Pharyngula

The Cambrian as an evolutionary exemplar

i-ccbc028bf567ec6e49f3b515a2c4c149-old_pharyngula.gif

I’ve been reading Valentine’s On the Origin of Phyla(amzn/b&n/abe/pwll) lately, and I have to tell you, it’s a hard slog. This is one of those extremely information-dense science texts that rather gracelessly hammers you with the data and difficult concepts on page after page. I am convinced that James W. Valentine is ten times smarter than I am and knows ten thousand times as much, and it’s a struggle to squeeze that volume of knowledge into my miniscule brain pan.

One thing I would like to greatly condense and simplify is his discussion of the Cambrian ‘explosion’. Misinterpretation of the Cambrian is one of the many prongs of the creationist assault on science; both old school Biblical creationists and the new stealth creationists of the ID movement have seized upon it as evidence of an abrupt creation—that a Designer poofed the precursors to all modern forms into existence suddenly, and without precursors, and that this observation contradicts evolutionary theory.

It doesn’t. Valentine has an excellent diagram that shows how wrong the creationists are.

i-9edd5317f60b6bde203f477bb13b8f6d-cambrian_timeline.gif

Let’s look at this from bottom to top, from oldest to youngest. There are two lessons here: one is that the Cambrian was a real transition event, but the other is that it looks remarkably natural and progressive—something best explained by material phenomena and not unsupported speculation about mysterious and invisible Designers.

Roughly 570-600 million years ago, fossils are sparse, but they include the phosphatized embryos of the Doushantuo formation in China and a scattering of trace fossils. Trace fossils are the remains of trackways and burrows, not the animals themselves, and tell us that there were small soft-bodied and multicellular animals living on the substrate; we even have a few fossils of more elaborate, bilaterally-symmetric animals, comparable to flatworms.

Here are some of these early trace fossils; they are small squiggles in the sediment, the faintest signs of living creatures once having crawled there.

i-be36852623b4aa226513fdc90cdbf315-neoproterozoic_trace.gif

Near the end of the Neoproterozoic, the larger and more complex and enigmatic Vendian and Ediacaran fossils turn up. There are also more and more complicated trace fossils. Animals are getting larger and making more substantial trackways; in addition, they’re beginning to burrow down into the sediment. We begin to see signs of a phenomenon called bioturbation, where the substrate is stirred and turned over by animal activity, which was absent before.

i-699a05ae0df46b3b2c1bf8612318ac17-cambrian_trace.gif

Another important feature begins to make its appearance: the small shelly fossils. These are little guys, only about 1 or 2mm across. The kings of creation at this time were scattered beasties the size of a baby’s toenail, but still, it was a step upward in size and durability from what had come before.

i-2026df3376063afcf79ad28d8a38111a-small_shelly.gif

The Cambrian itself begins 543 million years ago, and is broken up into periods several millions of years in length with their own distribution of fossils. The oldest, the Manykaian, is marked by more trace fossils, and a greater diversity of the small shelly fossils; the diagnostic fossil whose appearance is used to mark the beginning of the period is a trace fossil, the relatively large burrows of Treptichnus pedum. In the Tommotian, 530 million years ago, the first recognizable brachiopods and molluscs are found, and there are trace fossils that indicate something with many legs scurried by—the first arthropods. The first actual fossils of arthropods and echinoderms are found millions of years later.

It’s more than ten million years later that the spectacular and strange animals of the Burgess Shale make their appearance. It’s during the Middle Cambrian that we can say most of the modern phyla are present, although of course the representatives of those phyla don’t look much at all like their modern members.

One message from these data is that the Cambrian ‘explosion’ was real. It isn’t an artifact of poor sampling of ancient rocks—we have a range of good fossils from the period before, and it’s clear that the pre-Cambrian world was a different place than the post-Cambrian.

But another important lesson, and one that creationists like to hide, is that while this was a sudden event in a geological sense, it wasn’t actually all that rapid in human terms. The evolution of the canonical Cambrian forms was drawn out over tens of millions of years. They didn’t just come out of nowhere, either; while individual lineages are cryptic, we see a slow aggregate increase in the complexity of multicellular animals in the fossil record that culminated in the flowering of large-animal diversity in the Cambrian.

I’ve had many creationists try to use the Argument from the Cambrian Explosion as a fait accompli against evolution (most recently, just this week). It’s actually an argument from ignorance, though, since the data certainly does not fit a sudden creation by divine or alien fiat. It does fit with the idea of the appearance of these animals as a product of prior history, though…even though there are many mysteries about the details, the big picture does not require miracles or the supernatural.

Comments

  1. #1 tristero
    November 21, 2006

    “One message from these data is that the Cambrian ‘explosion’ was real. It isn’t an artifact of poor sampling of ancient rocks–we have a range of good fossils from the period before, and it’s clear that the pre-Cambrian world was a different place than the post-Cambrian.”

    Does Valentine hazard a guess as to what “triggered” the explosion? Was it a significant change in the environment? Or was some kind of a threshhold reached in the complexity of lifeforms which then quickly morphed to fill ecological niches? Anything else?

    I apologize if these are stupid questions. I’m not a scientist but I’ve been fascinated by Cambrian for years and read a few lay science books about it. I’ve also got a book of plates of a lot of the Burgess Shale critters and they are utterly incredible, and absolutely wonderful.

  2. #2 CCP
    November 21, 2006

    …the data certainly DO not fit…

  3. #3 Dennis
    November 21, 2006

    tristero,

    There’s no cause of the Cambrian explosion pinned down, but there are a few floating hypotheses. One is the newly-oxygenated atmosphere fascilitated life-by-cellular combustion (and probably accelerated the rate of genetic mutations: oxygen is really pretty nasty stuff!) Another is that the Cambrian was the beginning of the period in evolutionary history where organisms’ genomes had built up enough complexity and redundancy to make more mutations survivable, leading to increased speciation. Or, it may have been global warming.

  4. #4 llewelly
    November 21, 2006

    no, no, CCP, it’s ‘the datas do not fit’ … ūüėČ

  5. #5 AlanW
    November 21, 2006

    It’s actually an argument from ignorance, though, since the data certainly does not fit a sudden creation by divine or alien fiat.
    No! Whoda thunkit? Sounds like the cretinists’ usual MO to me. That and fingers in the ears ‘lalala I can’t hear you’

  6. #6 tristero
    November 21, 2006

    Dennis,

    Thanks for taking the time to respond.

  7. #7 Dave Newton
    November 21, 2006

    I’ve always been a fan of the “mutation was now survivable” camp and speciation was “easier” due to less complexity.

    Whatever it was the process is still more amazing to me than a random FSM going *poof*.

  8. #8 eyelessgame
    November 21, 2006

    The thing to keep in mind, too, is that the period of time we’re talking about — 600 to 520 MYA — is a longer period of time than from the extinction of the dinosaurs to today. I dunno that anyone else needs that reminder to put this in perspective, but I sometimes do. “Explosions” that take longer than the entire Cenozoic.

  9. #9 jimmiraybob
    November 21, 2006

    “Life” doesn’t even start at the Cambrian Explosion. What is made of the much much longer history of our little single-celled buddies – the Archaea, Eucharia, and Bacteria? I’m personally partial to the Archaea but I can understand that some may not find them all that sexy since they don’t sport arms and legs…..and, of course, I was referring to two AND eight arms.

  10. #10 TheBlackCat
    November 21, 2006

    Another hypothesis I heard was that the evolution of eyes made the fast-moving, hunting predator an effective strategy, resulting in rapid diversification as the now-ubiquitous evolutionary “arms race” went into high gear.

  11. #11 Molly
    November 21, 2006

    Disclaimer! I’m just an undergrad, and certainly no expert. One of my professors is an invertebrate paleontologist, and he believes that there wasn’t really a large explosion of life in the Cambrian, but that many preexisting life forms evolved hard parts which allowed them to be preserved more easily.

  12. #12 Steviepinhead
    November 21, 2006

    Evolution of vision, and the resulting increase in the efficiency of predation, might well have driven the evolution of fossilizable hard parts, so these last two comments may point to a synergistic process.

    There’s a good pop-sci book on the vision-driven theory, “In the Blink of an Eye,” by Oxford University zoologist Andrew Parker (Perseus 2003).

  13. #13 Julie
    November 21, 2006

    Thank you for this post, PZ. I know it’s odd to say this, but looking at those little line drawings of the burrows and shells was a religious experience for me (!).

    I am in awe of our world, in awe of life… in awe of how we got here… from there…

  14. #14 Scott Simmons
    November 21, 2006

    “Another important feature begins to make its appearance: the small shelly fossils. These are little guys, only about 1 or 2mm across. The kings of creation at this time were scattered beasties the size of a baby’s toenail, but still, it was a step upward in size and durability from what had come before.”

    Curious question: I’m in the middle of S.J. Gould’s collection Eight Little Piggies, and one of the hypotheses he floats was that those small shells were from larger creatures that had many disconnected shells, rather than from individual miniscule single-shelled creatures. This is from about fifteen years back–what’s current consensus on this? Does Valentine discuss it at all?

  15. #15 Jeff
    November 21, 2006

    I second your appreciation of Valentine’s book – I bought it sight unseen after seeing only the title on amazon, and have found it fascinating, and very challenging (I’m a pathologist, not a zoologist or developmental biologist), but it does keep the mind sharp, and nicely complements “Endless forms most Beautiful” in my library. I think that Gould’s valedictory tome is even a tougher read, especially for the non-biologist.
    – Jeff

  16. #16 Chris Nedin
    November 21, 2006

    JW’s works do tend to be packed with detail.

    The “Vendian” in relation to this period of time is now an informal term. There was a vote, we won. It’s now the Ediacaran Period (635-542 million years ago). Use it, damit, or we’ll sue.

    Not having read On the Origin of Phyla (I tend to rely on bibliophiles like PZ to keep me abreast of developments), I’m not sure if its JW or PZ that’s claiming, in regard to Small Shelly Fossils (SSF) The kings of creation at this time were scattered beasties the size of a baby’s toenail. A lot of the SSFs are actually pieces of body armour or other bits and pieces shed from larger animals. A number of Burgess Shale forms show this, and the modern velvet worm Peripitus grows a structure not unlike SSMs on it’s head during the mating period. Abundant coeval trace fossils show that larger animals were around at the time of the SSFs.

    As to the cause of the so-called “Cambrian Explosion” it was actually the coming together of a number of factors which all worked together. This included rising sea levels to open up new niche-space, the concentration of oxygen scavenging to particular parts of the body (gills) – this freed organisms from the passive adsorption of oxygen over the entire body, which in turn allowed the differentiation of body plans into jaws, antennae, legs, flaps, even better gills etc, improved biochemical pathways such as oxygen-carrying fluids in circulatory systems which could take oxygen deep into the tissues and allowed a circular instead of a flat body plan, and the ability to lay down minerals over certain parts of the body (probably originally as a mechanism to excrete excess minerals out of the body) no longer required for oxygen gathering, resulting in the production of solid platforms for muscle attachment and so better function, and also teeth and claws, but also mineralised body covering to protect against teeth and claws. Also mineralised body covering means that growth can only occur by shedding the old cover = moults = 1 animal could leave multiple copies of itself.

    And that’s just a few. It’s a case of right time, right place, right biochemical pathways.

  17. #17 Tatarize
    November 21, 2006

    Come now, everybody know it was the evolution of Hox genes, and development of the homeobox (my personal pet theory/probably wrong).

    The point is, there is a huge debate here… and nobody knows why evolution suddenly kicked into overdrive. So clearly, God did it. That’s the core of of Creation Science, find a point where there is some minor disagreement and toss in some God-did-its.

  18. #18 Rey Fox
    November 22, 2006

    “I’ll take the awe of understanding over the awe of ignorance any time.” -Douglas Adams

  19. #19 BC
    November 22, 2006

    So, did you finish the book, how was the rest of it? (I ask this only because this post is a repost from August 14.)

  20. #20 Lynne
    November 22, 2006

    Creationists are actually trying to co-opt Burgess Shale? One should have to mount a defense? I hate to see someone assume they find a book dense because they’re less competent than the author. Some authors write better than others, but then some have better pictures, so we look at everything we can get our hands on. Upside down hallucinogenic critters. Clusters of cells. Happy Edicaria!

  21. #21 MartinC
    November 22, 2006

    Its not just the creationists that have the wrong perspective on this matter though. Reading a lot of pro-evolutionary material you could easily get the impression that evolution BEGAN during or just prior to the Cambrian. Presumably this is simply carryover from the time practically all evolutionary evidence was in the form of fossils – and there just arent many from before this time.
    In reality the vast majority of evolutionary change occurred in the three billion year period prior to the Cambrian. More than three quarters of this time was spent evolving the cell, with all the rest – multicellular life, plants animas etc taking up only a small fraction of this time.

  22. #22 GuLi
    November 22, 2006

    Now come on, just _look_ – well, you have to zoom in quite a
    bit, but it’s obvious from the diagram: the Cambrian explosion
    happened on Nov. 22nd, 543 752 109 B.C.
    Oh, my – it’s today!

    Merry Nemakit-Daldynianmas, y’all!

  23. #23 Alex
    November 22, 2006

    Parker’s book comes strongly recommended from me, but I seem to remember reading that his theory has been criticised fairly heavily since then. (Would PZ perhaps consider giving us his assessment?)

  24. #24 Daephex
    November 22, 2006

    This is very interesting. I wish I knew where you were getting your fundamentalists, though. The ones that show up at MY door couldn’t attempt to put together an argument based on any sort of Cambrian anything. I get stuck with the ones who think that just because I don’t know how the universe got started, that they have won some sort of victory. I always have to say something lame, like… “well, I hope scientists can explain that someday, I know they’re working on it.” Does anyone have anything better than this? I only ask because I do my civic duty– I keep the door-to-door people at my house as long as possible– I figure I’m helping more gullible folks who may live down the road.

  25. #25 Pete Dunkelberg
    November 22, 2006

    As Chris Nedin said, the causes of the so-called “Cambrian Explosion” were actually the coming together of a number of factors which all worked together. (slightly modified) Here’s one more indication of the complex combination of factors.

    Parker’s book: good as a *part* of the picture, but hardly the whole cause; you might consider it an effect as much as a cause – or a factor which kept things going. Newer book: _Out of Thin Air_, only partly on the Cambrian, which stresses changes in atmospheric oxygen. Oxygen levels increased in the Cambrian, although only to a fraction of the current level.

  26. #26 whomever1
    November 22, 2006

    I have a vague question (prob. inspired by fuzzy thinking), but is there any sense to the idea that the “Cambrian Explosion” might be the result of the evolution of an ecology? I have this thought that at some date, the beasties stopped bumbling around engulfing whatever organic material they encountered, and became specialists, starting up the “evolutionary armsrace” referred to above. But also, ecological niches were established and filled, local environments could become stable and homey, and creatures could begin not only adapting to their local environment, but also adapting their local environment to them. Or to get all new-agey and annoying to my rational friends here, at what date was Gaia born?

  27. #27 Torbjörn Larsson
    November 22, 2006

    More than three quarters of this time was spent evolving the cell, with all the rest – multicellular life, plants animas etc taking up only a small fraction of this time.

    And, I assume, evolving photosynthesis, building up an appreciable oxygen atmosphere, and learning to live and love the poisonous stuff called oxygen, like Dennis said.

    Which BTW was part of one of the Gaia hypotheses concerning this. The details are vague to me at the moment, but I believe they thought increased oxygen content of near surface minerals made them more malleable and thinned the crust enough to get plate tectonics going in a selfsustaining process. (Explaining some of Mars and possibly Venus poor plate tectonics BTW.) Increased continental drift and evolutionary forcing climate changes would fit with the snowball earth explanation too. I wonder what become of that Gaia hypotheses? Most likely the mineral properties wasn’t as they thought.

    There is no shortage of hypotheses. Another reason this is fascinating of course.

    a circular instead of a flat body plan

    Nitpick: Changing dimensions between comparisons is confusing – the first moments I couldn’t understand why a flat body plan couldn’t be circular. Circular – linear or cylindrical – flat cuts right to a visualization, I think.

    improved biochemical pathways

    I assume one of the left out pathway causes may be that the increased collection of pathways should have included more various use and handling of minerals, involved in increasing the ability (and need) to excrete and mineralize preferentially. That generic idea has a lot going for itself.

  28. #28 Torbjörn Larsson
    November 22, 2006

    Note to PZ, if you are reading this and want to improve the blog format, among the other thousands of things you do:

    The blockquote seems to be mistreated by scienceblogs scripts. At least in my browser (Firefox) the next paragraph gets the same text style if all you do is a line break. (Which the scripts formatting encourage but isn’t proper HTML format otherwise.)

    And now, excuse me while I test a fix for mine own quotes:

    Test quote

    This is a text with only line break.

    Test quote

    This is a text with space before line break.

    Test quote

    This is a text with HTML line break before line break.

  29. #29 Torbjörn Larsson
    November 22, 2006

    Curious. The blockquote scripts seems to be severely buggy. Okay, never feed the trolls.. eh, bugs. An empty line between quote and text should work.

    I get stuck with the ones who think that just because I don’t know how the universe got started, that they have won some sort of victory. I always have to say something lame, like… “well, I hope scientists can explain that someday, I know they’re working on it.” Does anyone have anything better than this?

    OT here, but a fair question. The answer depends on what they are after.

    – We do know how the universe started, the accepted bigbang theory points back to a compressed and hot beginning.

    – But they are probably after an explanation for “origin”. Origin is an old philosophic and theological idea based on a flawed conception of causality. “Origin” has no place in science. The proper way to ask the question is “what was the initial conditions”and possibly also “why is the laws of physics as they are”.

    – If they still want answers, both of these questions have answers in some proposed theories. So while we don’t really know yet, we should be pretty confident that we can get an answer. It seems to be questions that science can answer.

    – If they want more specific answers, so much the better, they could be genuinely curious. Here you have to pick some ideas and present them as possibles. I have mine favorites, but for a general and professional view you could look at some astronomy blog. Or we could continue here, so you are not left hanging.

  30. #30 John Wendt
    November 22, 2006

    Valentines’s book is great, although after the general stuff in Part One it’s more a reference, not exactly bedtime reading. I don’t think Valentine is smarter than PZ, it’s just that he’s done a lot of reading.

    Some people think that the “explosion” is an illusion.

    One creationist Web site points out that 10MY is still just an eyeblink in geological time, so the “explosion” is still sudden. No realization that the only relevant time frame is the generation time of the organism. If the organism breeds once a year, that’s 10M generations.

  31. #31 Chris Nedin
    November 22, 2006

    Torbjörn Larsson wrote
    Changing dimensions between comparisons is confusing – the first moments I couldn’t understand why a flat body plan couldn’t be circular. Circular – linear or cylindrical – flat cuts right to a visualization, I think.

    The evolution of the circular cross-sectional body plan was very important and directly relevant to bioturbation. For organisms that passively adsorb oxygen through the surface, having a flat body plan is not barrier to growing large, provided that you remain very thin, and relatively sedate. Oxygen diffuses slowly through tissues and does not penetrate far, so tissues need to be within a few millimetres of the surface. An organism can be over a metre in length provided that it is no more than a few millimetres thick e.g. Dickinsonia. But circular cross-sectional organisms that passively adsorb oxygen cannot grow much more than a few millimetres in diameter because any larger (thicker), and oxygen cannot get to the deeper tissues, also the surface/volume ration problem starts to have an effect.

    This is important because mechanically-speaking, a circular cross-section is a much better body design for burrowers. So, the development of oxygen-carrying fluids and circulatory systems freed up circular cross-sectional organisms to become larger and more active burrowers (more active because passive oxygen diffusion is slow and so not conducive to energetic activities). Hey presto – bioturbation.

    As I wrote somewhere else, burrowing is right up there in the list of brilliant tactical manoeuvres, conferring protection from predators and allowing access to all that buried organic matter.

  32. #32 Torbjörn Larsson
    November 23, 2006

    Chris:
    “The evolution of the circular cross-sectional body plan was very important and directly relevant to bioturbation.”

    I can appreciate that. These diffusion limits are one of the more everyday aspects of biology – one can ask contacts wearer about risks for unwanted blood vessel growth, or why large cancers release growth factors.

    But I was mentioning a confusion that interrupted my reading. After all, a body plan isn’t only a cross section, but “essentially the blueprint for the way the body of an organism is laid out. An organism’s symmetry, its number of body segments and number of limbs are all aspects of its body plan.” ( http://en.wikipedia.org/wiki/Body_plan )

    “burrowing is right up there in the list of brilliant tactical manoeuvres”
    It certainly feels so, under the blankets a cold winter morning. ūüėČ

  33. #33 Dawid Mazurek
    December 12, 2006
  34. #34 Mytho
    January 27, 2008

    Biology is not my fiel of expertise, sometimes it’s a hard subject on it’s own merit, although very interesting nonetheless. After reading this exemplary article, I can only conclude that I’ve missed the best part of nature by neglecting a throughout study of this science in particular.

    Interesting, revealing, marvelous. What else can I say?

  35. #35 Mytho
    January 27, 2008

    Biology is not my fiel of expertise, sometimes it’s a hard subject on it’s own merit, although very interesting nonetheless. After reading this exemplary article, I can only conclude that I’ve missed the best part of nature by neglecting a throughout study of this science in particular.

    Interesting, revealing, marvelous. What else can I say?

  36. #36 Johannes Lochmann
    March 3, 2008

    Another very interesting one of my professors tended to stress is that before there could happen cellular (and multicellular, by that matter) evolution a phase of chemical and molecular evulutionary phase was required. So his guess was that at least 2 of the 3 billion years before the first known fossil traces where certainly free of cellular life and filled with all sorts of molecules engaged in strictly darwinian evolution in the sense of “how do I protect myself from reacting with my fellow molecular comrades and, after some time, that evil oxygen thingy!”, as he used to put it.

New comments have been temporarily disabled. Please check back soon.