Yesterdays science post on
UTI concerned the Cambrian Explosion and some possible solutions for it. But there was a time over 600 million years ago, just before the Cambrian, when the world was racked by environmental upheavals the likes of which have not been seen since. Organisms even more alien than our Cambrian friends were scattered across a weird seascape; life so strange that we cannot even classify it! This ancient era is known as
The Ediacaran Period.
Large Graphics Below!!!
At the start of the Ediacaran, about 640 million years ago, the earth's surface was encased in ice. Vast glaciers covered both land and sea, from the poles to the equator forming an endless frozen tableau under a crystal clear sky. The Snowball Earth represented a victory of sorts for cyano-bacteria over their methane emitting peers. As the blue-green algae flourished, they released a deadly new, corrosive poisonous gas which burnt the methanogens to a crisp where they lay. That poison was called, oxygen. When the population of methane producing microbes crashed, the atmospheric methane waste gas could not be replenished. Existing methane was taken out of the atmosphere over the next few millions years, the greenhouse effects it provided diminished, and the earth froze solid. Waves of glaciers marched down from the poles to the equator time and time again. Understandably, this time is called the Cryogenian. (The chemosynthetic methenogens would survive, and they can still be found today near black smokers and hydrosulfide seeps in the deep ocean, safely away from sunlight and the inevitable toxin of oxygen produced by photosynthetic bacteria.)
There was no rain 650 million years ago (MYA). The life giving atmospheric water vapor we take for granted lay frozen on the surface. Perhaps hardy bacterial psychrophiles thrived in the frigid wasteland. Here and there swaths of open water would wax and wane in cracks, dancing in resonance to the tidal influences of the moon and sun offering an oasis for evolution to work its magic; but this was a barren ice-world compared to our lovely blue-green planet today. This depressing state may have persisted with brief respites for 200 million years or more, starting over 800 mya.
But change was literally in the air. Volcanic outgassing delivered stores of greenhouse gases to the primeval atmosphere. And with no precipitation to clean them out, they would build, and build, the earth would grow warmer, until a tipping point was reached ... and the ice relinquished our mother earth from its unforgiving embrace. This may have happened several times during the Cryogenian. The final melt, around 640 mya, was probably quite sudden. Models show the ice would have disappeared in 100 years or less. The sudden gush of water vapor evaporating off the now exposed ocean would have acted as another greenhouse gas and the temperature could have risen ten degrees or more during that brief melt. The resultant change in weather would have been titanic. Great hyper-canes and super-typhoons would have raged across the global ocean for millennia.
By the time it was over, the earth's surface would be free of ice from pole to pole. On land, there may have been a crust of blue-green algae here and there near shores and tidal pools, but the real show would have been underwater, in the new, warm Ediacaran Seas. For there the first known multicellular creatures, what we bio geeks call 'metazoa' would shake off the vestige of cold, and invent a new world. These were probably early Cnidaria (Corals and Jellies). And they can only be described as bizarre.
The long plant-like objects may look like fern fronds but they're not. Ferns are still hundreds of millions of years in the future. These strange leaf-like critters are possibly colonial organisms made of smaller polyps (Polyps are about as simple as an animal can get. It's basically a tube. The inside of the tube is the 'gut', one end is the 'mouth' and the other is the 'anus'). They're grouped under the designation Rangeomorphs and may be distantly related to modern day corals and jellyfish (Cnidaria).
The oldest known metazoan fossils (Top) are ring like impressions dated to 600 million years ago and may represent the anchor points of early rangeomorph stalks. Or they could be primitive versions of later Ediacaran biota such as the strange Kimberella reconstructed in the middle. Other fossils from 590 million years ago are trace fossils such as the one on the bottom. The latter has been tentatively identified as the tracks of a worm.
Spriggina, 570 MYA. One of the first known bilaterally symmetrical critters. This ancient beastie may have been a close cousin to every snail, worm, insect, and vertebrate, alive today.
Another bilateral critter, Dickinsonia, dated to 560 mya. Dickinsonia are thought to be either an early annelid (roundworm) or a coral polyp similar to modern Fungiid Corals from Indonesia.
From the Burgess Shale: Top Row-Hallucinogenia next to possible lobopod. 2nd Row- Wiwaxia, Anomalocarus, and the five-eyed Opibina. Bottom Row-Early trilobytes
About 530 million years ago, on a shallow ocean shelf in what would one day be called Canada, a small underwater ridge collapsed and buried the sea bottom under tons of fine grained silt. Over time the creatures ensconced within would fossilize preserving with photolike quality the details of soft tissue and internal organs. By a quirk of fate this region was spared destruction from the relentless forces of geology and now lays conveniently near the surface. This one small formation, not much larger than a soccer field, is the basis of the classic "Cambrian Explosion" and is known as the Burgess Shale. There is no notable geological boundary between the Cambrian and the Ediacaran. No great extinction's, no telltale global layer of space-borne dust or volcanic apocalypse. Thus it's highly plausible that this amazing array of Cambrian Creatures preserved in the Burgess were present in the late Ediacaran for millions of years.
Why did multicellular animals develop when they did? To understand that we need to think about oxygen. We humans of course tend to think pretty dang highly of oxygen and find ourselves under great distress if we can't get enough. Yet oxygen is a terribly destructive gas which can reduce solid iron to dust. We're adapted to it, early life was not. The introduction of large amounts of oxygen in the primeval atmosphere was so catastrophic it's often referred to as the Oxygen Holocaust.
It took a long time for oxygen to build up in the atmosphere. At first the gas was taken out by chemical interactions with iron and other elements in the early oceans leaving behind thick bands of rust and other compounds in the strata. But eventually these elements were all locked up by the oxygen and it begin to accrue in significant quantities as a free gas. A likely scenario is that the increased oxygen in the atmosphere, courtesy of blue-green algae, had a devastating effect on earlier single celled organisms in two related ways: 1) It directly poisoned them and 2) It ended the greenhouse effect of the methanogens producing a period of glaciation ending at the start of the Ediracaran 630 million years ago. If so, those early oxygen producing microbes won the first chemical war in history, and they did it without so far as we can tell so much as moving an inch!
But oxygen may have also made possible a new tissue (In fact probably the basis of almost ALL tissue) for the growing repertoire of early life, a substance called Collagen. Collagen is the most abundant protein in all animals. It forms the basis of all epithelial tissue such as skin and the lining of internal organs. Collagen is a matrix, a tough, fibrous and flexible substrate in which early colonies of microbes could begin to organize into polyps. This might lead to the first corals and jellyfish, exceedingly simple animals by today's standard, but light-years ahead of their single celled antecedents.
Corals today often reproduce using free swimming larvae the size of a pinhead called planulae. The larvae float/swim until they find a good spot to anchor, and transform into a polyp which grows into a coral. It's plausible that these larvae themselves are what evolved into the first soft-bodied wormlike creatures. The Snowball conditions of the Cryogenian might have selected for mobility in the larvea; all the better to seek out a viable anchor point in the harsh conditions of that time. This period in the coral's reproductive cycle would then reasonably lengthen, until it may have been altogether discarded in some cases. The end result of that evolutionary process might be something like a primitive worm.
Take a wormlike critter which has discarded the sedentary coral stage of adulthood, add segmentation and you have an early arthropod. This is but one possible answer to the question: Where did trilobytes come from?
Encase an early worm with a carbonate shell and you might have the precursors of mollusks; clams and oysters (Add eyes and tentacles coming out of the shells and you have the ancestors of modern squid and octopi).
Take an early worm, add a sliver of stiff tissue running down the back and you have a chordate like Pikaia; the forerunner of vertebrates: Fish, amphibians, reptiles, and mammals like ourselves.
Sound crazy? Well the above is speculation to be sure and it's been greatly oversimplified due to my own lack of proper training in the underlying science. But that thumbnail sketch gives you five of the major phyla (Cnidaria-jellies/corals, Annelids-worms, Arthropoda, mollusks, and chordates/vertebrates) which creationists claim can't possibly be explained by evolutionary biology. And hey, it's at least plausible, a lot can happen in half a billion years. Events we transitory mortals living out our paltry 80 year lives would classify intuitively as improbable or impossible become inevitable. And the alternate idea, often pushed by purveyors of Intelligent Design Creationism, which can be summarized as the "Poof Theory", doesn't offer a hell of a lot of testable predictions.
Coral larvea to worms to the more familiar animals we know today? Or maybe just early polyps straight to worms with corals being a later development ... Who knows for sure. But if correct the worm did most certainly turn! So, the next time you spy a lowly night crawler wiggling through the soil, instead of dismissing it as a stupid slimey critter, good for little but aerating soil or baiting a hook, think of the Ediacaran and your own ancestors. Without that little guy's ancient cousins, 600 million years ago, the world you know wouldn't exist; and neither would you!