The world is facing a major sixth wave of extinction, experts say. The five previous waves occurred already millions of years ago: the first extinction wave about 443 million years ago, with a large sea level drop as a result of cooling (Appalachian formation probably played a role in this). This is also called the Ordovician/Silurian extinction. Many marine organisms such as corals and trilobites became extinct in this wave. The second occurred at the end of the Devonian era, about 359 million years ago. The cause was probably a large decrease in the oxygen content in the oceans, possibly caused by volcanism. Many marine organisms, such as precursors of current mollusks, also died in this wave. The third wave, which marked the transition from the Permian to Triassic era, meant the extinction of many animal species. This has been the largest extinction wave to date (almost all marine life and three quarters of life on land died out). This wave, some 251 million years ago, according to researchers is due to a large-scale release of carbon dioxide, much more than we currently release into the atmosphere. The fourth wave marked the transition from the Triassic to Jurassic era - some 201 million years ago. Here too, the release of a mega amount of CO2 plays a major role (as a result of magma released on the earth's surface). This extinction wave meant the beginning of the dinosaur era, which, with the impact of an asteroid, came to a sudden end some 66 million years ago and made way for the mammals. This transition, from Cretaceous to Paleogene (known as the K-Pg transition) is called the fifth great extinction and is most well-known to the general public. For more detailed information on extinction waves, see Greshko's article in National Geographic (2019), or the information on the website of the American Museum of Natural History. For a clear scientific article: Barnosky, Nature 2011.