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The Gaia hypothesis first proposed by James Lovelock (or Gaia theory as he and his supporters now call it, claiming that it is able to satisfy predictive tests) is fairly well known these days. Life is supposedly maintaining the biosphere in homeostasis, stabilising atmospheric composition, temperature, ocean chemistry and so forth thus maintaining conditions suitable for life; humans however are working against and destabilising the system.

Peter Ward is a palaeontologist who here proposes the opposite, that life by its actions works to its own detriment, even to the extent of its own mass destruction. He calls it the Medea hypothesis, after the legendary figure who killed her own children.

He begins in the first two chapters by considering the definition of life and the definition of evolutionary success. If Darwinian evolution is one of the defining qualities of life, it seems almost a logical conclusion that competition will cause life to work against itself.

The third chapter, "Two hypotheses on the nature of life on earth", introduces the Gaia hypothesis, and then the Medea hypothesis. He contends that whilst his Medea hypothesis is just that, a hypothesis, the Gaia model can be shown to be incorrect in many ways.

Chapter four, "Medean feedbacks and global processes" seeks to demonstrate that many of the supposedly negative feedbacks of Gaia which work to keep the biosphere in homeostasis are in fact the opposite, positive feedbacks. So for example temperature increase causes changes which result in yet further temperature increase, a runaway effect.

Then in the fifth chapter, "Medean events in the history of life", Ward asserts that all but one of the many mass extinctions in the history of life on earth (the exception being the end Cretaceous extinction accepted by most as being the result of a meteorite impact) were caused by life itself causing catastrophic changes to the conditions of the biosphere. These events include:

- 3.7 billion years ago: methane generation causing planetary cooling
- 2.5 billion years ago: oxygen generation killing off many life forms
- 2.3 billion years ago: consumption of methane and carbon dioxide causing global glaciation
- between 2 and 1 billion years ago: hydrogen sulphide generation poisoning the oceans ("eutrophication")
- 700 million years ago: second global glaciation
- 600 million years ago: rise of animals causing major temperature drop
- 395-365 million years ago: microbial mass extinctions
- 400 - 250 million years ago: global temperature changes due to colonisation of land by plants
- 251 million years ago: Devonian eutrophication

Chapter six, "Humans as Medeans" considers how we are causing biosphere destruction as if we were prokaryotic life form; for example our population is growing exponentially and we are creating waste which is poisoning many other life forms.

Chapter seven, "Biomass through time as a test" suggests a falsification of the Gaia hypothesis in that the earth's biomass peaked at somewhere between 1 billion and 300 million years ago and has been decreasing since. We are effectively already in planetary old age. This theme leads into chapter eight, "Predicted future trends of biomass". Decreasing carbon dioxide levels, a consequence of life itself, mean that many plants could be extinct in as little as 100 million years time. Grasses, which are more efficent at carbon dioxide absorption will last longer, but in 500 million years time all plants could be extinct, with obvious consequences for all other life.

After a brief summation are the chapters "Environmental implications" and "What must be done". Because life is "Medean", humans are in fact the only possibility for sustaining life on earth into the far future by actively changing the planet. Rather than a "back to nature" approach, we must take an active management approach.

For further related reading, see earlier works by Ward: Under A Green Sky concerning mass extinctions; (with Don Brownlee) The Life and Death of Planet Earth considering both past and future life. Other recommended works by Ward: Life as We Do Not Know it: The NASA Search for (and Synthesis Of) Alien Life and (again with Brownlee) Rare Earth: Why Complex Life Is Uncommon in the Universe.
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on 25 January 2014
I am so sure if I agree fully with the other reviews although I am honestly a big fan of Peter Ward having read few of his other books . I was really keen on reading and finishing this book but it came out as mixed feelings for me.

The start and end of the book are pretty good, well written and clear but somehow, I lost it in the middle part. I am not sure if the author really keep focus on the subject in some chapters. I felt lost a couple of time despite putting some real effort to follow. I think with a little bit more editing and re-reading, the book would have come out much better.

A couple of photos to illustrate all of this would have been a welcome addition as well.

Nevertheless, this does not really affect my liking of P. Ward work but perhaps this is a book slightly less in quality compared to the others.
Highly recommend "Rare Earth" which has been critically acclaimed.
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on 25 July 2009
Una visione rivoluzionaria e provocativa delle relazioni della vita con la biosfera terrestre. In contrasto con la visione dell'ipotesi di Gaia di James Lovelock (l'idea che la vita sostiene e mantiene condizioni di abitabilità sulla Terra) l'ipotesi di Medea (la madre della mitologia greca che uccise i propri figli) di Peter Ward sostiene che la vita ha tendenze biocide e causerà la scomparsa della vita sulla Terra nel futuro cosí come è già avvenuto nel passato... "Solo la capacità di modificare il futuro ci salverà..." sarà l'uomo che dovrà modificare le condizioni ambientali per evitare la scomparsa della vita, e dell'uomo, in un futuro i cui effetti "Medea" già sono rilevabili... Un'ipotesi che aprirà un acceso dibattito, scientifico e metodologico, sulle modalità di conservazione della natura...
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on 24 November 2009
Mi è piaciuto.
L'ho trovato scritto con estremo rigore scientifico.
Mi ha aiutato a riflettere su questo delicato tema e ad approfondire le mie conoscenze dell'argomento.
Soprattutto mi ha aperto un modo di giudicare diverso dal solito.
Spero che il testo possa essere presto pubblicato anche in lingua italiana in maniera che possa essere diffuso anche nel mio Paese con maggiore facilità.
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