PZ Myers. 2005 Aug 14. The Cambrian as an evolutionary exemplar. <http://pharyngula.org/index/weblog/the_cambrian_as_an_evolutionary_exemplar/>. Accessed 2008 Aug 21.
Posted on M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr on Sunday, August 14, 2005
The Cambrian as an evolutionary exemplar
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.
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.
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.
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.
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.
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- Excellent post. Really, really good stuff.
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... 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.
For contrast, here's what someone wrote about a different author and book
The sort of popular science writing that makes the reader feel like a genius
#: Posted by on 08/14 at 12:40 PM -
Delightful post, PZ. Nicest condensed description of the Cambrian explosion I've seen.
#: Posted by on 08/14 at 12:46 PM
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What is so surprising to me is that ID-creationists are using Valentine to support their claims about the Cambrian, despite Valentine's latest book.
#: Posted by on 08/14 at 01:12 PM
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Wonderful. This makes me want to get the book....
Last I read about the whole thing was "Wonderful Life" so an update is needed. -
PZ, your Amazon link does not work. It looks like you forgot to put in the ISBN.
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Anti-spam: Replace "user" with "harlequin2"#: Posted by on 08/14 at 01:34 PM -
It's not quite as readable as Wonderful Life.
And it's obvious why creationists don't cite it: it's got actual evidence, and none of it supports their nonsense. -
I am convinced that James W. Valentine is ten times smarter than I am and knows ten thousand times as much
Me
Z is as PZ:Valentine.
I don't think I'll be adding that one to my Amazon list. So, thanks for summarizing it so well, PZ!#: Posted by on 08/14 at 02:20 PM -
Blech. Damn smiley generator. That first line should read:
Me : PZ is as PZ : Valentine.
(Note to self: always preview first.)#: Posted by on 08/14 at 02:22 PM -
Fascinating, wonderful, excellent! Yes, I've read Gould's "Wonderful Life", too, but so long ago I can't remember the details. I occasionally read such popular science books as appear - have read Niles Eldredge's "Life Pulse", and must read Dawkins' "The Ancestor's Tale". Wouldn't a faithful popularisation of the subject of this particular episode (600 - 500 MY ago) from early life on Earth make a fascinating popular science book or a BBC Horizon programme - if it's not been done already? (USA-ians read "WGBH Boston Nova show" for that last!)
It's rather obvious that the evidence collected by Valentine and other scientists over the world gives the creationists a hard time - what, no fossils of the 'dog kind', the 'ape kind', or even the 'fish kind' in any Cambrian deposit!? I don't know the variety of 'Intelligent Design creationists' well enough to know how or if they attempt explain away this evidence, but if one insists upon a creator, one presumably has to explain why he/she/his noodly eminence apparently tinkered with precursor designs first, before finally 'getting it right'.
A matter of curiousity - who first used the term "explosion" for what went on in the later Cambrian? Gould writes on the first page of chapter 1 of "Wonderful Life" (in my edition) 'Modern multicellular animals make their first uncontested appearance in the fossil record some 570 million years ago - and with a bang, not a protracted crescendo. This "Cambrian Explosion" marks the advent (at least into direct evidence) of virtually all major groups of modern animals ...' Who was Gould quoting, or did he originate the phrase?#: Posted by on 08/14 at 03:34 PM -
The evolution of the canonical Cambrian forms was drawn out over tens of millions of years.
this is another place where i think much of the creationist critique comes from an inadequate appreciation of quantitative techniques and just plain-old number sense. a million years is a really long time. how long? we can't grasp it without putting it in relation to things we know about.
the paleomagnetists say in a million years each and every square meter of ground on the surface of Earth has been struck by lightning once.
a million years is long enough for the tectonic motion of the North American plate to carry it 20 some-odd miles southwest. 10 million years is long enough to carry it more than 200 miles, something that would be noticed (caution: 20 MB download!).
a million years is long enough for erosion to lop off 1500 feet from rock outcrops.
a million seconds is 11.6 days. -
PZ,
If you have a, "...miniscule brain pan.", Thhen I am in big trouble!. But the point that is clear is that evolution is a very complex puzzle. With its emense time sequences and catalog of geological, biological and catastrophic events that shape species it is a challenge to get ones brain around it. And that is where the easy fall back to it has to be "created" seems so attractive.
As I have read the dialog here and PT, I am now crystal clear that standards based, high stakes test driven science cirriculum is killing the teaching of science as a process verses as a list of facts, figures and trivia. Students are not learning how science functions as a process of constant questioning, analyzing and concluding. Rather we are getting children ready to latch onto any handle that has the correct combination of identifiers, e.g. penis enlargment or scientific based creation.#: Posted by on 08/14 at 04:43 PM -
The idiocy continues
he geological period known as the Cambrian is marked by the rather sudden appearance of all the basic forms of animals now in existence, sort of a biological Big Bang. There are no transitional forms between them, and no new basic forms have appeared since then.
#: Posted by on 08/14 at 05:09 PM - ooh, good post. thank you very much for that illustration (verbally and graphically).
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"I've read Gould's "Wonderful Life", too, but so long ago I can't remember the details"
The thing I remember most; (and mostly a paraphrase)is as he cleans up another fossil: "Oh fuck, another new species!"#: Posted by on 08/14 at 07:21 PM -
Me : PZ is as PZ : Valentine.
creationist : libertarian : sea urchin : Me : NelC : PZ : Valentine#: Posted by Chris Clarke on 08/14 at 07:46 PM -
It's just amazing what the process of science can figure out. Just stunning.
Did you hear about Harvard's OOL announcement?#: Posted by on 08/14 at 09:43 PM -
Oh goody a chance to get stuck into PZ - and they don't come around too often!
Firstly, there's a good chance that some of the early Cambrian trace fossils were made by arthropods - we just don't see them as fossils because they had no hard parts at the time.
Secondly, regarding the small shelly fossils, "The kings of creation at this time were scattered beasties the size of a baby's toenail" Actually a significant proportion of small shellys were individual pieces of 'armour' covering larger organisms like chain mail. They were the first attempt at what would become a complete covering by means of either 'scales' (eg Wywaxia), or permineralisation of the outer part of the body (trilobites).
Thirdly, "It's more than ten million years later that the spectacular and strange animals of the Burgess Shale make their appearance" Make their appearance? Make their appearance!!! Sloppy PZ, sloppy. It's things like this that give creationists ammunition. 'Make their appearance' IN THE FOSSIL RECORD. A very important distinction. The reason for the appearence of the Burgess Shale fauna is the exceptional preservation at the site, preserving soft bodied organisms that would not normally be preserved, and NOT, repeat NOT their first appearence on Earth.
Besides a number of forms from the Burgess Shale are also found at the geologically older sites of Emu Bay and Chengjiang. Why? because these sites are also site of exceptional preservation. The Burgess Shale fauna is a routine, ordinary assemblage, most elements of which range through the Cambrian, made exceptional by the fact that the majority soft bodied forms are usually not preserved.
Professor Steve Steve (currently residing in Canberra) is not please PZ. But he forgives you#: Posted by on 08/15 at 02:14 AM -
You evilutionists are just so mean.
Yeah, I'm greatly abbreviating Valentine's argument. Those wonderful lagerstatten do mean the record is patchy, but he is saying that what we know of the distribution of fossils supports the idea of gradual (over tens of millions of years) change, that is still nonetheless a geologically rapid event.
I assumed that since I was talking about the fossil record, everyone would understand that that's where the 'appearance' was. But you are right, you can't make even reasonable assumptions when dealing with creationists. -
With some modifications, this would make a good Talk.Origins FAQ.
#: Posted by on 08/15 at 07:25 AM
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Harvard to Investigate Origins of Life
The team of researchers will receive $1 million in funding annually from Harvard over the next few years. The project begins with an admission that some mysteries about life's origins cannot be explained.
"My expectation is that we will be able to reduce this to a very simple series of logical events that could have taken place with no divine intervention," said David R. Liu, a professor of chemistry and chemical biology at Harvard.
The "Origins of Life in the Universe Initiative" is still in its early stages, scientists told the Boston Sunday Globe. Harvard has told the research team to make plans for adding faculty members and a collection of multimillion-dollar facilities...
#: Posted by on 08/15 at 08:21 AM -
Campbell and Meyer didn't get the memo
Some scientists think the fossil record challenges the Darwinian idea that all organisms share a common ancestor. Events such as the "Cambrian explosion" show that new forms of life appear suddenly in the fossil record without evidence of connection to earlier forms — contradicting Darwin's picture of the history of life as a fully-connected branching tree.
#: Posted by on 08/15 at 02:55 PM -
Re: Campbell and Meyer didn't get the memo
Some scientists think the fossil record challenges the Darwinian idea that all organisms share a common ancestor. Events such as the "Cambrian explosion" show that new forms of life appear suddenly in the fossil record without evidence of connection to earlier forms — contradicting Darwin's picture of the history of life as a fully-connected branching tree.
Yep. They state,
Events such as the "Cambrian explosion" show that new forms of life appear suddenly in the fossil record without evidence of connection to earlier forms — contradicting Darwin's picture of the history of life as a fully-connected branching tree.
Evolution: Debate it
By John Angus Campbell and Stephen C. Meyer
Posted 8/14/2005 8:48 PM
http://www.usatoday.com/news/opinion/editorials/2005-08-14-evolution-debate_x.htm
However, in the same issue of USA Today, we find a brief analysis of the history of creationist movement in the US and a critique of ID's claim to the right for equal time in the classroom which is very well written and to the point:
Evolution: Just teach it
By Eugenie C. Scott and Glenn Branch
Posted 8/14/2005 8:51 PM
http://www.usatoday.com/news/opinion/editorials/2005-08-14-evolution-teach_x.htm -
I'm recalling an essay by Stephen Jay Gould, titled "Is the Cambrian Explosion a Sigmoid Fraud?"
I'll have to look that up when I get home.#: Posted by Karl Lembke on 08/17 at 09:33 AM -
Thanks PZ,
You have provided much light on a difficult subject.
If I were an ID proponent, and seeing all this evidence, I would begin paying more attention to [the intelligent design of] the building blocks of life: the unique physical properties of water and the amazing chemistry of hydrogen, oxygen, carbon and phosphorus. Ah, but followers of ID can't do that because their ideas are not based on science, they're based on religion.#: Posted by on 08/18 at 03:39 AM