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.
August 14th, 2005 at 2:44 pm
While perhaps peripheral to this posting, your noting that geologic time is very different from human time, even after years of geology in university (during which time for convenience I discounted relative geologic time, the actual enormity of the years involved didn’t really become “real” to me until I was doing differential thermal analysis of some Devonian clays, some years later.
As the temperature in the microfurnace containing the sample increase, the clay began to boil off the molecular water, which started spitting out of the vent at the top of the furnace. I suddently realized that I was seeing water which had been chemically bound on the order of 350 million years.
The lesson of geologic time had now been graphically demonstrated, for me at least!
August 14th, 2005 at 7:18 pm
In regard to creationists and IDers playing the “Cambrian card,” I had the same experience as a volunteer for the ACLU in Harrisburg, PA for bill 1007 — the “Intelligent Design” bill.
One of the state reps–in writing, mind you, as well as verbally–offered the “proof” that the Cambrian explosition refuted evolution.
For your amusment, the details–and the reply I prepared–here @ Yet another H 1007 story: Intelligent Design, Evolution and Science Education in Pennsylvania.
August 14th, 2005 at 8:08 pm
There you go again, using facts to explain reality. You must realize that facts have no bearing on this.
You want people who refuse to accurately remember the last three decades to conceptualize millions of years.
When you work in cryptology you realize that there millions of possible combinations that have to be tested to “break” the code, but you break the code because, with enough time, you can test all of those possibilities.
From a background in cryptology, if you look at what it takes to create DNA, you notice that the process accelerates at the end, with each intervening step having fewer possibilities than the step before. I think that is what this time line represents - the long build up to assemble the building blocks, but once the blocks are in place the process speeds up because the critical mass of biological material has been achieved.
August 14th, 2005 at 10:30 pm
it’s weird to me that evolutionists can’t see their own leaps of faith. too little philosophy in public schools, methinks.
how do you KNOW that all the stuff in that diagram is true, and that it all fits together to equal evolution?
frankly, you don’t know. you believe. that’s okay, but stop looking down on other people’s beliefs, then.
August 15th, 2005 at 3:13 am
JAMI - the ID folks are using an almost nihilistic skepticism to obliterate the distinction between faith and facts because they want a total overlap of science and theology:
http://radamisto.blogspot.com/2005/08/radio-tidbits_14.html
August 15th, 2005 at 6:00 am
It’s not a matter of belief. Buy the book; it’s almost 600 pages of detail, detail, detail. We’re talking about thousands of puzzle pieces brought together, all fitting together nicely, all assembled into a reasonable picture. There are still lots of missing pieces, but we’ve got enough to see the broad outlines…and the story it reveals is evolution, and it contradicts biblical accounts.
I don’t look down on people’s beliefs, but I do look down on their ignorance.
August 15th, 2005 at 7:08 am
PZ, your response to Jami is perfect, but you make one assumption about her buying the book. She’d actually try and understand it, or it’s underpinning in the science of Geology. Much, much easier to say “God is Great, God is Good, We thank him for our Earth…[poof], see, here it is!”.
I have not met an ID person/Creationist yet who can explain radioactive decay without resorting to “the devil does it” or “God has a plan”. Fundamentally obsevable phenomena like K40-Ar40 decay are so far outside their box, that it’s a real “head exploder” because it works back to when the Earth was cooling and those elements were literally “set in stone”.
Besides, given the vengengfullness and general all-around wrathful nature of their God (see: Moses fleeing pharoah; Noah, Adam and Eve being tossed out of Eden), wouldn’t said deity be more than a little pissed off about the state of the planet, I mean if we can fuck it up so bad in just 10,000 years, in 10,000 more it’ll be an intergalactic landfill or something. — Dumping on North America on alternate Sundays and Tuesdays, East Asia Wednesdays and Fridays, Africa and South America are open Tuesday thru Saturday every week, radioactive waste can be dumped in Greenland by permit only. No dumping in Australia, don’t want to hurt no kangaroos. (apologies to Randy Newman)
August 15th, 2005 at 7:51 am
Yeah, we believe in evolution - just like we believe there is a universe out there that is knowable by sensory phenomena. Sometimes senses enhanced by tools.
The fact of the matter is, those who consider observation the basis of knowledge and who examine the evidence in sufficient detail, almost invariably come to very similar conclusions about the origins of species. Darwin’s name is on the theory, but the same conclusions have been reached by quite a few other scientists, not all of whom liked or respected Darwin. Nobody who understands science takes anybody’s word on faith - the smartest and most honest scientist in the history of the world could be wrong. It’s all about verification. Huge amounts of new evidence - thousands of fossils, thousands of newly discovered living species - have been observed since Darwin, and all of it fits the basic framework. Not a single plant or animal has been discovered, living or hinted at by traces in rock, that does not fit somewhere in the tree, that does not fit the timeline.
There is only one leap of faith involved:
That there is a real universe, knowable through observation of sensory phenomena. In other words, that this is not all a dream.
There is one part of that leap which gives some people trouble: “knowable through observation of sensory phenomena”. Some people consider other sources (e. g. the Bible or Chairman Mau) more reliable than observation.
But the game of science is to rely entirely on phenomena observed through the senses. Science doesn’t even really make that one leap of faith, so even if one of the Genesis accounts is true, or the earth is, in fact, part of the corpse of a giant described in a Norse edda, or an illusion as described in certain Eastern traditions and the Matrix movie, the game of science is to figure out explanations of observed phenomena based on the observable evidence. Just as a lawyer in an American courtroom can’t use Japanese laws even if he believes those laws are better, a scientist can’t use anything but observable phenomena to explain phenomena. If it is all a dream, science is about how the dream works. If God created an old world recently, the job of science is to read the story God wrote in the stones.