Plant Colonization Triggered Global Cooling and First Mass Extinction

Generated: 2026-06-19 · API: Gemini 2.5 Flash · Modes: Summary

Plant Colonization Triggered Global Cooling and First Mass Extinction

Clip title: How Plants Caused the First Mass Extinction Author / channel: PBS Eons URL: https://www.youtube.com/watch?v=mAkjETPM1s4

Summary

Around 500 million years ago, during the Cambrian period, Earth’s landmasses were barren and devoid of complex plant or animal life. Terrestrial environments were primarily inhabited by microscopic life, specifically ancient microbial films composed mostly of cyanobacteria, and possibly some early fungi. These microbes had a monopoly on land life for billions of years. However, the eventual arrival of the first macroscopic life forms on land would trigger a series of dramatic changes, including a significant global cooling event and one of Earth’s “Big Five” mass extinctions.

Scientists have pieced together the story of these pioneering land plants through fossil evidence and molecular data. Fossilized plant spores with thick protective coverings, found in rocks from regions like Saudi Arabia, the Czech Republic (dated 462 million years ago), and Argentina (dated 470 million years ago), indicate the presence of terrestrial plants during the Ordovician period. Furthermore, using a “molecular clock” method, which tracks DNA changes over time, scientists suggest that plants may have colonized land even earlier, around 515 million years ago in the mid-Cambrian. These early plants were likely small, non-vascular, and moss-like, resembling modern green algae or liverworts, clinging to rocks near water sources.

The impact of these seemingly tiny plants was profound. By colonizing rocks, they formed what is known as “cryptogamic cover,” which actively weathered down rocks and released essential minerals, notably phosphorus. This phosphorus, washed by rainfall, flowed into the oceans. The sudden influx of phosphorus acted as a massive fertilizer for marine plants, leading to an explosion of algal blooms. When these vast algal populations died and decomposed, bacteria consumed enormous amounts of oxygen from the ocean waters, creating widespread hypoxic (oxygen-poor) or even anoxic (no oxygen) conditions. This environmental catastrophe, known as the End-Ordovician Extinction Event, wiped out approximately 85% of animal species and a quarter of all marine animal families.

Beyond ocean deoxygenation, the proliferation of early land plants also contributed to global climate cooling. Hypoxic oceans prevent carbon from cycling back into the atmosphere as carbon dioxide; instead, it becomes buried in sediments as black shales, a phenomenon observed in late Ordovician rock formations. This removal of atmospheric carbon dioxide led to a rapid and substantial drop in global temperatures, a “cold snap” that accelerated the extinction event. While tectonic activity, such as mountain formation and volcanic eruptions, also played a role in the Ordovician cooling by increasing rock weathering and potentially causing volcanic winters, the widespread colonization of land by early plants appears to be a crucial trigger for this global catastrophe. Paradoxically, despite causing a mass extinction, these pioneering land plants laid the groundwork for all subsequent terrestrial life by building up rich soil bases through death and decomposition, and eventually flooding the atmosphere with oxygen, transforming the planet into the diverse, green world we know today.

Video Description & Links

Description

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In the middle of the Cambrian, life on land was about to get a little more crowded. And those newcomers would end up changing the world. The arrival of plants on land would make the world colder, drain much of the oxygen out of the oceans and eventually, it would help cause a massive extinction event.

Thanks to Fabrizio de Rossi for the excellent illustration of early terrestrial plants. You can find more of Fabrizio’s work here: https://www.facebook.com/ArtofFabricious/

And special thanks to Paul Strother for sending us an incredibly cool photo of an Ordovician plant spore for this video. Check out Paul’s website here:https://sites.google.com/bc.edu/paulkstrothersbcwebsite/home

Credit for Paleogeographic Map: Scotese, C.R., 2019. Plate Tectonics, Paleogeography, and Ice Ages, YouTube video: https://youtu.be/UevnAq1MTVA.

Produced in collaboration with PBS Digital Studios: http://youtube.com/pbsdigitalstudios

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References: https://docs.google.com/document/d/13WpWZjQ_LjFeeFzJz8u7eGuiAAY6ZaoPqyhpEoAB3C0/

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Plant Colonization Triggered Global Cooling and First Mass Extinction

Plant Colonization Triggered Global Cooling and First Mass Extinction