The Elements of Evolution (5) Causes


Major extinctions happen when a set of causal factors that might not be of serious consequence by themselves become aligned in time. ~ Norman MacLeod

The causes of extinction events are various. A major meteorite impact creates a planetary shock, as it did 66 MYA (the K–Pg event), finishing off the large dinosaurs.

Comet storms and bolides occasion extinction events, as has happened in at least 3 events, with shock waves causing raging wildfires, massive floods, acid rains, and withering winters.

Radiation from supernovae sterilizes and kills surface life, as do solar flares. Geomagnetic reversals forge a flux of cosmic rays to similar effect. Disruptive radiation factored into the extinction events that ended the Ordovician, Devonian, Permian, and Cretaceous periods.

Continental drift has been another facet in mass extinction via global glaciation or warming, increased aridity, and volcanism. The configuration of continents has a pronounced effect on the viability of macroscopic life everywhere.

Volcanoes spew toxicity into the atmosphere, on land, and in the oceans, affecting ecology worldwide, setting up stepwise extinctions. Tectonic plate movements and volcanoes are the 2 sides of the same coin. Volcanism played a role in many mass extinction events, notably the most severe.

Changes in sea level, salinity, and oceanic oxygen levels contributed to several extinction events, as have pattern disruptions in ocean-atmosphere circulation. Rising sea levels from deglaciation can prompt volcanic activity as mantle plumes are put under more pressure.

Finally, life itself creates extinction events: by disease, predation, and other changes into the food web. Most notable is the evolution of new plants, with better protections against herbivory: depriving animals reliant upon the plants of the past.

All mass extinctions stem from a selfsame biotic dynamic: relatively rapid changes in the environment with which life forms are unable to cope quickly enough via adaptation. An extinction event ensues by a cascade of ecological dependencies, or by rapid environmental changes that simultaneously decimate numerous species. Extinction events invariably involve both dynamics.