The term, "Cambrian Explosion", refers to the period of time in the fossil record in which there is an apparent rapid diversification of living organisms. Anti-evolutionists, particularly Intelligent Design proponents, have often presented the Cambrian Explosion as a insurmountable problem for Modern Evolutionary Theory claiming that it more aptly supports creationist (including Intelligent Design) theory.
Up to this point in this series of posts I have been laying a foundation to show that this characterization is not true. I hope to show that Modern Evolutionary Theory is the ONLY theory that has a chance of explaining the Cambrian Explosion. No from of creationism has any satisfying answers to the questions that the phenomenon presents, while Modern Evolutionary Theory does.
In this, the fourth part of this series of posts I will introduce the metazoan fossils of the Ediacaran period and discuss how they fit in with the upcoming Cambrian Explosion. I will leave links to the three previous posts in this series at the end of this blog.
In Darwin’s time the Cambrian Explosion presented two major problems for his theory. (1) The age of the earth didn’t seem to be old enough to support the slow pace of evolution that he postulated, and (2) no potential intermediate fossils were known from which the complex fossils of the Cambrian could have arisen.
Since then the problem of time has been completely solved. The earth is not only 20,000,000 years old (the estimated age of the earth in Darwin's time). It is 4.6 billion years old. Life has been on the earth for at least 3.6 billion years and probably longer. That life remained bacteria-like for almost 2 billion years. It remained single-celled for almost another billion years. There were multicellular organisms for 160,000,000 years before the Cambrian Explosion and these multicellular organisms are becoming more and more complex and diverse as we approach the Cambrian Explosion. Doesn’t that solve the precursor problem as well? It certainly goes a long way to solving it, but it doesn’t go all the way.
The reason it doesn’t go all the way is because the relationship between the Ediacaran fossils and the fossils of the Cambrian Explosion is difficult to determine. For instance, look at this fossil:
It is Dickinsonia, perhaps the best known of all the Ediacaran fossils. The first thing to notice about it is that it is segmented. Most paleontologists believe that it is an early example of an annelid worm (a bilaterarian animal). However, it bears striking similarities as well to Cnidarians (radiolarian animals) like this Indonesian coral:
This is Spriggina:
It too is segmented and was classified as an early annelid. It is now thought to be more likely an early arthropod (i.e. a precursor to crustaceans, spiders, and insects).
This is Kimbrella:
Early specimens were classified as early box jellyfish. Newer specimens from Russia show that the outside is a hard shell. Now these are thought to be early molluscans (clams).
Thus, the Ediacaran fossils have been and still are undergoing an intensive reclassification as more and more fossils are found. There is a lot of uncertainty as to how they relate to the Cambrian fossils. Some biologists think we do not have enough information and we are unlikely to ever have enough to determine the relationship. Some of these believe the Ediacaran fossils are best left as an enigma and should not be considered as precursors to Cambrian fossils. Most biologists would agree that we do not have enough information to determine relationships definitively. But the Ediacaran fossils DO fit with the expected overall complexity of intermediate fossils and they display a variety of intermediate-like characteristics. So most biologists also believe that we do know enough to make tentative hypotheses as to relationships. They also believe these hypotheses will be subject to testing with the advent of new fossils.
Creationists side with the contrarian group and extend their assertions to imply there definitely are NO precursors. They say that the Cambrian fossils suddenly appear out of nowhere already complex. This is grossly misleading, not only because there are potential precursors in the fossil record but also because the Cambrian fossils didn’t appear that “suddenly” either.
Let’s take a close look at the fossil record of the Cambrian Explosion. To do that we need to subdivide the Cambrian period more finely. I will use the time scale of the International Commission on Stratigraphy (http://www.stratigraphy.org/cheu.pdf). The Cambrian period encompasses a period from about 542 Ma to 488 Ma. It can be subdivided into ten STAGES of between 2 and 14 million years. If one goes solely by the fossil record, the Cambrian Explosion appears to encompass only Stage 2 (approximately 535 to 521 Ma) and mostly the last 10,000,000 years of that.
While we have seen that clear representatives of the Poriferans (sponges) and jellyfish (Cnidarians) show up in the Precambrian, it is this 10 million year period (Stage 2 of the Cambrian) that representatives from most of the other major phyla clearly show up. This includes the clams (Mollusca), crustaceans (Arthropoda), velvet worms (Onchyphora), round worms (Annelida), starfish relatives (Echinodermata) and vertebrates (Chordata).
We classify phyla on the basis of certain characteristics of an organism’s body plan. For instance, all members of the largest phylum (Arthropoda) have bilateral (as opposed to radial) symmetry, segmented bodies, an exoskeleton, and multiple jointed appendages. To be classified as an arthropod, a species must possess all of those characteristics. If a typical member of a species only possesses some of those characteristics it is not an arthropod. For instance, onchyphorans possess bilateral symmetry and multiple legs but no hard exoskeleton, segmented body, or jointed appendages. Thus they have a different body plan and are therefore classified in a different phylum from the Arthropods.
An interesting finding among the fossils of the Cambrian Explosion are certain animals called Anomalocarids like Opabinia that show signs of being transitional between the two phyla.
Opabinia has bilateral symmetry, a segmented body, a hard exoskeleton, and multiple appendages just like Arthropods. However, those appendages are not jointed. They look just like the appendages of the onchyphorans.
So here is the problem of the Cambrian Explosion at the moment. Based on the fossil evidence as we know it, there were numerous Precambrian metazoan organisms. However, for the most part their classification is uncertain. While the Cambrian fossils can usually be classified clearly into appropriate phyla, the Precambrian ones cannot. The question then arises, “Are these the ancestors of the fossils of the Cambrian or not?” We simply cannot say with any degree of certainty one way or the other.
Creationists, including ID proponents who present the Cambrian Explosion as an insurmountable problem for modern evolutionary theory claims they aren’t the ancestors. They then further claim that the phyla must have arisen instantly. They claim that natural processes could not have done it. So they conclude that this must have been accomplished by an intelligent agent.
The strength of this argument lies in its absolute assertions and in the idea that the only other option to what they claim could not have accomplished this rapid diversification is an intelligent designer. But they ALWAYS leave the intelligent design hypothesis unexamined. Before I get to some of the explanations of the fossil record from an evolutionary point of view, I would like to actually examine the intelligent design hypothesis a bit.
First, they vastly overstate their case. NO ONE says the Precambrian Ediacaran fossils are DEFINITELY not the ancestors of the Cambrian. In fact, as I indicated above many of the Ediacaran fossils show at least partial affinities to Cambrian forms. Unfortunately we cannot tell with certainty yet exactly how close that affinity is.
Second, the phyla DID NOT appear instantaneously. They appeared over a time span of 13 million years. That is plenty of time for naturalistic evolutionary processes to accomplish amazing things.
Third, what Intelligent Designer did it? This is a basic question that is actively avoided by the ID proponents. The obvious answer is a god of some type. But this sends us directly to the philosophical question, “Where did the god come from?” The answer to this leads to either infinite regress (from a super god who came from a super-super god who came from …) or to the god not being a part of this universe and therefore not constricted to natural law. But what does not being constricted to natural law really mean? It means that the god is not understandable. Trying to explain the unexplained by invoking the inexplicable is no explanation at all.
Fourth, how did the Intelligent Designer do it? This is another basic question that is actively avoided by the ID proponents. The obvious answer would be by supernatural means. But this is no different than just saying it was done magically. It explains nothing.
Fifth, what were the Ediacaran organisms then if not ancestors of the Cambrian ones? Did they evolve naturally or were they the product of the Intelligent Designer? If the former, then naturalistic evolution can produce complex organisms. What is stopping naturalistic evolution from producing the Cambrian organisms? If the latter, then did the Intelligent Designer screw up? Did he decide that after he made them they really didn’t have a future, destroy them, and start over from scratch?
Sixth, while the modern phyla made their first definite appearances in Cambrian, none of these organisms are like organisms of today. For one thing they are all aquatic. None lived on dry land. For another thing, not even modern aquatic organisms existed during the Cambrian. There has been a GREAT DEAL of change in the diversity of organisms since then. How did this come about? Were naturalistic evolutionary processes sufficient to accomplish this? If naturalistic evolutionary processes are capable of doing that, then producing the organisms of the Cambrian Explosion seems almost trivial. If an Intelligent Designer was required for that, then what were the organisms of the Cambrian Explosion? Was the Designer just practicing with those? Why did he seem to allow one organism to go extinct and then replace it with a similar organism immediately after that? Why does he seem to feel an urge to let one of his creations die off every million years or so only to magic a similar one up in its place? Why do it in such a way that looks to all the world that naturalistic evolution did it?
These are all basic questions. They are also all questions that ID proponents have no interest in trying to answer. In fact, they are questions that are actively avoided by ID proponents. If asked ID proponents tell us that at THAT POINT we are injecting religion into the situation. This is not so. These questions NATURALLY fall from their proposal. They NATURALLY fall out of their theory, which they claim is scientific. If they REALLY want to redefine science as they claim to permit supernatural explanations then they are OBLIGATED to look at these supernatural explanations critically. If this is done the obvious answers become uncomfortable for their theology. The Intelligent Designer begins to look not too intelligent. He looks wasteful, an incompetent bungler who is working his way up to the present-day diversity of organisms by a process of trial and error. The only way to save him from embarrassment is to claim that his plan is inscrutable and we are too stupid to see it. But that is the same thing as admitting THERE IS NO OBSERVABLE PLAN OR PURPOSE. But ID proponents claim that purpose is the hallmark of intelligent design.
Now let’s contrast that approach with a REAL scientific approach. There is a real phenomenon here that is in need of explaining. According to fossil record record, almost all modern body plans make their first unquestioned appearance over a 13 million year period (Stage 2 of the Cambrian period). How did this come about? With respect to evolutionary theory the situation is MUCH better than it was during Darwin’s day.
We know that life had plenty of time to diversify up to the Cambrian. We know that it DID diversify up to the Cambrian. The only problem we really need to solve is exactly HOW did it diversify. That is the thing we still are not sure of. But we DO have ideas … lots of them really. Furthermore, these ideas came about specifically because we did not feel like a supernatural being magicking the Cambrian organisms into existence was a satisfactory explanation. The ideas came from and have led to productive research.
The potential naturalistic ideas are often classified into two broad categories. (1) Those involving the environment of the Cambrian … EXTRINSIC FACTORS, and (2) Those involving factors within the organisms themselves … INTRINSIC FACTORS.
In my next blog I will examine some of these intrinsic and extrinsic factors that help give us insights into why the Cambrian Explosion occurred. Under an Intelligent Design scenario there would be no impetus to even investigate them. The only reason we have these fruitful leads to investigate is because REAL science is trying to find naturalistic explanations for observable phenomenon.
Cheers,
Darwin's Beagle
======
Links to other posts in this series:
http://www.progressiveu.org/152714-the-cambrian-explosion-part-1
http://www.progressiveu.org/185137-the-cambrian-explosion-part-2
http://www.progressiveu.org/000634-the-cambrian-explosion-part-3
