Was the Cambrian Explosion Really an Explosion?

Donald Prothero, a paleontologist, knows his fossils. And, in 2007, Columbia University Press published his book, Evolution: What the Fossils Say and Why It Matters. For it’s scope, clarity of writing, and visual attractiveness (it has lots of illustrative drawings, photographs, and charts), it’s certainly one of the best book on the subject of evolution vs. creationism out there.

Here’s part of what Prothero says about the Cambrian explosion (he devotes a whole chapter of his book to the subject):

Of all the distortions of the fossil record that the creationists promote, the worst is their version of the “Cambrian explosion.” . . . The major group of invertebrate fossils do not appear suddenly at the base of the Cambrian but are spaced out over strata spanning 80 million years—hardly an instantaneous “explosion”!

Donald Prothero emphasizes that early evolution followed a logical progression from a world of “pond scum” to multicellular life (represented by “The Garden of Ediacara”) to shells to “mineralized, fossilizable skeletons.” According to Prothero:

. . . the earliest stages of the Cambrian (known as the Nemakit-Daldynian and the Tommotian stages, from 520 to 545 million years ago) are dominated by tiny (only a few millimeters) fossils nicknamed the ‘little shellies’ . . .

And things become larger and more mineralized from there. Prothero sums up this way:

[T]he 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 largely shelly organisms (especially trilobites) finally develops. . . . Apparently, creationists persist in presenting a version of the Cambrian that is at least fifty years out of date either because they don’t know any better (the ‘clueless’ hypothesis) or because they do know better (the ‘deceiver hypothesis). Either way, it is bad science.

So, are we done? For the reasonable lay person, should the case be effectively closed on the Cambrian explosion?

Maybe not.

In 1993, Stephen Gould edited a book for Norton titled The Book of Life: An Illustrated History of the Evolution of Life on Earth. Like Prothero’s book from a decade later, it’s an attractive and high production affair. And the chapter that includes a discussion of the Cambrian was written by a paleobiologist of equivelent expertise to Prothero, John Sepkoski of the University of Chicago. In general terms, the chapter hits the same bases as Prothero, but there are some tantalizing qualifiers to the tidy picture that Prothero draws. Here’s what Sepkoski writes:

The new story is more coherent than the old, but it raises new questions, and it still leaves hundreds of millions of years of metazoan ancestry during the late Precambrian without a fossil history.

Metazoa is a synonym for the kingdom of Animalia (or animals), so what Sepkoski is saying is the following: prior to the Cambrian, we really don’t see all that much evidence of animals in the fossil record. But the ancestry of Cambrian animals, for mutation and natural selection to do its magic, must surely go back “hundreds of millions of years.”

And the tidy picture of “The Garden of Ediacara” that Prothero depicts as an evolutionary stepping stone toward at least some Cambrian animals may not, in fact, be all that easy to establish. Sepkoski notes that the Edicarian fauna may be “a separate development, not related to modern animal phyla.”

Yikes.

In other words, “The Garden of Ediacara” may have been largely an evolutionary dead-end (or, in any case, not a very big piece in the puzzle of the evolution of metazoa; that is, of animals).

So, maybe the Cambrian explosion was an explosion. Sepkoski, at least in 1993, was willing to use the term:

What is extraordinary about animal phyla is that nearly all of them seem to have evolved around the Cambrian-Precambrian boundary. Furthermore, that evolutionary explosion also produced a horde of unique and sometimes outlandish-looking animals whose fossils cannot be assigned to any living phylum.

So, at the Cambrian-Precambrian boundary, a tipping point was reached in which animal phyla suddenly, and quite dramatically, radiated, but prior to this period the fossil record for the ancestors of metazoa is just not that good (even though it should span hundreds of millions of years).

That sounds like an explosion to me.

But maybe it was an explosion driven, not directly by God, but by simple biomineralization. Here’s a recent speculation offered for the Cambrian explosion in a Physorg.com science news article:

[E]arly shell-bearing creatures help to resolve Charles Darwin’s concern over the sudden appearance of so many new animal species during the Cambrian explosion. The fossil record gives the impression of a “Creation” event, but in reality, animals had evolved prior to the explosion. They just didn’t leave much for paleontologists to find until they developed the skeleton-making trait.

And once a few animals started building with minerals, a “housing boom” erupted across the animal kingdom.

I’m sorry, but this sounds like an assumption in want of proof: did animals have a vastly long and gradual evolutionary history prior to the Cambrian or not? If, after all, the fossil record is almost wholly absent, then on what is the article’s confident pronouncement based?

It appears to be nothing.

So, I’ll try again: did the Cambrian constitute an evolutionary explosion or not? In search of an answer, I’m now opening yet another very attractive coffee-table style book on evolution, published by the University of California in 2009 (Evolution: the Story of Life, by Cambridge lecturer Douglas Palmer). Like Gould’s book from the 1990s and Prothero’s book from 2007, it’s suffused with helpful visuals.

But what does Douglas Palmer say about the Cambrian? Here it is (page 48):

Right from the earliest times, around 542 million years ago, an extraordinary explosion of life in the seas prompted an arms race between predators and prey. Within 20 million years or so, most major groups of animals had evolved, including our remotest vertebrate ancestors.

Houston, we have a problem. Palmer (2009) says it took 20 million years for the evolution of the various phyla of the Kingdom of Metazoa (animals); Prothero (2007) says it took 80 million years (and calls the evolution of animal phyla a “slow fuse”); and Gould and Sepkoski (1993) say it must have taken “hundreds of millions of years.”

What’s a lay person like me, dipping into science websites like Physorg.com and books produced by highly esteemed academic presses (Columbia, Norton, and the University of California), to make of this?   

My conclusions: 

  • Physorg.com is blowing blue pipe smoke.
  • Donald Prothero overstates his case (that the Cambrian explosion is a misleading myth, and ought to be called a “slow fuse”);
  • John Sepkoski is even-handed: the Cambrian explosion really was an explosion, but, if evolutionary gradualism is true, then animals have an unpreserved evolutionary history that goes back “hundreds of millions of years” before the Cambrian.
  • Palmer tells it like it really appears to be: the period between 542-522 million years ago constituted “an extraordinary explosion of life.” 

Can evolution by random variation and undirected natural selection (absent any target whatsoever) really account for what happened during the Cambrian explosion?

Why should anyone believe it?

About Santi Tafarella

I teach writing and literature at Antelope Valley College in California.
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7 Responses to Was the Cambrian Explosion Really an Explosion?

  1. Longtooth says:

    The phenomena of the Cambrian era, relative to the pre-Cambrian state of biological evolution, seem at times altogether too extraordinary in the complexity and sophistication of the various life forms that emerged. How is it that biological life on this planet could limp along in a relatively unsophisticated state for maybe three billion years and then all of a sudden spew forth with all the highly advanced life that the Cambrian era evidently produced? Like many lay people I’ve pondered this mystery for a long time. Last fall, while rail-birding on a friend’s introductory biology class, an answer came to me, or part of an answer, or at least a conceptual piece of the puzzle I believe.
    I’m several decades removed from my own introduction to biology. It had therefore been a long time since I last took anything approaching a studious look at the structure and function of plant and animal cells, most of which I either forgot or never learned. This time though, I was jaw-dropping astounded by their complexity and sophistication. The level of functional capability of the basic animal cell is, I assume, prerequisite to supporting all higher life forms. These tiny little cells are so advanced that it seems that the mechanisms of evolution could easily have needed several billion years alone to bring them to a state that could effectively support higher life. So then, could not the Cambrian “explosion” or “fuse” have been set off by a crystallization in capability on the single cellular level? Is this too simple and uninformed of an explanation? Glad to be corrected if it is.

    • santitafarella says:

      Longtooth,

      That’s a clever idea—that something happened within the cell that then flowered, but what drove the cell to machine-function in this way in the first place?

      The only strictly materialist explanation on offer (for now) is random variation and natural selection, but that doesn’t seem adequate to the task.

      —Santi

  2. Longtooth says:

    Santi,

    What is the inscrutable property of ultimate reality that affords the potentiality for even the simplest forms of biological life let alone the more complex? What is the ultimate nature of the life force behind it all that motivates biological entities to persist and strive in the ways they do? Well, these seem to be the fundamental mysteries underlying everything that breaths. I certainly fall short of having any decisive answers. Still, arguments from ignorance are as unconvincing for me as “mechanistic” explanations are for others.

    I’m well over my head in writing about the nuances of the theory of biological evolution or being an apologist for it. However, as I understand things, random variation and natural selection are not the only determinants of biological evolution. They are the most celebrated, but do not alone adequately account for all the biological phenomena and potentially intervening environmental factors involved. With Wikipedia as my quick and dirty source, there are four primary things at work rather than just two: natural selection, genetic drift, mutation, and gene flow. More comprehensively, evolutionary theory has progressed substantially since Darwin’s time, having matured into what is called the “modern evolutionary synthesis” which is touted to provide a “coherent and unifying explanation for the history and diversity of life on Earth”. So, maybe modern evolutionary theory is a bit more adequate than it might first appear to be.

    –Longtooth

  3. santitafarella says:

    Longtooth,

    Mutation, genetic drift, and gene flow are the random variations upon which natural selection operates.

    My own best-guess of what we’re embedded in is by way of analogy: the universe is akin to an apple tree. Just as an apple tree “branches” and “leaves” and “apple blossoms” and “apples,” the universe bears its fruit as well (it “stars” and “planets” and “lifes” and, ultimately, “minds” and “peoples”). We should no more treat the universe as a random phenomenon that just happened to produce these things as we should treat an apple tree as a random phenomenon that just happened to produce its things.

    That is not to say that nothing random or contingent is in play in the universe. Like the apple tree, the universe also has its peculiarities. If you played the program again, you would get variations.

    But underlying this low entropy, highly improbable, universe is some sort of “genetic” blueprint that is, in general terms, playing itself out. It may be an alien or it may be God who is behind it, but I suspect that mind, not blind matter, undergirds the universe in some fashion.

    The only alternative to this is to give the universe, prior to the big bang, a vast multiverse history. If you do this, then you are positing a natural history for the universe akin to the natural history that gave us the apple tree. Absent this, you’ve just got a universal “apple tree” with a genetic-like blueprint running, but lacking a history to account for it.

    The great contemporary leap of faith, in my view, is the multiverse vs. God. Which will you choose?

    —Santi

    • Longtooth says:

      –Santi

      “The great contemporary leap of faith, in my view, is whether you will come down on the side of belief in the multiverse or God. Which will you choose”.

      A mind-closing choice between a godless multiverse verses a god-possessed single universe? Hmmmm. Although I reviewed the multiverse blog I still don’t quite grasp how a multiverse precludes the existence of some great intelligence or intelligences. To the contrary, it would seem if anything to make such entities more plausible, not less. .

      Giving a few alternative speculations some rope, why not a multi-god multiverse verses a single god universe, or a multi-god universe verses …..? The single universe speculation carries the baggage of assuming one great almighty unmovable mover who masterminded and triggered the whole enchilada. I’m just a dummy, but my suspicion is that multiple unmovable movers might provide an easier accounting for both the complexities and stability of the physical laws governing our cosmos, multiverse versus universe issues notwithstanding.

      Perhaps god or gods (great cosmic intelligences) do exist, but themselves were nevertheless the offspring of ancient causes unattended by any self aware designing mind. I could never really understand the necessity of mind over matter in the god debate. Why must a mindful god or gods be the first cause or causes behind this realm of physical being and becoming that we occupy; a long-standing ideological dogma? It only has traction if god (some god) is always characterized as the first cause. That, however, might just as well mean that the postulated cosmic god began as an unconscious triggering event with an emergent cosmic consciousness resulting a-posteriori.

      Maybe mind and matter are codependent for existence if not ultimately just different facets of the same thing. Evidence of “mind” is only found in the presence of matter or its energy equivalent. Intelligence (as in information) is a component of and prerequisite in some accumulation to supporting a mind that manifests thought and self awareness. The presence of any modicum of intelligence presupposes some coherent organization of something. At least down to the level of elements and molecules, matter is also characterized by a coherent organization of something. Making a long leap from these observations, maybe we humans (who conceive of “gods” and other intangible things and ponder notions about their nature) are local manifestations of a cosmos striving to come to knowledge of itself, but with multiple participants and maturation quite a long way off.

      –Longtooth

      • santitafarella says:

        Longtooth,

        Well, your speculations above are interesting, but they don’t foreclose the choice: either our big bang universe has a natural history that preceeded it or it doesn’t. If it doesn’t, you’ve got to account for the very far from entropy quality of it; if it does, you’ve still got to account for the multiverse’s contingent and conditional existence. At some point, you’ve got to posit (it seems to me) one thing that is unlike all other conditioned things: one unconditioned thing.

        In other words, all things in the universe have conditions for their existence. You are conditioned upon cells; your cells on molecules; molecules on atoms; atoms on quarks; quarks on strings, etc.

        There must be one thing that is itself unconditioned that undergirds all that is conditioned (otherwise there would be no conditioned existence at all). And, as Aquinas would say, we call this singular unconditioned Being “God.”

        As for mind, I’ve always thought it must have been odd for God to awake into consciousness and say, “Wow! This is trippy! I’m God!” The minute you think of mind in any way related to time, God must have had a first moment of consciousness as well. But theists would say that God is outside of time. But then, the moment he thought anything discreetly, such as “I think I’ll create a material universe,” that would be a moment in time, wouldn’t it? He would be aware of himself and of an imagined material universe. But these thoughts would all be conditioned on his mind. So would the thought be God, or is God behind the thought? Hmm. It seems God has the same problem humans do: how many people are in your head? One or two? Who is speaking in your inner talk and who is listening?

        By the way, I’ve blogged a couple of more chapters on The Molecule book. I’d be curious to know if you think I’ve made any obvious errors there.

        —Santi : )

  4. hbhatnagar says:

    The Cambrian explosion is a big unanswered question in our history on this planet. I wish it could be answered with the finality you seek, but it cannot at present. 🙂
    As for the apple tree analogy, it falls short in my view in that we know the blueprint behind it. We don’t find any such behind the universe per se. It might be argued that we are just ignorant of the Grand Plan and we might discover it in the future. Perhaps. In the interim we need to make the best of what we do know of the universe and deduce accordingly.
    With regards to Multiverse vs God, I don’t find enough evidence either way. Multiverse just seems to bypass the question of creation while God confounds it by positing the most complex creator at the onset of creation itself.

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