NARRATOR: In the middle of the Cambrian Period about 500 million years ago, the face of our planet looked completely different. There was land, but there weren't any plants or animals living on it, anywhere. Instead, the dry land was rocky and barren, with no shrubs or trees or grasses. But clinging to the rocks in the thin ancient soils was life, just a paper-thin film of microbes. These microbes were most likely the only terrestrial life around, and had been for several billion years. Scientists think that these ancient microbial films were probably made up of cyanobacteria, and maybe some of the first fungi. And each bacterium was likely doing what cyanobacteria do today, sending out tiny filaments of cells from the main bacterial mat to start new colonies. So the fact is, for a good chunk of Earth's history, cyanobacteria had a monopoly on the terrestrial environment. But life on land was about to get a little more crowded, and those newcomers would end up changing the world. Their arrival would make the world colder, and fast. And it would drain much of the oxygen out of the world's oceans. Eventually, it would help cause a massive extinction event in which around 85% of animal species, including a quarter of marine animal families, disappeared from the planet forever. This environmental catastrophe is known today as the End-Ordovician Extinction event. And it was the first of what we often call the Big Five mass extinctions in the history of our planet. So what could have caused such a massive global calamity? Well, scientists think it may have been kicked off by the world's first tiny terrestrial plants.

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(Describer) Title: Eons.

Now, we don't exactly know what the first terrestrial plant on Earth was, but we have a good idea of what it looked like and how it lived. Unlike animals, plants tend to leave behind a terrible fossil record. You might get a leaf or a stem, but rarely a whole plant. So the earliest fossil record of land plants isn't parts of their bodies; it's their spores, the particles that ancient plants used to reproduce. Pollen didn't exist when plants first made the move onto land, but there were spores like the ones you'd see today on a moss or a fern. Back in the 1990s, scientists found lots of plant spores in rocks from Saudi Arabia and the Czech Republic. These spores were dated to 462 million years ago, during that cooling event that took place in the Ordovician Period. And they could tell they came from land plants and not aquatic plants because the spores had a thick covering that all land plant spores have today. This covering protects the spores as they deal with environmental stressors like wind or flowing water. And aquatic plants don't have that because they don't need it in their environments, which tend to be less harsh. And this covering is also what allows spores to fossilize, along with the fact that they are produced in huge quantities in a variety of habitats. In 2010, even older spores were found in Argentina, and they dated to 470 million years ago. But paleontologists think that the arrival of plants on land actually happened even earlier, based on dates produced by the method known as the molecular clock. By looking at the average number of changes in DNA over time, scientists can calculate when a type of organism evolved on Earth. And this method puts plants on land at least 515 million years ago, right in the middle of the Cambrian period. And it looks like land plants started diversifying almost as soon as they left the oceans. The fossil spores in Argentina weren't just from one kind of plant, but from at least five different kinds, a little community of Ordovician plants. It's hard to know what those plants were based on spores, but scientists can tell they were non-vascular, meaning that they didn't have the system of roots and tubes that many modern plants use to move water and nutrients around. Paleobotanists are still debating on what exactly the first type of land plant actually was, but they agree that it was small and moss-like, probably some kind of green algae or liver wart. And these were pioneering little plants, venturing from the water into conditions where they were at risk of drying out. Scientists think that these early plants probably clung to rocks near the water. There they released their spores, taking advantage of the tide to disperse those spores, like their ancestors had done for generations, and gradually transitioning from aquatic to terrestrial life. Over time, through natural selection, they acquired adaptations for life on land, like hard-walled spores and waxy coverings called cuticles that allowed them to become more fully terrestrial. And it looks like their tendency to cling to rocks is what would have spelled disaster for life in the oceans. Today, the scientific name for living material that clings to rocks is cryptogamic cover. And this cover doesn't just sit there, it interacts with the rocks, wearing them down over time and releasing minerals like phosphorus, potassium, and iron. Scientists have used modern cryptogamic covers to see how the first plants might have worn rocks down 500 million years ago. By growing moss on rocks and measuring the minerals released, they found that moss-covered rocks released 60 times more phosphorus than rocks without moss. Once it's freed from the rocks, the phosphorus gets washed away by rainfall, traveling over landscapes and eventually flowing into the oceans. And geologists have found evidence of this very phenomenon in the deep past. In rock formations in modern-day New Mexico and Texas, they found phosphorus in deposits dating to the Late Ordovician, when the American Southwest was underwater and just as plants were getting a foothold on the land. And those ancient deposits doomed animal life in the oceans. That's because phosphorus is one of the nutrients that plants need for growth, but it's usually in short supply. Plants can only get it from the breaking down of rocks. So a major influx of phosphorus into the oceans would have caused an explosion of marine plants in the form of huge algal blooms. After algae bloom, they eventually die and are broken down by bacteria. And this process uses up a lot of the oxygen in the water. As a result, the ocean becomes oxygen-poor, or hypoxic, or even anoxic, where there's no oxygen left. And since marine animals need oxygen, they can't survive. But that's not the only change that was caused by the phosphorus runoff. A hypoxic ocean can also cool the climate, because carbon needs to bind with oxygen to cycle out of the ocean and into the atmosphere as carbon dioxide. But when ocean water is hypoxic, the carbon just gets buried in the sediments and stays there. In the geologic record, buried organic carbon with no oxygen shows up as black shales. And there are extensive black shale deposits in places like China and northern Africa dating to the Late Ordovician. So an oxygen-poor ocean and a cooler climate could have certainly been behind the major extinction of ocean life. Now, in fairness to the plants, experts know that there were other things going on that likely contributed to the extinction event. Namely, it was also a time of massive tectonic activity. New mountains were forming, like the Appalachians, and huge volcanic eruptions took place as tectonic plates of the supercontinent Gondwana moved and folded against each other. Some researchers even suspect that all of the gases spewed out by those volcanoes cooled the earth, causing volcanic winters. Plus, acid rain likely caused weathering of the new mountains, which removed even more carbon from the atmosphere and drove even more global cooling. But what stands out in the geologic record is how sudden this cold snap was. Starting around 488 million years ago, the planet began to cool, and the temperature continued to drop over the next 44 million years, which is pretty fast in geologic terms. So something else must have been at work to cause that amount of cooling in such a short timeframe. And based on the evidence in modern experimental work, it looks like that trigger might have been plants moving onto land. But there's no need to hate on plants because of all of the downstream effects that came with their big terrestrial transition. Sure, the first land plants were the spark that wreaked havoc on ocean biodiversity, but they also paved the way for all the terrestrial life that came after, because those tiny plants set up the conditions for more sophisticated terrestrial life to evolve. They built up a rich soil base through death and decomposition and they gradually flooded the atmosphere with oxygen. And over time, the plants themselves took over the land. Their roots became longer to tap deeper for nutrients, vascular tissues began to carry water and minerals around the plants, supporting the growth of much bigger plants. Later, huge changes, like the evolution of flowering plants, transformed the vegetation on Earth into the ancestors of the plants that we see today and use for food. If it weren't for the pioneering little plants that got a foothold on land half a billion years ago, our planet might still be barren, rocky, and populated by nothing but microbial films. So maybe we can give them a pass for getting the ball rolling on the world's first mass extinction.

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(Describer) Titles: Eons Kallie Moore: Content Consultant, Host PBS Accessibility provided by the US Department of Education.