryotic lineage into deep time due to our mitochondria, a result of endosymbiosis (see short blog on mothers and mitochondria). So life did not first appear in the Cambrian. 


The Cambrian is now divided into Early, Middle and Late. Traditionally, the Cambrian was marked by the appearance of the trilobites, but now we know these forms occur at the end of the Early time and other fossils preceded them in the Cambrian. The Cambrian “explosion” was a fast evolutionary diversification event that occurred over about 20 million years in the last part of the Early section of the Cambrian. As will be discussed later, other relatively fast evolution examples have been noted before in the fossil record - times of quick diversification geologically.



The Ediacaran Period 


This is the time just before the Cambrian and it contains a weird assortment of flora and fauna. Nearly all are soft bodied organisms without skeletons. “These impressions have reminded some paleontologists of the impressions made by sea jellies, worms, soft corals, and other nonskeletonized organisms. Over 2,000 specimens are known, usually placed in about 30 - 40 genera and about 50 - 70 species, so they were relatively diverse.” (2). Some are very different from today’s species. Other paleontologists suggest they are unique because they lack modern patterns and may represent early failed experiments in multicellularity. The Ediacaran fossils are known from soft sea floor bottoms only and few important features are present. Thus, where they fit into classifications is controversial. They are definitely multicellular but paleontologists are not sure even what kingdom to place them into or even if they represent a new kingdom (2). 


Scientists have calculated molecular clock estimates for the divergence times of the major invertebrate groups and these figures indicate that major branching points go back as far as 800 - 900 million years ago well into the pre-Cambrian times (4, 2). This means that advanced multicellular life (without fossilizable skeletons) were present on the earth 600 million and even 900 million years ago - at least 50 million years before the Cambrian. The Ediacaran fauna represents the next logical stage bridging single celled life to the appearance of soft bodied life. 



Early Cambrian Stages


The fossil record indicates it took 3 billion years for life to develop the ability to mineralize shells which is what we finally see in the early Cambrian stages. When they did develop mineralization it was calcium phosphate, the mineral that makes up bones, and not calcium carbonate that most of today’s marine invertebrates utilize.


Prothero writes about what followed after the Ediacaran: for almost 25 million years many tiny phosphorous fossil shells (a few millimeters only) were abundant in the earliest stages of the Cambrian. The Nemakit-Daldynian and Tommotian stages from 520 - 545 mya. revealed that some looked like coiled mollusks, some like primitive clams, and others like tubular forms or even like miniature jacks. The earliest sponges had already appeared in the Ediacaran, and sponges have always been considered to be the most primitive animals we have today. By the Tommotian stage 530 million years ago larger fossil invertebrates appear such as the brachiopods. Abundant burrowing can now be seen indicating a true internal cavity or coelom. Thus, the earliest Cambrian shows evidence of a gradual increase in diversity from the Vendian, but no “explosion” (2).


Another author notes: “…the Cambrian explosion does not document the “sudden appearance” of all animal phyla. A significant number of animal phyla are already present prior to the explosion. It is simply that we remain unsure of their exact classification precisely because they are so different from living animals.” (4)



Later Cambrian Stages


The third stage is known as the Atdabanian stage and there is a significant development of diversity found. Much of it is the genera of trilobites. “Most of the other animal phyla had already appeared by this time (including mollusks, sponges, corals, echinoderms) or would appear later in the Cambrian (vertebrates) or even in the Ordovician Period that followed (e.g., the “moss animals or bryozoans)… Then in the Middle Cambrian we have the extraordinary soft-bodied preservation of fossils…Some are still complete mysteries to zoologists, not fitting into any living phylum. Others are apparently soft-shelled arthropods.” (2)



Summary


1. What “exploded” in the Cambrian? When fossils were first found in this period it was mostly just the hard shelled trilobites - lots of them. They and other mineralized fossils seemed to appear suddenly without precursors. They were fairly complex multicellular organisms. But that is an old and early observation. The early Cambrian has 25 million years of small shelled animals before the arrival of the larger forms. The fossils of the Ediacara have since been discovered and it’s obvious that there was a gradual increase in diversity from the Ediacaran stages to and through the Cambrian. 


Many feel that the appearance of so many new forms in the Cambrian was more a function of an “End-Ediacaran" mass extinction that happened before the Cambrian. “This implies that the "explosion" did not represent animals "replacing" the incumbent organisms, and pushing them gradually to extinction; rather, the data are more consistent with a radiation of animals to fill in vacant niches, left empty as an extinction cleared out the pre-existing fauna” (8)


2. The Cambrian “Explosion” was just a fast diversification event, an adaptive radiation period. We know that this has happened in other parts of the fossil record such as the 10 - 15 million years of whale evolution, (see the actual overwhelming evidence for whale evolution in 3 relatively short videos here) and the mere 10 million years from the Cretaceous-Paleocene extinction that produced all our different orders of birds. The “explosion” is basically a myth and hold-over from early observations and catchy titles. As in the Big Bang; it was really not an explosion either. 

3. Although the Cambrian diversification event (“explosion”) is normally stated to have occurred over a 10 - 20 million span, if we include the precursors in the Ediacaran to the large shelly organisms in the Tommotian, like the trilobites, we are really looking at 80 million years (2). In addition it follows a series of logical stages from simple and small to larger and more complex mineralized forms. The development of more complex life from 600 to 540 mya is hardly an “explosion”. 


“The theory of evolution has no issue with the Cambrian explosion, as the explosion spans such an extraordinary long period of time.” (5)

4. The tiny fossils found in the early stages of the Cambrian and span 25 million years are nicknamed the “little shellies” because they are so small and so abundant. Their shells are calcium phosphate instead of calcium carbonate like our marine invertebrates today. This is not a killer argument against evolution. Some scientists think this could have resulted from an environmental condition such as low oxygen at the time. This is a gap in our knowledge and gaps are by definition lacking information - which is different than finding actual evidence that disproves something. Despite the denial by Intelligent Design advocates, the movement is centered around a well known logical fallacy involving trying to use gaps which by definition are lacking in evidence as evidence. Absence of evidence is not evidence of absence.


5. What about all the Cambrian forms that are extinct? Is not that a problem for evolution since many did not end up being precursors to the later fossils and today’s species? This is probably a misunderstanding of classification. “Many of these ‘extinct phyla’ from the Cambrian were really part of the arthropod phylum. Some are clearly brachiopods, and some were mollusks. Although these animals are anomalous when compared to today's SPECIES, they really are not anomalous when compared to today’s PHYLA.”(6)


 6. All major life groups did not appear at the Cambrian. It was not a “creation” event for life on earth. 

Biologos notes:

“The major animal body plans that appeared in the Cambrian Explosion did not include the appearance of modern animal groups such as: starfish, crabs, insects, fish, lizards, birds and mammals. These animal groups all appeared at various times much later in the fossil record. The forms that appeared in the Cambrian Explosion were more primitive than these later groups, and many of them were soft-bodied organisms. However, they did include the basic features that define the major branches of the tree of life to which later life forms belong. For example, vertebrates are part of the Chordata group. The chordates are characterized by a nerve cord, gill pouches and a support rod called the notochord. In the Cambrian fauna, we first see fossils of soft-bodied creatures with these characteristics. However, the living groups of vertebrates appeared much later. It is also important to realize that many of the Cambrian organisms, although likely near the base of major branches of the tree of life, did not possess all of the defining characteristics of modern animal body plans.” (7)



Conclusion


Thus, the Cambrian did not represent the sudden appearance of many major groups that are present today. It does not support Young Earth Creationist and Intelligent Design claims that life as we know it appeared suddenly in the Cambrian, but rather a proper examination of what came before, during, and after the Cambrian supports instead evolution. A very thorough evaluation of the amazing discoveries in the Cambrian and pre-Cambrian fossils including challenges in interpreting them can be found in Budd's 2013 review (9). Nothing in the Cambrian discounts evolution at all: "Together with a remarkable growth in knowledge about the environments that these early animals lived in, these discoveries have long exerted a fascination and strong influence on views on the origins of animals, and indeed, the nature of evolution itself. Attention is now shifting to the period of time just before animals become common, at the base of the Cambrian and in the preceding Ediacaran Period. Remarkable though the Burgess Shale deposits have been, a substantial gap still exists in our knowledge of the earliest animals. Nevertheless, the fossils from this most remarkable period of evolutionary history continue to exert a strong influence on many aspects of animal evolution, not least recent theories about developmental evolution." (9)


An early simile for Intelligent Design was that finding a watch implied a watchmaker. The Cambrian explosion has been thought by anti-evolutionists as a good example against evolution. However, a close review demonstrates that it instead fully supports evolution albeit with much that remains unknown. As if often the case when discussing anti-evolution claims, it is vital to ask what is being withheld from the conversation. Note below the Cambrian in context.


From: https://www.britannica.com/science/Cambrian-explosion. Cambrian Explosion; paleontology. Fair use claimed; educational purpose. This article is a good brief review of the Cambrian.




Literature Cited


1.Cyanobacteria Fossil Record.


2. Prothero, Donald R. 2017. Evolution: What The Fossils Say and Why It Matters. Columbia University Press. 2nd ed. New York. 427 pp.


3. Timeline for the Flood


4. Why “Sudden Appearance” Is Not as It Appears


5. Doesn't the sudden appearance of all the "modern groups" of animals during the Cambrian explosion prove creationism? (Hassan Lahiri) 


6. How Would You Explain the Cambrian Explosion? (David Rosen)


7. Does the Cambrian Explosion Pose a Challenge to Evolution?


8. Doesn’t the sudden appearance of all the “modern groups” of animals during the Cambrian explosion prove creationism? (Paul Lucas)


9. At the Origin of Animals: The Revolutionary Cambrian Fossil Record







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