Around two and a half billion years ago, at the fringes of thriving microbial communities, some groups had evolved to use photosynthesis instead of chemistry to acquire their energy. These new photosynthesizers could colonize everything the light touched. But the gift of photosynthesis came with an unwanted by-product: oxygen. Oxygen in the archaean ocean was as lethal as mustard gas. It had filled the oceans and then the air. The great oxygen extinction laid the foundation for the emergence of eukaryotes.
The reactive element poisoned everything in its path. Then it transformed the atmosphere stripping it of its insulating blanket of methane and plunging the planet into an ice age for 300 million years. By this process, thick ice extended from the pole to the equator, and along the shallow shorelines, photosynthesizers died in their droves. Indeed the vast majority of ecosystems today rely on the very photosynthesis that brought about such destruction life clung on by the thinnest of threads. Just one percent of all life in arcane oceans persisted into the next age, the proterozoic. And it’s from this one percent that the living world was rebuilt.
How plants caused the first mass extinction?
About 500 million years ago, In the middle of the Cambrian Period, the face of the earth 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 and thin ancient soils was life, just a paper-thin film of microbes. These microbes were most likely the only terrestrial life around. And they had been for several billion years.
Scientists think these ancient microbial films were probably made up of cyanobacteria and some of the first fungi. Each bacterium was sending out tiny filaments of cells from the main bacterial mat to start new colonies. For a good chunk of Earth’s history, cyanobacteria had a monopoly on the terrestrial environment. And those newcomers would end up changing the world.
Their arrival would make the world colder and fast. It would drain much of the oxygen out of the world’s oceans. Eventually, it would help cause a massive extinction event. 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 planet’s history.
So, what could’ve caused such a massive, global calamity? Scientists think it may have been kicked off by the world’s first tiny terrestrial plants. Unlike animals, plants tend to leave behind a terrible fossil record. 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 a move onto land.
Aquatic plants effect
In the 1990s, scientists found lots of plant spores in rocks from Saudi Arabia and the Czech Republic. These spores were dated 462 million years ago. During that cooling event that took place in the Ordovician Period. They came from land plants and not aquatic plants because they 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.
Aquatic plants don’t have that because they don’t need it in their environment, which tends to be less harsh. And this covering is also what allows spores to fossilize. They are produced in huge quantities in a variety of habitats. In 2010, even older spores were found in Argentina and dated to 470 million years ago. But paleontologists think that the arrival of plants on land happened even earlier. It is 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.
This method puts plants on land at least 515 million years ago, right in the middle of the Cambrian Period. 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 at least 5 different kinds: a little community of Ordovician plants.
It’s hard to know what those plants were based on spores. But scientists can tell that they were non-vascular. It means 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 what exactly the first type of land plant was. But they agree that it was small and moss-like, probably some green algae or liverwort. 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, as 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.
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 the cryptogamic cover. It interacts with 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 modern-day New Mexico and Texas rock formations, they found phosphorus in deposits dating to the Late Ordovician Period. At this time, the American Southwest was underwater, just as plants were getting a foothold on land.
Those ancient deposits spelled doom for ocean animal life. 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. This process uses up a lot of the oxygen in the water. As a result, the ocean becomes oxygen-poor, hypoxic, or even anoxic, with no oxygen left. 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. 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 sediments and stays there. In the geologic record, buried organic carbon with no oxygen shows up as black shales. There are extensive black shale deposits in places like China and northern Africa, dating to the Late Ordovician. So, a cooler climate and an oxygen-poor ocean could certainly have been behind the major extinction of ocean life.
Massive tectonic activity
In fairness to the plants, experts know that other things 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 the 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 rock weathering of the new mountains. It 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.
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. Based on the evidence and modern experimental work, it looks like that trigger might’ve been planted moving onto land. There’s no need to hate on plants because of all of the downstream effects that came with their big terrestrial transition.
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.
Over time, the plants themselves took over the land. Their roots became longer to tap deeper for nutrients. Vascular tissue 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.
If it weren’t for the pioneering little plants that got a foothold on land half a billion years ago, the earth might still be barren, rocky, and populated by nothing but microbial films.
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