Medea hypothesis

The Medea hypothesis is a term coined by paleontologist Peter Ward[1] for the anti-Gaian hypothesis that multicellular life, understood as a superorganism, is suicidal. In this view, microbial-triggered mass extinctions are attempts to return the Earth to the microbial-dominated state it has been for most of its history.[2][3][4] The metaphor refers to the mythological Medea (representing the Earth), who kills her own children (multicellular life).

Past "suicide attempts" include:

• Methane poisoning, 3.5 billion years ago
• The oxygen catastrophe, 2.45 billion years ago
• Snowball Earth, twice, 2.4–2.1 billion years ago and 790–630 million years ago
• At least five putative hydrogen sulfide-induced mass extinctions, such as the Great Dying, 252.28 million years ago

The list does not include the Cretaceous–Paleogene extinction event, since this was, at least partially, externally induced by a meteor impact.

Peter Ward also believes that the current man-made climate change and mass extinction event may be considered to be the most recent Medean event. As these events are anthropogenic, he postulates that Medean events are not necessarily caused by microbes, but by intelligent life as well. He believes that the final mass extinction of complex life, roughly about 500–900 million years in the future, will also be considered a Medean event. Plant life that will still exist by 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 be due to the increasing heat from the Sun gradually speeding up the weathering process that removes them 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.