Punctuated Equilibrium, the Cambrian Explosion and Falsifying Evolution

October 17, 2008

Creationists will commonly claim that evolution cannot be falsified and therefore should be disregarded as science. To ‘prove’ their point, they insist that the theory is capable of being reworked to absorb any and all new evidence, oftentimes citing the neo-Darwinian Synthesis or the Cambrian explosion as examples of evolution’s infinite malleability in action. This recently came up in the comments section here, and has lead to this post by Eric Kemp.

Before I go any further I’d like to object, as Penfuinfactory already has, to the use of the phrase ‘molecules-to-man evolution’. While I can see the childish alliterative appeal it might hold for some people, it’s a massive oversimplification and implicitly misogynistic. Kemp has given a bizarre defence of using the word ‘molecules’ by pointing out that the first living organisms were likely some sort of very basic prokaryotes (I’m confused too), while also stating that ‘man is the highest form of evolution’. This is not how evolution works, and it’s a very common misconception among Creationists. There is no reason why the evolution of humans from ‘molecules’ is any more difficult to explain than the evolution of other extant animal species.

Anyway, on to the debunking:

The Cambrian is a geological period that began somewhere around 570 million years ago. Most people know of it only because of the famous ‘Cambrian explosion’ (sometimes referred to as the ‘Cambrian radiation’), a major evolutionary event during which almost all of the modern phyla appeared. ‘Phylum’ is a fairly loosely defined taxonomic term that means, roughly, ‘body plan’. As an illustrative example, spiders and crabs are both members of the phylum Arthropoda, while the phylum Chordata comprises fish, reptiles, birds and mammals – including, of course, humans. Note that the definition of ‘phylum’ and its relationship with other taxonomical rankings change depending on whether one is discussing morphological similarity or evolutionary relatedness.

The Cambrian period has long fascinated scientists, in part because it contained creatures that, compared with modern animals, looks absolutely bizarre. Opabinia is a good example of what I mean. However, that fascination has long been haunted by a certain amount of unease, as the Cambrian explosion is unlike anything else in the fossil record and appears to have been much less ‘gradual’ than subsequent evolution (although I should point out that Creationists frequently exaggerate how ‘sudden’ it actually was). Is it really an example of scientists ‘moving the goalposts’ and ignoring what should be evidence that directly contradicts evolution?

Unfortunately, the late Stephen Jay Gould often gets wheeled out when this topic comes up, a practice that he has actually asked Creationists to refrain from (keep this in mind, as some YECs in particular like to act as if Gould tacitly agreed with them). The story goes that he came up with the idea of ‘punctuated equilibrium’ in order to ‘save’ evolution, which is complete nonsense. I actually read some of Gould’s work, and in The Structure of Evolutionary Theory discovered a sustained and blistering assault on the scientific community for assuming the truth of gradualism. (Misconception alert: As Richard Dawkins has pointed out, when Gould says criticises ‘gradualism’ he is not implying that saltation or special creation are true.) But wait, you might ask, wasn’t Gould supposed to be acting as the theory’s saviour by suggesting punctuated equilibrium? Why then is he so critical of the scientific community, and why has punctuated equilibrium had such an enormously hard time getting the attention he feels it deserves?

As always, the Creationist brigade has offered a woefully simplistic version of the real story. Gould believed that punctuated equilibrium (or something like it) should have been realised by Darwin himself and that the entire fossil record is ample evidence of it. Not because the fossil record isn’t evidence of evolution – he obviously agrees that it is – but because it is evidence of evolution happening in a less gradualistic fashion than is commonly believed. I cannot stress this point enough: Gould was not making a knee-jerk reaction to the realisation that the fossil record disproves ‘gradual’ (as in slow) evolution, he was proposing a different type of gradual evolution. The factuality of evolution in its most basic conception has not been under serious threat of being deposed by the evidence for many years.

This leads me to the second important point, one which Creationists frequently ignore: the evidence in favour of evolution is so strong at this point that, despite their simple-minded protests, it will take something thar directly contradicts evolution to throw it into serious doubt. Again, I need to make the distinction between evidence which raises questions about how evolution happens and evidence which raises questions about whether it happens. The basics of evolutionary theory – common ancestry, change via mutation, natural selection, genetic drift, etc. – are in no way threatened by the Cambrian explosion. (Somebody is inevitably going to claim that, even if  punctuated equilibrium explains the Cambrian explosion, it requires spontaneously rapid mutation rates. This is not the case, and I suggest you read Gould’s own writing on the matter.)

Rather than enter into a discussion on how to explain the Cambrian explosion, I’ll link to the Wikipedia article (for convenience sake – obviously there are far more authorative works out there), along with an interesting list of discredited hypotheses. The existence of the latter is one source of evidence that scientists will not simply latch onto any explanation for an apparent deficit in their precious theory.

What I’m trying to say with all of this is that the Cambrian explosion does not contradict the vast amounts of evidence which suggest that evolution has occured, even if it does raise some questions over its methods. With that settled, I’ll go into more detail over some of the specifics of Eric Kemp’s argument:

The Cambrian Explosion represents the rapid appearance of most forms of life on Earth with no evolutionary predecessors.

In about a 5 million year period, most of the life as we know it suddenly appeared with no evolutionary ancestors.  As few as twenty and as many as thirty-five of the worlds’ forty phyla (the highest category of the Kindom Animalae) appeared out of no where (J.W. Valentine Development 126, 1999).  This means that entirely novel and highly complex body plans shows up in the fossil record with no ancestors.  If we compress all of the Earth’s history into twenty-four hours, the Cambrian explosion would last only about one minute.  Put another way, seventy-five percent of all life shows up in about .07% of the time the Earth has existed.

Firstly, the term ‘phylum’ is not tied only to the Kingdom Animaliae. Every kingdom is divided into one or more phyla (although the word ‘division’ is more frequently used in certain Kingdoms).

Secondly, the ‘clock’ analogy is one that should be avoided; it gives the impression that every phyla differentiated only during this period, ignoring entirely all of those were either pre- or post-Cambrian.

Thirdly, the fact that many phyla have no fossil ancestors prior to the Cambrian explosion is not surprising, given that hard body parts are generally thought to have first developed during this period.

The claim that ‘seventy-five percent of all life shows up in about .07%  the Earth has existed’ is incredibly misleading. The Cambrian period contains no land plants at all – indeed, the majority of species during the Cambrian were entirely aquatic. It certainly did not contain ‘all life’.

Finally, Eric is ignoring the very real debate over how sudden the Cambrian explosion actually was and how many of the extant phyla first evolved during it.

The fossil record literally goes from fungi and simple worms to the trilobite with an articulated body, complicated nervous system and compound eyes, fully formed and novel, in the blink of an eye!  The most astounding thing about the Cambrian explosion is that it’s followed by stasis.  That is, zero new body plans have evolved in the 500 million years since the Cambrian.

This is a good example of a red herring. Although the majority of phyla appeared during the Cambrian, that is not the same as claiming that the majority of speciation or differentiation occured during it. As I’ve pointed out, most Cambrian-era life was aquatic and land plants were entirely absent; clearly, life has been doing a lot of ‘evolving’ in the last 500 million years. I suspect that the word ‘stasis’ here is co-opted from Gould, in which case it is mis-used.

There is nothing in evolutionary theory which states that new phyla must contiously appear. Although the relatively sudden appearance of all major phyla certainly raises some interesting questions about early evolution, it does not invalidate the theory.

The Cambrian Explosion is evidence of ‘top-down’ differentiation.

Neo-Darwinian theory predicts a “bottom up” pattern where small differences develop before the large differences in form and body plan are seen much farther down the line.  For instance, pre-Cambrian sponges should have produced a myriad of varieties and those varieties would then eventually lead to different species, forms and body plans.

However, the fossil record from the Cambrian shows a completely different “top down” pattern.  Massive differences in form and body plan appear suddenly with nothing simpler preceding them.  Then, after the Cambrian, only minor variations arise within the framework of the body plans set down in the Cambrian.

Minor variation? In what sense is the evolution of virtually all land animals and literally all land plants ‘minor’? Eric is once again applying the concept of the phylum across the board, as if it’s the only way to categorize the different types of living organism. Life in the Cambrian era was very different to what exists today, as even a casual examination of the fossils will reveal.

Eric goes on to make the erroneous claims about Gould and puncuated equilibrium that I’ve already covered, before giving us this gem:

To explain how life as we know it evolved from a single cell, evolutionary theory says, “It had billions of years to do it”.  The fossil record contradicts this by telling us it did most of the work in only 5 million years.  To explain the contradiction, Stephen Jay Gould comes up with punctuated equilibrium, which STILL doesn’t explain the fossil record.  Then to explain why life went into body plan stasis after the Cambrian, evolutionary theory goes back to “well, evolution IS really slow”.

Eric, you directly contradict yourself here. You say that evolution ‘did all of its work’ in 5 million years, implying that it all life differentiated over the course of the Cambrian explosion. Not only have your claims about what the event actually is steadily grown more exaggerated, but you’ve (yet again) confused ‘phylum’ for ‘all categories of life’. Immediately after this you go back to talking only about body plan ‘stasis’ – should I assume that your earlier comment about evolution doing all of its work in 5 million years was also referring only to phyla?

Before I wrap this up, I’d like to bring things back to where they started: the question of whether evolution is falsifiable. As I’ve spent the last while explaining, the Cambrian explosion does not represent a threat to the veracity of evolutionary theory and so is not an example of something that scientists have simply swept under the rug. But what about Penguinfactory’s example of ‘rabbits in the pre-Cambrian’? Would such a find represent a contradiction to the theory of evolution in its entirety? I’ll quote PF on the matter (keep in mind that ‘bunny’ here is a stand-in for any modern mammal):

A few other observations that would invalidate evolution if a pre-Cambrian bunny was discovered:

a) How would a mammal survive in a world with no plants? Unless pre-Cambrian plant fossils were found as well, the most likely explanation for this fossil would be that the evolutionary model is incorrect and that the seemingly chronological sorting of the fossil record is an illusion.

b) If pre-Cambrian plants were found, that would still leave the issue of time. How could life have gone from almost microscopic soft-bodied marine organims to land-dwelling mammals in such a short space of time, given our estimates of when life on Earth began.

c) There’s also the issue of the later fossil record- why would our Pre-Cambrian bunnies vanish abruptly off the face of the Earth for over a billion years and then re-appear in more recent times?

If any Creationists reading this could provide a hypothetical scientific response to these questions that makes even the slightest amount of sense, I’d love to hear it.