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- A book, a dire prediction: "The Sixth Extinction: An Unnatural History" is a 2014 non-fiction book by Elizabeth Kolbert that argues that the Earth is in the midst of a modern, man-made, sixth extinction. It chronicles previous mass extinction events, and compares them to the widespread extinctions during our present time. The author received the Pulitzer Prize for General Non-Fiction for the book in 2015.
- Mass extinction events: An extinction event (a biotic crisis) is a fast decrease in the biodiversity on Earth. It occurs when the rate of extinction increases with respect to the rate of speciation. Biosphere suffers as far as macro-species are concerned.
- When do these events occur? Fossil studies indicate that extinction events occur at an uneven rate. The background rate of extinctions on Earth is about two to five taxonomic families of marine animals every million years. Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land animals.
- First extinction event: Perhaps, it was the "Great Oxygenation Event" (GOE) is also called the Oxygen Holocaust. It was the biologically induced appearance of dioxygen (O2) in Earth's atmosphere. Evidence suggests that this major change happened around 2.45 billion years ago (2.45 Ga or 245 crore years ago), during the Siderian period, at the beginning of the Proterozoic eon.
- What did it do? The increased production of oxygen set Earth's original atmosphere off-balance, because free oxygen is toxic to obligate anaerobic organisms, and the rising concentrations may have destroyed most such organisms at the time.
- Who did it? Oceanic cyanobacteria, which evolved into coordinated macroscopic forms more than 2.3 billion years ago (approximately 200 million years before the GOE), are perhaps the first microbes to produce oxygen by photosynthesis. Before the GOE, any free oxygen they produced was chemically captured by dissolved iron etc.
- Cambrian explosion: The Cambrian explosion or Cambrian radiation was an event approximately 541 million years ago in the Cambrian period when most major animal phyla appeared in the fossil record. It lasted for about 20–25 million years.
- Next extinction events: In a landmark paper published in 1982, Jack Sepkoski and David M. Raup identified five mass extinctions. Since the Cambrian explosion five further major mass extinctions have significantly exceeded the background extinction rate. The most recent was the the Cretaceous–Paleogene extinction event, which occurred approximately 66 million years ago (Ma), and was a large-scale mass extinction of animal and plant species in a geologically short period of time. In addition to the five major mass extinctions, there are numerous minor ones as well, and the ongoing mass extinction caused by human activity is sometimes called the sixth extinction.
- Which are the five?
- FIRST - Ordovician–Silurian extinction events (End Ordovician or O–S): 450–440 Ma (million years ago) at the Ordovician– Silurian transition. Two events occurred that killed off 27% of all families, 57% of all genera and 60% to 70% of all species.
- SECOND - Late Devonian extinction: 375–360 Ma near the Devonian–Carboniferous transition. At the end of the Frasnian Age in the later part(s) of the Devonian Period, a prolonged series of extinctions eliminated about 19% of all families, 50% of all genera and at least 70% of all species.
- THIRD - Permian–Triassic extinction event (End Permian): 252 Ma at the Permian–Triassic transition.[9] Earth's largest extinction killed 57% of all families, 83% of all genera and 90% to 96% of all species (53% of marine families, 84% of marine genera, about 96% of all marine species and an estimated 70% of land species, including insects). The highly successful marine arthropod, the trilobite, became extinct.
- FOURTH - Triassic–Jurassic extinction event (End Triassic): 201.3 Ma at the Triassic–Jurassic transition. About 23% of all families, 48% of all genera (20% of marine families and 55% of marine genera) and 70% to 75% of all species became extinct. Most non-dinosaurian archosaurs, most therapsids, and most of the large amphibians were eliminated, leaving dinosaurs with little terrestrial competition.
- FIFTH - Cretaceous–Paleogene extinction event (End Cretaceous, K–Pg extinction, or formerly K–T extinction): 66 Ma at the Cretaceous (Maastrichtian) – Paleogene (Danian) transition interval. The event formerly called the Cretaceous-Tertiary or K–T extinction or K–T boundary is now officially named the Cretaceous–Paleogene (or K–Pg) extinction event. About 17% of all families, 50% of all genera and 75% of all species became extinct.
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