Hundreds of millions of years before our Miocene primate ancestors scampered through the rain forest canopies, before the dinosaur stars of Jurassic Park really walked the earth, before the mammal-like synapsid reptiles of the Permian clunked across the land, before the Devonian Sharks ruled the sea, before the Ordovician and Silurian squid-like Nautiloids ruthlessly preyed on our primitive vertebrate ancestors and cousins, there was an alien water-world full of horrors; and it was right here on Earth! The ocean was rife with bizarre animals and plants, and 'critters' that defy classification. Geologists call this time The Cambrian Period. Creationists call it a miracle.
We might have recognized the Cambrian as our home planet long ago, just barely. For the earth was a very different world then. The land was virtually barren, scorched by an unfiltered sun, and whipped by desert winds clocking in at over 100 MPH. But the warm seas were brimming with alien looking marine life, weirder than any sci-fi movie monsters dreamed up in Hollywood. Terrifying super-predators, bristling with pincers, sporting nasty mechanical grinding mouths, whip tentacles, and armored with chitinous exoskeletons, roamed in the thousands, among clouds of photosynthetic and chemosynthetic bacteria.
It would have been a sight to behold, as those ghastly creatures weaved in and out between the stalks of sponges, brachiopods, and incongruous Rangeomorphs, which formed the primeval oceanic forest, looking for prey, and trying to avoid being someone else's dinner. A patient, sharp-eyed, time-traveler might have even caught a glimpse of a drab, reclusive creature, hiding nervously in the mud. It was a worm-like marine organism, the first known which featured an odd, stiff sliver of tissue, running the length of it's body, just underneath the dorsal surface. The unassuming Pikiaia, or something like it, would eventually give rise to all vertebrates; lampreys, sharks, bony fish, eels, salamanders, lizards, Dimetrodons, raptors, snakes, triceratops, birds, whales, hippos, dogs, and of course, humans.
The Cambrian was once thought to represent a definitive boundary between the Proterozoic Eon, (Simple one-celled creatures), and the first quasi-modern ecosystem, composed of the familiar array of complex, multi-cellular animals and plants, predators and prey, much more like what we see in the oceans today.
For over a century in classical paleogeology, Cambrian bugs and snails and early Cnidarians, seemed as though they popped up out of nowhere in the fossil record, about 540 million years ago. This geologic landmark was thus christened the Cambrian Explosion. Creationists of all flavors have long used the explosion as evidence for their views. Both by claiming the cause of the Cambrian Explosion is unknown (More or less correct), that it cannot be explained by science (Highly speculative), and that it is evidence for Creationism (Incorrect). Recent finds have shown that the event wasn't nearly so sudden as once thought. And trace fossil evidence of animals preceding the Cambrian by tens of millions of years have now been found in relative abundance. But, the majority of the solid, irrefutably multi-cellular fossil evidence does lie within roughly twenty-million years forming the classic Cambrian Explosion itself. How do evolutionary scientists and paleobiologists explain this event?
There are several candidate possibilities, a few of which I'll briefly review, with the caveat that it could be a confluence of any or all of them, or something we don't know about at all may be solely or greatly responsible. And I apologize to the geologists and biologists in advance for no doubt simplifying and/or garbling any of the geology and biology below. PLease feel free to weigh in with corrections or opinions.
The SnowBall Earth Theory- The entire planet, from poles to equator, was locked in sheets of ice right before the Cambrian Explosion. The ice started advancing towards the tropics about 600 million years ago, and ended with a global melt about 550 million years ago. The glaciers were thin and translucent enough in the tropics, and/or broken up in some places from tidal action and wave mechanics, for photosynthesis to continue in a few scattered spots around the planet. And biomes were also preserved around thermal vents and in the vicinity of cold hydrocarbon and hydrosulfide seeps, just as they are today. After the ice ruled for about 10 to 20 million years, the CO2 would have built up in the atmosphere from volcanic outgassing to a critical threshold. In the absence of liquid water precipitation, there was no process to clean the CO2 out. This eventually led to a greenhouse tipping point, raising the global temperature well above 0 C. And when that ice started melting in the heat, models show the melting process would have been rapid, taking perhaps as little as 100 years to complete the process. As the heat worked into the ocean, it would have produced hyper hurricanes and enormous mega-typhoons, unlike anything we see today in scope and severity. The weather would have gone crazy. Storms could have lasted for centuries. Photosynthetic cyano-bacteria rapidly radiated into the now vast swaths of open water and into new coastal shallows which were free of thick ice, and carried the entire ecosystem along with it, producing a burst of evolutionary adaptation we see in the record as the Cambrian Explosion. There is good evidence the earth did experience vast glaciation at this time followed by a rapid melt in which CO2 was taken out of the apmosphere and fixed in huge quantities into the strata being laid down just before the Cambrian. I'd give this explanation a star.
Sampling Bias-We've only a few pages out of the volumes of time covering the periods in question. All early life was in the ocean, or so we believe. Oceanic strata from the Cambrian and before is extremely rare due to the nature of Plate Tectonics. Fresh sea floor spreads from the mid-ocean ridge which encircles our planet like the seams on a baseball. New material extrudes from the ridge on a virtual conveyor belt, spreads away until it hits the boundary of a continent over the course of a few hundred million years, at which point the heavier oceanic basaltic crust usually subducts beneath the lighter granitic continental rock, and disappears back into the hot, molten, mantle. Only rarely does the proper kind of fossil bearing sediments from that long ago get uplifted and preserved intact. Rarer still is it from the right time and the right kind of place where ancient life was thriving. And rarest of all is it found conveniently on or near the surface, and possible to access. Most of our classical, detailed, knowledge of the earliest complex biota comes from one, small, lode of Cambrian rock, located in the Canadian Rockies called the Burgess Shale. The Burgess is miracle of luck and geology. It contains incredible, photo like quality fossils, showing exquisite soft tissue details or Cambrian life. This layer of Shale is a treasure. If not for the Burgess, we would know much less about Cambrian life. I'd also give this explanation some serious weight, because it works hand in hand with the SnowBall Earth theory. IOW, the snowball idea predicts that fossil life would be rare during the glaciation because whatever was there was restricted and thus less likely to be found preserved today.
The Biology Theory 1- Predation evolved for the first time just before the Cambrian, setting off an evolutionary arms race between predators, prey, and scavengers.
The Biology Theory 2- Complex organisms did exist in quantity tens or maybe even hundreds of millions of years prior to the Cambrian, but it was mostly soft bodied, worm like, or jellyfish like, critters, which left poor fossils. There is solid fossil evidence for such creatures prior to the Cambrian, although limited to 50 to 100 myb (Million years before). I'd have to say this remains a possibility. Absence of evidence is not evidence of absence.
The Biology Theory 3- Sexual reproduction became widespread. And sex provided a great deal of variety for evolution to act on. Biodiversity took off like a rocket (No sexual, phallic puns intended, you sick freaks). My guess is that sex is significantly older than the Cambrian, but maybe something changed
The Intelligent Design Hypothesis-God/Allah/Odin/Cthuhulu/Aliens created all phyla during the Cambrian and let it evolve from there, using unknown methods, for unknown reasons, which in fact cannot ever be known. (Actually I just threw that one in as piece of raw meat in gratuity for the hordes of furious fundies (Furious fundie, hell of a name for a drink and a taxa?). I've been assured by several relatively anyonymous e-mails from time to time that I'm being 'watched', and one of which even threatened that they're going to do to me 'what they did to Dan Rather'. Heh, I'm a big shot Ma, a big shot!)
My own wild speculations - Sampling bias and a global bottleneck from the snowball earth conditions prior to the Cambrian prevents us from seeing the diveristy of earlier life. And the bio theory 2 also plays a role. The Cambrian was merely the earliest evolutionary radiation, following the earliest global extinction event, we've been able to discern with confidence in the fossil record. In extinction events, it's the larger, specialized, longer living organisms that usually go first. Poor sample availability prevents us from knowing of the earlier life in great detail.
And there was plenty of time before the Cambrian, billions of years in fact, almost ten times as long as the interval between that primitive worm-like chordate and humans, for all kinds of even weirder and more alien looking life to have existed on earth. I can imagine a period prior to the Cambrian in which giant precursors to worms the size of whales roamed the seas, filtering bacteria. And enormous primitive jellyfish the size of small islands floated in the ocean, with millions of tentacles lined with stomachs, hanging down into the water snaring bacteria and smaller prey. A nice crusty layer of fried skin on top protecting the critter from the blistering UV. There is no evidence for it, but I can envision it as a possibility. (The Darwinian Mafia is probably going to bust down my door soon for that lapse of discipline. Although, you never know when ... no one expects the Darwinian Mafia!)
Go back another few hundred million years, through an even earlier hypothetical extinction event or two. There could have been huge conglomerations of microbial colonies, along the lines of modern siphonophores, that stretched for miles. Go back about another billion years or so, perhaps 'microbes', single celled creatures, the size of pinto beans or footballs, in a bewildering variety of shapes and sizes, were flitterring through the ancient ocean powered by rows of whirring cilia?
Who knows. Enormous worms, jellyfish, siphonophores, and even giant one celled creatures, would have left almost no fossil traces, even if we had lots of strata to examine, which we don't. And even if we did, would we recognize the imprint of a primitive worm the size of a killer whale in a fossil bed as an early annelid? Or a blobbish walnut-sized indentation in a piece of rock from one billion years ago as a single, one-celled, giant amoebae? I doubt it.
For that matter, the evidence we have that life remained at the single cell or simple bacterial level for 3.5 billion years may be misleading and incomplete. The fossil evidence through that vast expanse of time is meager. So we almost certainly are missing a lot. For all we know, large, robust ecosystems full of exotic, complex, and even more alien biota than the ghastly creatures that roamed the ancient Cambrian seas, could have flourished on a radically different planet earth, two or three billion years ago. Lots of really unusual and massive extinction events can happen in that length of time. Maybe a nearby super nova cut loose, or a giant Gamma Ray burst lit up a couple of hundred light years away, or a rogue Kuiper Belt Object ten times the size of Pluto crashed into the sun and set off titanic solar flares lasting years; any or all of which would have cooked off half the atmosphere, boiled the surface of the ocean, and left only bacterial slime as the lone survivors, shielded from the apocalypse in the cool dark depths near Black Smokers and methane seeps. Slime, which would carry the evolutionary torch for new, future lineage's that would develop, again, into a new round of complex life forms.
Or just the development of photosynthetic plants unleashing oxygen into the air, a corrosive poison, we know to much of the planet's early life, may have wreaked havoc, killing much of the ancient bacteria that couldn't adapt to the new toxin. And any ecosystems that that bacteria supported would have crashed into oblivion.
The earth of two-three billion years ago would have been unrecognizable. The planet spun almost twice as fast as it now spins. The moon was about 100,000 miles away. The tectonic plates would have been skidding around on roller skates compared to their present stately movement, over a thinner crust and on a much hotter and broiling surface of lava, and the sun would have been about 20% dimmer than we see it now ...
Fossils are precious; if not for a quirk of geochemistry, we wouldn't have them at all and we'd be limited to only extant organisms to puzzle out the past; think how much we would not know! But fossils formed even under the most ideal conditions in the best mineral matrix possible can be lost over time. They're not immune to the endless, relentless, effects of geology. They can be eroded into dust when exposed. They can be smoothed right out of the strata by heat as easily as you press a shirt flat with a hot iron. Fossils can be crushed into an unrecognizable, jumbled set of wrinkles by pressure. Or they can be chemically etched away by many processes, and literally fade from existence without even leaving their rocky tomb. And of couse it's important to underscore that the vast bulk of oceanic crust get recycled into the mantle of the earth. There may be some layer of strata being unearthed right now bearing evidence that touches on some of these science fiction musings above. So who knows how crazy my speculations are (They're less crazy than the Creationists). Who knows what kind of strange creature's remains are still locked away, hidden in stone graves for a billion years or more, waiting to be brought to light and tell us of their existence.
Leaving the realm of speculation and returning to the confines of science, the answer to the creationists claim that the Cambrian is not explained in detail by science, confidently and in consensus, is correct. The claim there are no candidate explanation is false. It's likely going to be a confluence of phenomena and we will likely never know for sure the precise combination. The claim it's not explainable by science is unsupported and, when you consider the implications of it, awfully extraordinary; and the claim the Cambrian Explosion is evidence for any alternative without supporting evidence for that alternative is a clear logical fallacy.
Why do I maintain it's plausibly explainable by science? The one fact we have learned about life on this little ball of mineral and metal we call the earth, the one thing we know from experience we can count on; life is very tenacious, it's incredibly adaptable, and it exceeds the scope of our wildest imagination, every chance it gets. This is true of life both now and in the remote past. It's therefore plauisble to extrapoloate those qualities of life into the Cambrian and before. And looking forward, if the past is any guide, the future will be wild.
| Tell a Friend About this Article |
