The metaphor refers to the mythological Medea (representing the Earth), who kills her own children (multicellular life).
In this view, microbial-triggered mass extinctions result in returns to the microbial-dominated state Earth has been in for most of its history.
[2][3][4] Possible examples of extinction events induced entirely or partially by biotic activities include: The list excludes the Cretaceous–Paleogene extinction event, since this was, at least partially, externally induced by a meteor impact.
As these events are anthropogenic, he postulates that Medean events are not necessarily caused by microbes, but by intelligent life as well and that the final mass extinction of complex life, roughly about 500–900 million years in the future, can also be considered a Medean event: "Plant life that still exists then will be forced to adapt to a warming and expanding Sun, causing them to remove even more carbon dioxide from the atmosphere (which in turn will have already been lowered due to the increasing heat from the Sun gradually speeding up the weathering process that removes these molecules from the atmosphere), and ultimately accelerating the complete extinction of complex life by making carbon dioxide levels drop down to just 10 ppm, below which plants can no longer survive."
However, Ward simultaneously argues that intelligent life such as humans may not necessarily just trigger future Medean events, but may eventually prevent them from occurring.