The Exotic Oceans of Early Earth
Approximately 541 million years ago (mya), mysterious events unfolded in the earth’s ancient oceans. Weird, unusual, and almost unimaginable lifeforms seemed to suddenly appear in a brief period of hyper-evolution. This event is known as the Cambrian explosion, and it marked a major evolutionary shift. During the next eleven million years (541 mya — 530 mya), most of the animal kingdom’s major branches sprang into existence and gained a foothold in the biosphere.
Deciphering geological data to envision life half a billion years ago is somewhat like looking at a couple of pyramids, with fuzzy partially worn away hieroglyphics, and trying to understand the nuances of life in early Egypt. But even under those circumstances, you would still probably get a more complete view of early Egyptian society than we have of Cambrian age life. Notwithstanding the challenges, however, we have a better understanding of life in the Cambrian oceans than we do of life in the Precambrian. Investigating the earth’s ancient past is a tenuous affair, requiring some degree of speculation to fill in the gaps.
Fossil evidence is a rare commodity
Uncovering evidence of life in the geological record relies on a combination of unusual preservation circumstances and the random luck of fossil deposits being at the earth’s surface present day. One of the problems with preserving fossilized materials is the vicious competition for food within all ecosystems. Life takes dinner wherever it can be found and always devours it as completely as possible. All that is edible will be eaten as predators eat prey and scavengers scoop up what is left. Bacteria will, of course, feast on any remaining organic debris. The biosphere always tries to recycle life into new life. This fact makes the chance of dead animals remains being fossilized very small indeed.
But occasionally, environmental circumstances arise where critical components of the organic carbon recycling system are missing. Then preservation becomes a possibility. One example is when anoxic conditions develop. If all oxygen disappears in a body of water, then a dead organism, which ends up in this environment, will not decay. As it is buried beneath layers of sediment and slowly turned into solid rock, a fossil record of the creature develops. The catch to this process in action requires animals being in an environment where nothing can live. For fossils to accumulate, dead animals must often be transported to their final resting place. But despite the odds against fossilization, life does occasionally conspire to preserve large numbers of animals at the same time and eventually fossilize their remains in the rock record.
So, on occasion, throughout the vast expanses of the earth’s history, insights into ancient life are preserved, buried in geological strata within the earth’s bowels. Thanks to the workings of plate tectonics, buried layers of rock are also sometimes pushed to the surface of the planet when colliding plates force one set of strata over another during the process of mountain building. When this happens, the occasional rare treasure trove of well-preserved fossils comes to the surface. Of course, erosion immediately goes to work, whittling down the mountains and washing away the newly surfaced rock. So today, we find ourselves left with the rare preservation events, which have been forced to the earth’s surface recently enough to avoid obliteration through erosion. Thus fossil windows into the past are rare and unusual. The German language has a name for the fossil treasure troves, Lagerstätten, meaning “load places,” or “mother lodes.”
These rare lagerstätten are windows in time, allowing us to catch a glimpse of our primordial past. Through these deposits, like the Burgess Shale, we start to gain insights into the rare period of evolution known as the Cambrian explosion. The origins of the animal kingdom as we know it today popped into existence in the geological equivalent of the blink of an eye — or so the fossil record would have us believe. But is there more to this story than meets the eye? Undoubtedly there is much more, for we cannot decipher all activity in the modern world by looking through a single window of our home, nor can we fully understand the ancient past through examining the occasional lagerstätten.
Skeletons, the hard parts
One of the evolutionary leaps forward at the beginning of the Cambrian was the development of skeletal structures upon which multicellular life could rest. Mobility, speed, protection, and more were provided by exoskeletons and later endoskeletons. No longer was life solely limited to soft-bodied bundles of cells, dominated by ocean current vagaries. Both predators and prey gained evolutionary advantages as skeletons morphed into all manner of new life. But essential to the fossil record, skeletons, the hard parts, were much more prone to preservation than purely soft-bodied creatures.
So, what part did the evolution of skeletons play in our perception that life exploded onto the scene in the early Cambrian? After all, we don’t usually find fossilized dinosaurs; we find their bones, the hard parts.
The earth itself has been around for about 4.5 billion years and life, in some primitive form, probably arose about 3.6 to 4.0 billion years ago. For about three-quarters of the planet’s history, life consisted of single-cell organisms. It took a long time for evolution to work out how to join up a group of cells and make multicellular lifeforms. Life didn’t hop straight from single-celled bacteria to hard-bodied trilobites roaming the ancient ocean floors. For approximately a hundred million years before the Cambrian explosion, soft-bodied organisms called the Ediacaran fauna dominated Precambrian sea bottoms.
Fossil preservation favors hard body parts, but it doesn’t completely ignore soft-body preservation. However, where fossilized troves of skeletal remains are rare, Lagerstätten consisting of only soft-bodied creatures are even rarer. Because of this difference in preservation rates, the probability of seeing a smooth fossil transition across the Precambrian-Cambrian boundary is unlikely. The nature of the fossilization process may be why we perceive an explosion of life at the beginning of the Cambrian. Before the Cambrian, we rarely see any fossil evidence of life, but after the development of skeletons we see a multitude of life. Perhaps the Cambrian explosion is more of a statistical phenomenon than a comment on the abundance of life.
Ediacaran Period
The Ediacaran Period occurred at the end of the Precambrian between 635 mya and 541 mya, and it followed the Late Cryogenian — a period when the earth froze over from pole to equator (Snowball Earth). As the planet thawed out, soft-bodied life took over the shallow oceans. The Ediacaran was a critical period when life mapped out new pathways like guts, legs, and complex behaviors such as burrowing.
But when Darwin wrote his masterpiece “On the Origin of Species” in 1859, no fossils older than the Cambrian were known. It was 1957 before definitive evidence of complex Precambrian lifeforms was confirmed. Another 30 years passed before Adolf Seilacher proposed that Ediacaran biota were not truly animals, but a failed evolutionary branch he called ‘Vendobionta.’
Today scientists are more inclined to see the Ediacarans as an eclectic group of organisms, representing multiple evolutionary pathways — mother nature was rolling the dice to see what might turn up. Included within this grab bag of lifeforms were the early precursors of animal life as we know it today.
Studies of Precambrian fossils involve more than simple visual analysis and comparison to ‘see’ what might be an animal versus something else. Geobiologists have entered the academic fray and developed techniques to isolate fat molecules from Ediacaran fossil remains. This line of investigation identified chemical products from the breakdown of cholesterol — a product common in animal cell membranes.
Another group utilized 3D laser scanning technology to identify tiny fossilized Ediacaran creatures (Ikaria wariootia) with left-right symmetry and identifiable frontends and backends. As much as we might not like to be reminded, a mouth, guts, and an anus are the basic defining characteristics of multicellular animal life, be it a worm or a human being.
The evidence is mounting and pointing to a past where the Cambrian explosion was not a big-bang event with animal life suddenly appearing from the void of non-existence. Before the explosion, life was doing what it does best, evolving, changing, and morphing along weird and wonderful evolutionary pathways — creating increasingly complex lifeforms. Perhaps the Cambrian started as an evolutionary inflection point as opposed to an explosion. Some of evolution’s experimental pathways proved wildly successful, and creatures representing the beginning of animal life as we know it today proved to be the winners in a game of genetic adaptation. The losers are only vague imprints in rare fossil deposits — failed branches of life in our planet’s ancient past.
Sources:
These bizarre ancient species are rewriting animal evolution (By Traci Watson; Nature)
Wonderful Life — The Burgess Shale and the Nature of History (by Stephen Jay Gould) — 1989, published by W. W. Norton & Company, Inc.
Feature Image:
Ediacaran Seafloor (Modified by ArcheanWeb) — Original Credit By Ryan Somma — Life in the Ediacaran SeaUploaded by FunkMonk, CC BY-SA 2.0, Wikimedia Commons
