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The Medea Hypothesis
Is Life on Earth Ultimately Self-Destructive?

A review by Gert Korthof   30 Sep 2009 (updated: 25 Nov 2009)

The Medea Hypothesis. Is Life on Earth Ultimately Self-Destructive?

The Medea Hypothesis. Is Life on Earth Ultimately Self-Destructive?
by Peter Ward (2009), Princeton University Press,
hardback 180 pages.
My view of life on earth has been updated considerably after reading Peter Ward's The Medea Hypothesis (1). Basically, there are only two ways how life on earth could end: either it would be destroyed by a large meteorite impact or by the dying sun. At least, that was what I thought. But, now I know it need not be an external factor at all, it could be an internal factor: life itself. Moreover, according to Ward, this is not an accident; it follows from the essence of Darwinism. That is a surprising claim, and seems hard to swallow.

My view of life was contradicted profoundly by Ward's hypothesis because I believed Lovelock's Gaia hypothesis was basically right. Gaia hypothesis says that life on earth is beneficial for life on earth and furthermore, there are a lot of feedback mechanisms helping to sustain life on earth. Yes, I knew that mass extinctions happened, many ice ages, even Snowball earth. But still life survived. The prime example of a beneficial effect was the production of oxygen by single-celled plants long before animals existed (2). An atmosphere with 21% oxygen enabled animal life on this planet. We owe our existence to photosynthetic life. The mechanism is this: (1) plants produce oxygen which is consumed by animals (they can't produce oxygen themselves); (2) animals produce CO2 which is consumed by plants. This reciprocal dependence on each other can go on forever, as long as herbivores don't wipe out plants. I don't know of any species wiping out another species, world-wide (except humans of course). For me, the oxygen story was the paradigm for the beneficial nature of life.

Is the Medea hypothesis true?

Ward did not entirely convince me. He showed some evidence, but he sometimes exaggerates his case, sometimes ignores facts contradicting Medea hypothesis, and in other cases too easily interprets facts as favourable to his Medea hypothesis and falsifying Gaia. Nobody can deny that life still exists after some 3.5 billion years despite Medea. This is not an insignificant observation. Secondly, all animals on earth depend on oxygen that was produced by plants during the long history of the earth (4). Ward claims that the first atmospheric oxygen caused massive extinctions. However, proof is needed for the amount of extinction. In any case it was a temporary effect. Plants have an enormous positive effect on the biodiversity of our planet. So it is a perfect example of an understatement when Ward writes about an 'oxygen catastrophe':
"Only the children of the bugs that could tolerate oxygen – and the cyanobacteria that learned to make it, and the bugs that later learned to breathe it – would thereafter enjoy the sunlight" (p.75).
Exactly! Those creatures that learned the trick were tremendously successful! Ward knows this very well:
"simply because the physics and chemistry of the universe could produce no better way for life to get energy than oxygen-dependent systems" (p.103).
These facts count against the Medea Hypothesis. Amazingly, Ward attributes the ice-ages (Snowball earth) to the activities of living organisms (removing greenhouse gasses carbon dioxide and methane). But, the causes of Snowball earth are still controversial (see wiki). Ward mentions that at the time the Sun was notably weaker (p.76), so whatever microbes are guilty, they could not have been the only cause of Snowball Earth. Ward uses data about biodiversity to support Medea. However, palaeontologists cannot even agree whether diversity was higher 10,000 years ago than it has ever been, or whether it plateaued hundreds of millions of years ago (3).

The human species

The Medea Hypothesis throws a new light on the destructive nature of the human species (environmental pollution, extinction of species, destruction of ecosystems). It seems the human species is the best example of a Medean force. By *her* advanced technology, the human species is able to finish the job: exterminating life on earth. We can now place human impact on life in a historical context. Ward argues that the human species is a Medean species, but in the final chapter "What Must Be Done" we read: "we simply cannot let the ice caps melt"!

Darwinism and Medea

Again Ward makes an intriguing and unexpected claim: Medea follows logically from Darwinism. His argument is this: Darwin's theory of natural selection is based on Malthus idea of biological populations growing faster than food sources. Thus depleting food resources. There are two effects. First: since food consists of other organisms (at least for animals), the Medean effect is that one species lowers population numbers of other species. That indeed is a negative effect. Second effect: competition arises for scarce resources ("struggle for survival"). Competition is thus an inherent attribute of Darwinian life (p.27). In the end a population grows beyond the "carrying capacity". Individuals of the same species must die. I think Ward has a point here that cannot be easily dismissed. The question is: is life self-destructive? For example: is a predator not only exterminating its prey? If a predator would exterminate its prey, the predator itself would go extinct. But there are still predators and prey around, so predators are not that destructive (Medean). Carnivores don't kill all prey animals, and herbivores don't eat all plants, because we do see carnivores and herbivores walking on earth today.
My conclusion is that Ward is right by pointing out that competition between members of the same species logically follows Darwinism, and maybe self-destruction would follow from a simple version of Darwinism, but since we observe that life on earth has not been self-destructive, we need a more sophisticated version of Darwinism to explain why life is not self-destructive. Probably we need to include some modern ecological theory into Darwinism.

The end of life on Earth?

The subtitle of the book is: Is Life on Earth Ultimately Self-destructive? It is not so difficult to predict that life on earth will disappear. It is more difficult to predict whether it will be caused by biological or nonbiological factors. Climate models predict that after 500 - 800 million years photosynthesis is impossible. This follows from decreasing levels of atmospheric CO2. (Current increasing levels of CO2 are temporary). Plants need a minimum level of CO2. Below 150 ppm plants cannot photosynthesize. Current levels are 380 ppm. It seems that decreasing levels of CO2 is an inevitable trend of the last 200 million years. Ultimately CO2 levels will drop to 10 ppm and not even very simple plants will be able to run photosynthesis at that level. Without plants no animals. No herbivores, no carnivores. According to Ward the cause of declining levels of CO2 is life itself. I need detailed evidence for this claim.

The Medea Hypothesis is an eye-opening idea. It is a very useful addition to the evolution literature and an alternative to the Gaia hypothesis, which so far was the only game in town. I think that life on earth has both Gaian and Medean aspects. After 4 billion years of life on earth Gaia still is the victor and Medea a loser. That may change in the (far) future.


  1. Princeton University Press, hardback 180 pages. References are located at the end of the book, organised per chapter. So, there is no complete alphabetical listing of all references and also references are repeated.
  2. Great Oxidation Event as trigger for the Cambrian explosion: "The greening of ancient Earth could thus be indirectly responsible for the sudden evolution, beginning about 600 million years ago, of larger respirating animals with oxygen-hungry cells, say geologists Paul Knauth of Arizona State University in Tempe and Martin Kennedy of the University of California, Riverside. "This is a profound event," says Kennedy. "It explains the rise of oxygen, and the timing of that rise." Other researchers argue that the oxygenation of Earth and the explosion of animals 600 million years ago arose from sudden and drastic changes in ocean water chemistry around the same time. There isn't much evidence for widespread plant life until around 400 million years ago." (Nature). Info.
  3. Douglas Erwin (2009) 'A call to the custodians of deep time', Nature 462, 282-283 (19 November 2009)
  4. The creation of new niches by species for other species is called Niche Construction: John Odling-Smee, Kevin Laland, Marcus Feldman (2003) Niche construction. The neglected process in evolution (review). Niche construction (positive for life) is ignored by Peter Ward (2009). Cyanobacteria created a huge niche for aerobic metabolism: including all animals, including us.


       Further Reading  

  • Peter Douglas Ward (1993) On Methuselah's Trail: Living Fossils and the Great Extinctions.
  • Peter Ward (1995) The End of Evolution: A Journey in Search of Clues to the Third Mass Extinction Facing Planet Earth
  • Peter Ward (1998) The Call of Distant Mammoths: Why The Ice Age Mammals Disappeared.
  • Peter Ward and Donald Brownlee (2000) Rare Earth. Why complex life is uncommon in the universe.
  • Peter Ward (2001) Future Evolution.
  • Peter Ward, Donald Brownlee (2004) The Life and Death of Planet Earth: How the New Science of Astrobiology Charts the Ultimate Fate of Our World.
  • Peter Ward (2006) Out of Thin Air: Dinosaurs, Birds, and Earth's Ancient Atmosphere.
  • Peter Ward (2008) Under a Green Sky: Global Warming, the Mass Extinctions of the Past, and What They Can Tell Us About Our Future.
  • Peter Ward (2012) The Flooded Earth: Our Future In a World Without Ice Caps.
  • Peter Ward, Joe Kirschvink (2015) A New History of Life: The Radical New Discoveries About the Origins and Evolution of Life on Earth:
    "In this revolutionary book, leading scientists Peter Ward and Joe Kirschvink rewrite the principal account of the history of life on Earth. They show not only how the rise of animals was delayed for billions of years, but also what it was that first forced fish out of the sea and onto the land."
    Short review in Nature: "Here, palaeobiologist Peter Ward and geobiologist Joe Kirschvink weave decades of findings into an audacious retelling, hingeing on catastrophic transformation; the roles of oxygen, hydrogen sulfide and carbon dioxide as well as carbon; and the importance of ecosystems. They speculate chillingly about future impacts of the biodiversity drain, and query our own evolutionary capacity.".
  • Peter Ward (wikipedia)
  • Douglas H. Erwin (2006) Extinction: How Life on Earth Nearly Ended 250 Million Years Ago
  • Filip J.R. Meysman , Jack J. Middelburg and Carlo H.R. Heip 'Bioturbation: a fresh look at Darwin's last idea'

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Copyright ©G. Korthof 2009 First published: 30 Sep 2009 Updated: 25 Nov 09 (typos) F.R./N: 19 Feb 15