The Cambrian Explosion

[achilles12604]

Besides the apologist answer that God was responsible for this phenomina by some method, does secular science have a theory as to the cause of this sudden explosion of new life all at once? (Remember I do not fall for that God of Gaps theory)

I am looking for science answer to this mystery. Anyone care to enlighten me?

[Reasoned]

I believe the Cambrian Explosion is another of the blows to Common Descent that evolutionists have failed to explain. ... During this period of 50 million years, "most extant body forms are found". And even more, after the Cambrian explosion, no new phylum-level body plans evolved.

This seems to be more-or-less a direct transcript from one of the creationist texts. The impression it leaves is that nothing much happened after the Cambrian Explosion. This impression is wrong.

At the end of the Cambrian Explosion, there were no mammals, no birds, no reptiles, no amphibians, no modern fishes. All those evolved later. The above statement that "most extant body forms are found" could only be considered true if you count a shark and an elephant as having the same body form, and a trilobyte to be the same form as a praying mantis.

There were no land plants. They came later. Flowering plants - which represent most of the plant life we see today - did not even appear until 140 MYA, late in the age of dinosaurs.

So, I think it's fair to say, there has been quite a bit of evolution since the Cambrian, and some of it was quite non-trivial.

I'll leave discussion of the C.E. itself for another post.

[Reasoned]

Besides the apologist answer that God was responsible for this phenomina by some method, does secular science have a theory as to the cause of this sudden explosion of new life all at once?

The Cambrian Explosion was a very complicated event spread over about 50 million years. There may never be a "perfect" explanation, but here are some factors to consider.

First, around that time was when large numbers of organisms first started leaving hard fossils. The evolution of hard body parts gave some obvious advantages for both attack (e.g. teeth) and defense (e.g. shells). Probably organisms which had this feature rapidly expanded at the expense of organisms that didn't have it. When there is rapid expansion, species tend to specialize into multiple niches, with each niche having different requirements (e.g. different food sources or predators to avoid) and hence different selection pressures. This could lead to fairly rapid speciation.

Also note that the proliferation of fossil types probably overstates the actual proliferation of species, since there is little record of the soft-bodied creatures which came before or survived during or after.

Another factor may have been some kind of breakthrough in "the evolution of evolvability". Some gene systems have the ability to control body plan at a fairly high level - the homeobox genes for example (see http://en.wikipedia.org/wiki/Homeobox for a good explanation). These are typically active during morphogenesis. Any major development in such a system would enable new body types and create a large number of new shapes to try out. (Actually triggering these shapes would require other separate mutations, but their potential could all lie in one morphogenetic system.) We can't be certain about this without seeing the genes of these ancient organisms, but it seems plausible given that such systems exist in all their descendants.

Finally, we have to consider that things don't evolve in a vacuum, but rather co-evolve with their environment, food sources, predators, parasites, and diseases. At the time of the C.E., because of the huge advantage of hardness or other developments, there may have been a period where almost any body type with some new feature was superior to any body type without it. This would have allowed a great flowering of multiple body types such as we see in the C.E. But eventually, competitive pressures would have normalized. Then we would see two things: (1) the least effective of the great variety of types would die out as the competition got tougher, and (2) further "experimentation" in body types would slow down greatly as it became harder for new "experimental" body types to compete with the existing "tried and true" ones. Both of these are consistent with what we see in the C.E. and immediate post-C.E. record.

So, if I could summarize, while we don't (and may never) have a detailed explanation of what DID happen in the Cambrian Explosion, we have some reasonable ideas about what MIGHT HAVE happened. It's certainly wrong to say that science is completely baffled by this event and has no explanation at all.

However it does still contain mysteries worthy of further study. Our best hope for additional insight seems to lie with comparative genome analysis (which may allow guessing at the genome of an ancient common ancestor from that period), or perhaps with the discovery of further fossils.

[otseng]

The above statement that "most extant body forms are found" could only be considered true if you count a shark and an elephant as having the same body form, and a trilobyte to be the same form as a praying mantis.

I perhaps should've said "body plan" rather than "body form".

My main point is that all the phyla are found in the Cambrian Explosion. I'm not discounting (in this thread) that considerable amount of evolution could have occurred after the Cambrian. But why did all the (35+) phyla appear during the Cambrian and why didn't any more phyla appear afterwards (with the exception of Bryozoa)?

There are two evolutionary scenarios that I see to explain the origin of all the phyla in the Cambrian - gradualism and saltationism.

Gradualism is the most popular view of evolution. But, if evolution occurred incrementally, but in hyperdrive, there should exist numerous different species in the Cambrian. But, this is not observed in the fossil record.

Saltationism is the other possibility, which fits in better with what we see in the fossil record. And I think Rob will be in the future going into more detail about this.

And why phylum level evolution only occurred during the Cambrian also remains a mystery.

[Galphanore]

And why phylum level evolution only occurred during the Cambrian also remains a mystery.

That's more to do with our own failings then those of evolution. We routinely place things with different body plans into the phylum of their evolutionary relatives because they are, quite obviously, genetically related. The question you should be asking is, why are we so stuck on putting these creatures into phylum when it requires twisting the distinctive characteristics of the phylum?

“Our results raise fundamental questions about the validity of characteristics used to distinguish the phyla of lower invertebrates. A sponge is defined as a ‘sedentary, filter-feeding metazoan which utilizes a single layer of flagellated cells (choanocytes) to pump a unidirectional water current through its body. Except for being sedentary, the cave Asbestopluma and presumably all Cladorhizidae lack these basic sponge attributes. In an extreme environment where active filter-feeding has a low yield, cladorhizids have developed a mode of life roughly similar to that of foraminiferans or cnidarians. Their feeding mechanism relies on passive capture of living prey and on transfer of nutrients into the body through intense cell migrations, the analogue of cytoplasmic streaming in foraminiferan pseudopodia. This may be compared to the emergence of macrophagy in abyssal tunicates, also accompanied by a reduction of the filtering system although in Cladorhizidae the result is more extreme, with a main body plan different from Porifera and resembling no other modern anatomical design.”
“Such a unique body plan would deserve recognition as a distinct phylum, if these animals were not so evidently close relatives of Porifera. Their siliceous spicules show clear similarities to several families of poecilosclerid Demospongiae.”

The problem is not with evolution, it is with our propensity to try to label things in a way that we can understand and associate to other similar things. Not even scientists can escape that.

[McCulloch]

My main point is that all the phyla are found in the Cambrian Explosion. I'm not discounting (in this thread) that considerable amount of evolution could have occurred after the Cambrian. But why did all the (35+) phyla appear during the Cambrian and why didn't any more phyla appear afterwards (with the exception of Bryozoa)?

There are two evolutionary scenarios that I see to explain the origin of all the phyla in the Cambrian - gradualism and saltationism.

Gradualism is the most popular view of evolution. But, if evolution occurred incrementally, but in hyperdrive, there should exist numerous different species in the Cambrian. But, this is not observed in the fossil record.

Saltationism is the other possibility, which fits in better with what we see in the fossil record. And I think Rob will be in the future going into more detail about this.

And why phylum level evolution only occurred during the Cambrian also remains a mystery.

This is not my field of expertise. But could the alleged Cambrian explosion be simply an illusion created by an extreme lack of fossils from pre-Cambrian eras?

[Galphanore]

That's the view espoused by the site I got the quote directly above yours from. Here.

The Cambrian/Precambrian boundary is no longer considered as the place where life suddenly appears. There is a continuum of life across this boundary. Grotzinger et al (1995, p. 603-604) write:

"Once held as the position in the rock record where the major invertebrate groups first appeared, the Precambrian-Cambrian boundary now serves more as a convenient reference point within an evolutionary continuum. Skeletalized organisms, including Cambrian-aspect shelly fossils, first appear below the boundary and then show strong diversification during the Early Cambrian. Similarly, trace fossils also appear first in the Vendian, exhibit a progression to more complex geometries across the boundary, and then parallel the dramatic radiation displayed by body fossils."

Evidences of macroscopic life forms are now found as early as 680 myr ago in the form of worm burrows (Pagel, 1999, p. 881). And several modern phyla are now claimed to appear in the Precambrian and thus are not part of the supposed 'Cambrian Explosion.'

[otseng]

This is not my field of expertise. But could the alleged Cambrian explosion be simply an illusion created by an extreme lack of fossils from pre-Cambrian eras?

I don't think you're alone in suspecting this a possibility.

Even in the article I quoted earlier, The Origin of Animal Body Plans, the author brings this idea up:

The trigger of the Cambrian explosion is still uncertain, although ideas abound. If the evolutionary trees are right and the fossil record is not deceptive, then many different lineages must have acquired complex anatomies and hard parts at about the same time.

Also, it's not just a lack of fossils in the pre-Cambrian, but all the other fossils as well in the Cambrian and post-Cambrian. Why did all the the phyla arise in the Cambrian? Why no new phyla post Cambrian? It'd be hard to believe that those are illusory also.

From my viewpoint, if the evidence and the theory don't line up, the approach shouldn't be questioning the evidence, but questioning the theory.

[Galphanore]

This is not my field of expertise. But could the alleged Cambrian explosion be simply an illusion created by an extreme lack of fossils from pre-Cambrian eras?

I don't think you're alone in suspecting this a possibility.

Even in the article I quoted earlier, The Origin of Animal Body Plans, the author brings this idea up:

The trigger of the Cambrian explosion is still uncertain, although ideas abound. If the evolutionary trees are right and the fossil record is not deceptive, then many different lineages must have acquired complex anatomies and hard parts at about the same time.

Also, it's not just a lack of fossils in the pre-Cambrian, but all the other fossils as well in the Cambrian and post-Cambrian. Why did all the the phyla arise in the Cambrian? Why no new phyla post Cambrian? It'd be hard to believe that those are illusory also.

From my viewpoint, if the evidence and the theory don't line up, the approach shouldn't be questioning the evidence, but questioning the theory.

There are though. The page I linked to poitns out just a few of them.

[QED]

I would dearly like to know if we are all agreed here that at this time (the Cambrian Era) animals were of a restricted size compared to subsequent Eras (e.g. Devonian). As I see it, a simple observation such as this should force a change in the creationist mindset. It stands out clearly to me as a signature of some sort of "learning curve", a hallmark of evolution in its most common sense.

[otseng]

As I see it, a simple observation such as this should force a change in the creationist mindset. It stands out clearly to me as a signature of some sort of "learning curve", a hallmark of evolution in its most common sense.

You completely lost me there. What does "signature of some sort of 'learning curve', a hallmark of evolution in its most common sense" mean?