An extinction event is a rapid (in geological terms), widespread loss of species, or biodiversity around the planet. With the advent in the Cambrian of life widespread across the planet, both in the sea and on land, the conditions appeared for large-scale changes in the environment to wipe out those life forms dependent on conditions as they were, and unable to adapt.
There is no set definition of what constitutes sufficient biodiversity loss to be considered an exctinction event. The fossil record–our only means of gauging these–can be interpreted by different methods, though the broad magnitude of an event is clear enough. Biological taxonomy, the study of organism types, is the analytical lens. There are seven traditional taxa, or groupings, of organisms of all varieties. From the broadest to the most specific, those are: Kingdom; Phylum; Class; Order; Family; Genus; and Species.
One extinction event might have wiped out a particularly high number of genera (plural of genus); another might have wiped out nearly all species, though left a broader smattering of orders, classes or phyla. Distinctions like those are pointless in our present context but are useful in pointing out that there is no one solitary means of understanding such ancient, huge events. The same goes for the causes of these events, which in most cases are hazy at best. The fossil record speaks clearly about the global loss of biodiversity, whether we have identified any physical cause or not.
Theories behind such massive die-offs tend to fall into one of six categories:
1.) Atmospheric change. Huge, sustained addition of new gases to, or removal of previous gases from, the atmosphere can create an environment inhospitable to organisms already in existence, and also alter the climate.
2.) Massive volcanic events, with three effects: (a) ejection of ash and aerosols, including sulfur dioxide (SO2), into the atmosphere, disrupting photosynthesis for years; (b) SO2 aerosols falling to earth as acid rain and acidifying bodies of water; (c) CO2 emissions warming the climate.
3.) Sea-level falls, sometimes due to a lowering of tectonic activity, diminishing mid-ocean spreading ridge volume and lowering sea level worldwide.
4.) Bolide impacts. The major asteroid strike at 65.5 mya, the Chicxulub Impact, is best known for helping along, if not accomplishing, the extinction of the dinosaurs. One school of thought, the Shiva Theory, holds that every 27 million years or so the earth is subject to such meteors because of the Sun’s passage through the plane of the galaxy then.
5.) Global cooling.
6.) Global warming.
Tomorrow: Extinction events 2: a brief list of major events.
This post was previously published on dailykos.com and is republished on Medium.
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