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The Cambrian Explosion that Wasn’t September 7, 2009

Posted by Bill in Evolution, Science.
Tags: , , , , ,

Bill and I spend a lot of time hanging out on the Creation Evolution Debate Forum. I also moderate on the Evolution versus Creation Forum. That means we spend a lot of our time debating with Creationists. One argument that comes up frequently is the myth of the Cambrian Explosion. Creationists claim that it is evidence against evolution. They say that the sudden appearance of new types of life is proof of a creator. In the course of researching a response to someone on the forum, a guy who says he is actually more of an ID-er than a creationist, I put the following together about the Cambrian Explosion. One of the other members of the forum liked it so much, I decided to post it here. So here it is, with some slight modifications:

Creationists/ID-ers claim that scientists are unable to explain the [apparent] sudden diversification of life in the Cambrian. First we have to look at whether there is actually anything that needs to be explained. The Cambrian Explosion has a kicky sort of name but it is not as unique as is commonly thought. It is commonly termed as being the sudden appearance or rapid diversification of life in the Cambrian period. Yet we know it isn’t the sudden appearance of fossils because of the Ediacaran fossils. Nor is the rate of diversification greater than that of other eras.


Part of the reason the Cambrian Explosion was termed as such was because of the sheer number of fossils found where there were supposedly few found from prior time periods. This is no longer true with the discovery of all the Ediacaran fossils (fossils of the earliest known multi-cellular life dating from the Pre-Cambrian).

From Darwin’s On the Origin of Species:

“There is another … difficulty, which is much more serious. I allude to the manner in which species belonging to several of the main divisions of the animal kingdom suddenly appear in the lowest known [Cambrian-age] fossiliferous rocks … If the theory be true, it is indisputable that before the lowest Cambrian stratum was deposited, long periods elapsed … and that during these vast periods, the world swarmed with living creatures… [But] to the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods before the Cambrian system, I can give no satisfactory answer. The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained”

It was this that focused attention on two things: the huge number of fossils in the Cambrian and the seeming lack of fossils from the Pre-Cambrian. Now, however, we have a vast wealth of fossils from the Pre-Cambrian- the Ediacaran fossils. So that part of the so-called “Cambrian Explosion” is simply no longer a factor. The other part, the seemingly rapid diversification is also a fallacy for many reasons.


Despite claims to the contrary, only 12 out of 35-37 animal phyla (or body plans) appear in the Cambrian, nowhere near the common Creationist claim that “almost all body plans” are present in the Cambrian. Cambrian life was largely unlike anything alive today. Most of the phylum-level body plans appear in the fossil record MUCH later. In fact, if we define the number of cell types as a measure of the complexity of a life form, the increase in complexity has been constant since the beginning of the Cambrian.

In fact, statistical analysis shows that the diversification of the Cambrian Explosion was no faster than any other radiations. Whereas the popular idea once was that diversification was high in the Cambrian and decreased afterwards, we now know that diversification  started at a relatively low level throughout the Cambrian and has been increasing since.





So what is the Cambrian Explosion actually?

It is a rise in diversity of hard-bodied marine animals. There are actually two phases of it—the first phase in which animals spread into niches on the sea floor (which actually occurred Pre-Cambrian) and then a second expansion in the early Cambrian as they filled the niches on the sea floor and moved into the water column.

It is true that the rate of diversification in the Cambrian is unique among MARINE animals, but it in no way is true for ALL animals.

So what caused this Rapid Diversification of Marine Animals?

We don’t know for sure. I would say there is no one answer but a combination of factors. First let’s look at the progression of the diversification of life. In the Pre-Cambrian you have life spreading out into the ecological niches on the sea floor. As those became filled, the animals had to go somewhere else so they moved upward, into the sea column. As the sea column became occupied, there was little room for animals to go because the incumbent in an ecological niche has a huge advantage. As empty niches become available, the animals continue diversifying. As the new niches are filled, the animals that fill those niches have little opportunity (or need) to modify their life style or form UNLESS the environmental conditions of the niche change.


Evolution occurs because of changes in the environment. If the environment never changes, evolution is going to be extremely slow. The more rapidly the environment changes, the faster the rate of evolution. So what was going on in the environment at the time of the Cambrian? For one thing, the oxygen levels were increasing. Earth’s earliest atmosphere contained no free oxygen. The oxygen we breathe today is the result of billions of years of photosynthesis. As a general trend, the concentration of oxygen in the atmosphere has risen gradually over about the last 2.6 billion years. As more oxygen became available for animals, the animals would increase in size (hence we see increased size of animals in the Ediacaran.) In addition, as more oxygen became available, metabolic pathways would be able to construct more complex structures such as a hard exoskeleton, which are extremely oxygen expensive.




The development of hard-shelled animals alone then creates a huge change in the environment because a hard-shelled animal is going to have a huge advantage over a soft-bodied animal. An effective predator is going to create a lot of opportunity for evolutionary change. Prey MUST adapt or die.


In addition, as the size of animals increased due to increased oxygen levels, this created more ecological niches as the size of plankton increased. Before the start of the Cambrian, the plankton was too small to fall rapidly toward the sea bed, and so they were destroyed by chemical processes or by larger plankton before they reached the ocean floor. As plankton grew larger, its corpses would fall more quickly and more of it would reach the sea bed. Ditto with its droppings. This changes the environment of both the sea bed and the sea column—whoo hoo! New Prey! New Prey means adaptation in predators and adaptation in predators means adaptation in prey which means more adaptation in predators and so on and so forth. Keep in mind the effect of co-evolution ( a trait in one organism causes another to evolve in response. A number of responses are possible and a different species can potentially emerge for each response.)







Another thing going on at the end of the Cambrian was a mass extinction event: the Ediacaran animals and small shelly fossils disappear from the fossil record, indicating some type of extinction event. The niche previously occupied by the Ediacaran animals (the sea floor) was suddenly available for the Cambrian animals.




So we have animals moving into the niches previously occupied by the Ediacaran animals and we have animals developing hard-shelled bodies. What else is going on at this time that contributes to the increased diversification? Well, for one thing this was about the time that animals developed eyes. Consider the huge advantage an eye confers upon both the predator and the prey. Before eyes, animals had to be close to each other to hunt prey and to evade predators. The development of the eye may, in turn, be responsible for the development of body features such as armor and spines.




So in answer to the question about what caused the so-called Cambrian Explosion (which we have shown was not actually an explosion), I would say a combination of factors all contributing to the increased availability of ecological niches. These factors include the mass extinction of the Ediacaran animals, the increased level of oxygen, increase in food supply, increase in predator-prey relationships. As animals evolved in response to these changes, the adaptations in turn created a need for more adaptations resulting in bigger and better adaptations.

Information for this post was taken from the following:






1. pelagian7 - September 7, 2009

Have you read Dowd’s, Thank God for Evolution? He does a decent job of combining spirituallty and science. He, specifically, explains how the brain evolved in steps. He, myself, and many others think that the early scripture writers used symbolic language to explain new concepts. Had they known of evolution, or the term big bang, we’d probably find them used.

It’s really a shame people become stuck defending doctrine, evolution could add a really cool chapter to Genesis.
pelagian7 over and out.

2. Dindy - September 7, 2009

Thanks! I haven’t read it– I’m not a theist so I don’t need to find a way to combine the two world views. However many people and scientists are able to balance their faith in God with their knowledge of evolution. I’ll check this book out when I have some time.

3. pelagian7 - September 7, 2009

I’m not sure what I am. I certainly don’t believe in the Paternal God of monotheism. Yet, I am open to spirituality and my God is the interrelated-ness and unexplained.

Thanks for responding, Pelagian7

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