Hello and welcome back to extinctions past, present, and future. Given knowledge of how people realize the long history of the earth and its life, it seems appropriate to review extinctions of the pre-human past, and in particular mass extinctions. The five mass extinctions of the past 542 million years or so were substantial enough to have left their mark in the geologic record telling of great losses of life on Earth followed by recoveries. As we'll review an upcoming lectures all mass extinctions were the outcome of global ecological crises. These crises were triggered by Earth processes such as volcanism or sea level changes or biological processes such as the spread of land plants, or extraterrestrial impacts, or a combination of these factors. By global ecological crises, I mean that species in all environments were adversely affected leaving behind only small percentages of the original biodiversity. But what about extinctions before 542 million years ago in that vast expanse of Earth history before the Cambrian Period that geologist appropriately nicknamed the Precambrian. Considering that life has been around for about 4 billion years, you would think at least a few global ecologically crises happened in the 3.5 billion years before the Cambrian Period. And you would be right, so we'll discuss two mass extinctions that took place during the Precambrian. Sometimes geologists and paleontologists neglect dimension these extinctions because the geological and paleontological evidence for them is just not as clear as those of the five most recent pre-human mass extinctions. Nevertheless, they're worth discussing because they demonstrate the fragility of biodiversity in the face of global change. The first mass extinction was likely of microbes. Yes, that's right, simple one-celled organisms, like bacteria can go extinct too. How could such a unicellular tragedy have happened especially billions of years before the invention of antiseptic soaps and antibiotics. Blame it on oxygen. You see the first 2 billion years or so of life, most of it was anaerobic. Living and thriving in marine environments to avoid of oxygen. These organisms which included primitive bacteria like archaeans and true bacteria, fuel their metabolism by converting chemicals without using oxygen. For example, let's say you step into a muddy area of a swamp or a salt marsh, in such shoe Rooney Mischief, you are often further rewarded with a sulfur's rotten egg smell. If so, you're breathing in the waste products of archaeans or bacteria that converted sulfur to sulfide which they do in anaerobic mud. Other anaerobic archaeans produce methane, which is a powerful greenhouse gas that will discuss in a bit and later on in the course. Now for a long time these anaerobic organisms dominated all environments, but they also lived alongside bacteria that produced oxygen called cyanobacteria. Cyanobacteria, where the first organisms to use photosynthesis, this was a revolutionary biological process for one-celled organisms because it use sunlight to produce food. Vonverting carbon dioxide and water into simple sugars and oxygen. So starting about 2.4 billion years ago photosynthesizing bacteria became more common. This produce more oxygen and organic carbon was buried at faster rates in world oceans, which also produced even more oxygen. Well, this was bad news for anaerobic organisms. Chemically speaking, these photosynthesizers radically change the Earth's atmosphere and oceans shifting them from reducing to oxidizing. An oxidizing atmosphere and oxidizing notions probably decrease methane by converting it to carbon dioxide. Because carbon dioxide is much less effective at retaining heat compared to methane. This was like the Earth switching from a thick wall blanket to a thin nylon bedspread. What happened next? A combination of global cooling and yet more oxygen likely caused a mass extinction of anaerobic organisms. This time in Earth history from about 2.4 to 2 billion years ago has been called the great oxidation event or the great oxygen event. But if you look at from the perspective of anaerobic organisms that maybe it's more accurate to call it the oxygen crises or the oxygen catastrophe. Sadly, no one was around at that time protesting with signs saying save the anaerobes, not to worry there anaerobic descendants are still around and enjoying oxygen-free conditions wherever they can find them, including in your guts. The second Precambrian mass extinction worth discussing is that of the Ediacaran biota. What is the Ediacaran is a period of geological time from about 630 to 542 million years ago that was signified by the first abundant fossil evidence of multicellular life. This fossil record is especially well represented in the last 20 million years or so of the Ediacaran period. Starting around 565 million years ago until the start of the Cambrian period. These life-forms were diverse and widespread with body and trace fossils in Ediacaran Age rocks of Australia, the United Kingdom, Russia, China, Namibia, Mexico, Canada, Argentina, California, and even the far-off exotic land of North Carolina. To say Ediacaran fossils were strange compared to modern life forms is an understatement. For one notice that I call them the Ediacaran biota and not fauna. Fauna implies that all of the body fossils were animals, but paleontologist can't even agree on this. Yes, most of them have been interpreted as animals, but some have also been identified as large one-celled organisms. Multicellular algae, fungi, or even lichens, and interpretation of most paleontologists are not lichen. Their forms vary from disk like to bag like to tube like to quilt like, well, you name it. What this uncertainty shows is that Ediacaran fossils are often compared to fossils from much later in geologic time and to modern organisms, but they still don't quite fit in those categories because they were just too weird. Trace fossils give us a little more certainty that at least a few mobile animals were part of this biota, because they left trails or shallow burrows. The Ediacaran though was a time when all of complex life was superficial. Living on or just below the surface in the oceans whether in deep water, shallow warm seas. And then the Ediacaran biota suddenly vanished with nearly all representatives gone by the start of the Cambrian period around 540 million years ago. What happened to them? Part of their extinction was probably related to the evolution of animals with new behaviors such as predation. Nearly, every Ediacaran life form was soft body and most were sedentary. So if they would have had no defenses against any hard-bodied animals that want to eat them all. Also most of these organisms lived on bacterial or algal mats, also known as biomats, which cover the sea floors like carpets and stabilize them. Once animals evolved to graze on these biomats, they would have ripped out the very foundation of any life attached to them. Another factor was the evolution of burrowing animals which dug deeper into ocean floor sediments and literally undermined surface life. Burrowing animals also would have been ecologically engineers change in the geochemistry of the oceans by pumping oxygen down into sediments that were previously sealed off by biomats. These again would have killed lots of anaerobes that it successfully evaded the oxygen dwellers. Poor anaerobes, [FOREIGN]. Nonetheless, the Cambrian period is when animals began to diversify in occupy all of the world's oceans. A proliferation continuing for nearly 100 million years well into the next geologic period, the Ordovician. With only a few ups and downs along the way life was good, until it came crashing down in a big way toward the end of the Ordovician period. And that is the topic of the next lecture.
