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Life and death on Earth – the Cronus hypothesis

Bradshaw, C.J.A., & Brook, B.W. (2009). The Cronus Hypothesis – extinction as a necessary and dynamic balance to evolutionary diversification Journal of Cosmology, 2, 201-209 (free online access)


As described on ConservationBytes, we (Corey Bradshaw and Barry Brook), and I recently published a paper in the very new and perhaps controversial online journal , the Journal of Cosmology. Cosmology? According the journal, ‘cosmology’ is:

“the study and understanding of existence in its totality, encompassing the infinite and eternal, and the origins and evolution of the cosmos, galaxies, stars, planets, earth, life, woman and man”.

The journal publishes papers dealing with ‘cosmology’ and is a vehicle for those who wish to publish on subjects devoted to the study of existence in its totality.

Ok. Quite an aim.

Our paper is part of the November (second ever) issue of the journal entitled Asteroids, Meteors, Comets, Climate and Mass Extinctions, and because we were the first to submit, we managed to secure the first paper in the issue.

Our paper, entitled The Cronus hypothesis – extinction as a necessary and dynamic balance to evolutionary diversification, introduces a new idea in the quest to find that perfect analogy for understanding the mechanisms dictating how life on our planet has waxed and waned over the billions of years since it first appeared.

In the 1960s, James Lovelock conceived the novel idea of Gaia – that the Earth functions like a single, self-regulating organism where life itself interacts with the physical environment to maintain conditions favourable for life (Gaia was the ancient Greeks’ Earth mother goddess). (see here for a book review on BNC of Lovelock’s latest, “The Vanishing Face of Gaia“) Embraced, contested, denounced and recently re-invigorated, the idea has evolved substantially since it first appeared. More recently (this year, in fact), Peter Ward countered the Gaia hypothesis with his own Greek metaphor – the Medea hypothesis. Essentially this view holds that life instead ‘seeks’ to destroy itself in an anti-Gaia manner (Medea was the siblicidal wife of Jason of the Argonauts). Ward described his Medea hypothesis as “Gaia’s evil twin”.

One can marvel at the incredible diversity of life on Earth (e.g., conservatively, > 4 million protists, 16600 protozoa, 75000-300000 helminth parasites, 1.5 million fungi, 320000 plants, 4-6 million arthropods, > 6500 amphibians, 10000 birds and > 5000 mammals) and wonder if there might be something in the ‘life makes it easier for life’ idea underlying Gaia. However, when one considers that over 99 % of all species that have ever existed are today extinct, then a Medea perspective might dominate.

Enter Cronus. Here we posit a new way of looking at the tumultuous history of life and death on Earth that effectively relegates Gaia and Medea to opposite ends of a spectrum. Cronus (patricidal son of Gaia overthrown by his own son, Zeus, and banished to Hades) treats speciation and extinction as birth and death in a ‘metapopulation’ of species assemblages split into biogeographic realms. Catastrophic extinction events can be brought about via species engineering their surroundings by passively modifying the delicate balance of oxygen, carbon dioxide and methane – indeed, humans might be the next species to fall victim to our own Medean tendencies. But extinction opens up new niches that eventually elicit speciation, and under conditions of relative environmental stability, specialists evolve because they are (at least temporarily) competitive under those conditions. When conditions change again, extinction ensues because not all can adapt quickly enough. Just as all individuals born in a population must eventually die, extinction is a necessary termination.

We think the Cronus metaphor has a lot of advantages over Gaia and Medea. The notion of a community of species as a population of selfish individuals retains the Darwinian view of contestation; self-regulation in Cronus occurs naturally as a result of extinction modifying the course of future evolution. Cronus also makes existing mathematical tools developed for metapopulation theory amenable to broader lines of inquiry.

For example, species as individuals with particular ‘mortality’ (extinction) rates, and lineages with particular ‘birth’ (speciation) rates, could interact and disperse among ‘habitats’ (biogeographical realms). ‘Density’ feedback could be represented as competitive exclusion or symbioses. As species dwindle, feedbacks such as reduced community resilience that further exacerbate extinction risk (Medea-like phase), and stochastic fluctuation around a ‘carrying capacity’ (niche saturation) arising when environmental conditions are relatively stable is the Gaia-like phase. Our Cronus framework is also scale-invariant – it could be applied to microbial diversity on another organism right up to inter-planetary exchange of life (panspermia).

What’s the relevance to climate change and other human impacts on the Earth System? We’re struggling to prevent extinction in the face of multiple challenges, so understanding how it works is an essential first step. Without the realisation that extinction is necessary (albeit, at rates preferably slower than they are currently), we cannot properly implement conservation triage, i.e., where do we invest in conservation and why?

We had fun with this, and we hope you enjoy it too.

CJA Bradshaw and BW Brook

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By Barry Brook

Barry Brook is an ARC Laureate Fellow and Chair of Environmental Sustainability at the University of Tasmania. He researches global change, ecology and energy.

18 replies on “Life and death on Earth – the Cronus hypothesis”

Former New South Wales premier Bob Carr says there needs to be a clear-headed debate in Australia about nuclear power

The hearts, minds and pockets of the community need to be engaged in the debate.

It may come from left field but one thing that I have recently seen mentioned by other commentators is the “Homer Simpson” effect on much of the mind set of societies view of the Nuclear Industry.

I have watched with my children the Simpsons throughout their formative years where they and I dare say many adults were subtly programmed to focus through humour on a less favourable view of the nuclear industry. (I suspect it may well have had a greater influence than many anti nuke demonstrations on societies position in the late eighties and nineties and now).


A couple of points from a Tasmanian perspective. Remnant populations may survive one traumatic change but not the next one. On mainland Australia the thylacine is seen in cave deposits and aboriginal rock art but it was exterminated well before European arrival. However in the island refuge of Tasmania they were not able to adapt when the European invaders did arrive. Unlike macropods they couldn’t ‘breed up’ and they had also become inbred. This may yet happen to Tassie devils a related species. Conceivably remnants of H. sapiens may survive round one (the energy crisis, overpopulation etc) but succumb to the next blow, perhaps wide geographic separation of complementary resources like water and metals.

Linking this to the perennial topic of pumped hydro there is long cut in late Permian-early Triassic rocks where the high voltage cable extends from Gordon Dam to Hobart. That dam is 40% full despite 3 metres of rain and has vacant slots for 300 MW turbines. As you walk along this cutting you can see how marine life went though short lived revivals then became totally barren for tens of millions of years. Layers with fish put in an appearance, then later rocks elsewhere show reptiles, early mammals and so on. The point being that the dominant taxa in the revival were not descended from the dominant taxa before the extinction event. Perhaps Earth II will belong to the insects. Since there won’t be enough time to form more oil they won’t make the mistake of burning it.


Announcements of the impending extinction H. sapiens are premature.

Unlike most species our intellect allows us to adapt to change much more rapidly and much more successfully than any of the other higher organisms. Furthermore, as the hair-shirt Greens are always telling us, there are populations that manage to thrive using very little energy or materials. Consequently while standards of living could plunge if we keep pretending that we can do without nuclear energy, it is unlikely that we will be threatened with extinction over lack of energy.

Histrionics and hyperbole aside, what the energy debate is about is what standard of living we wish to attain, not about the survival of the human race.


There is no need of us in the Universe. Let`s take the death of our species lightly, and so we are doing. Why to cry when we humans are gone? No more misery, no more evil rogues and idiots.

Our days are counted, let AGW be the Devil, no problem.


Finntotal – just because you can’t get any, and are so self absorbed that you think the Universe should care, doesn’t mean the rest of us think humanity is a lost cause. YOU are a lost cause – put yourself out of your own misery as you like, but how dare you be so arrogant and conceited to suggest that the whole human race should follow.

I am sick of these self-loathing nothings who haven’t made a life for themselves waxing on about how Man should vanish and how perfect the world would be without us.

The meaningless of your existance is not a lien on the rest of us, like the old song says, we have a right to be here.


I’ve always thought Gaia was a rather silly fairy story … the idea that the planet optimises for
things we think important (life friendly conditions) seems as superficially silly as directional
Darwinism. Cronos is a closer descriptive fit to what really happens. But does it have
predictive value?


Clearly humans are more complex than any earlier dominant species. In trying to justify exports of brown coal (1.4 kg CO2 per kwh) from Victoria to India the rationale is altruistic

Victoria cannot unilaterally limit global emissions, the coal products could help developing countries overcome poverty, and emissions may be even higher if countries are denied Victorian coal and use dirtier fuels

Well that’s OK then not sure if they could find a dirtier fuel, old tyres maybe. T. rex never claimed to be helping its fellow dinosaurs. Humans are different to anything before in biological history; they can recognise signs that the commons are being overused but talk themselves out of evasive action.

As we speak the Lake Eyre pelicans could be providing an example of what’s in store


Geoff said “But does it have predictive value?”

We hope so, but the idea needs to be developed further to generate some useful hypotheses for testing. We wrote this when speculating about that:

“Analogous to Lovelock’s parable of Daisyworld for applying a mathematical framework to the Gaia hypothesis (Lenton & Lovelock 2000; Watson & Lovelock 1983), a Cronus view of evolutionary and extinction dynamics could be modelled by modifying existing metapopulation tools (Hanski 1998, 1999). For example, species as individuals with particular “mortality” (extinction) rates, and lineages with particular “birth”; (speciation) rates, could interact and disperse among “habitats’ (biogeographical realms). “Density” feedback could represent anything from competitive exclusion to parasitic, mutualistic or commensal symbiosis.

As a “population” (species) declines, perverse feedbacks such as inbreeding depression can induce Allee effects (Courchamp et al. 2008) that further exacerbate extinction risk – this is one Medean-like phase of the population analogy represented by Cronus. In contrast, stochastic fluctuation around a “carrying capacity‟ (niche saturation; energy limitation) achieved through compensatory population dynamics arising when environmental conditions are relatively stable becomes the Gaia-like equilibrium embedded with Cronus. The Cronus model also has the advantage of being scale-invariant – it could be applied to the turnover of microbial diversity inhabiting a single macro-organism through to inter-planetary exchange of life. When combined with more theoretical development (and, ideally, experimental or numerical testing) of the thermodynamic model of biological entropy, Cronus mathematics can be used by evolutionary ecologists, palaeontologists and exobiologists to pose and test novel hypotheses regarding the ever-changing patterns of life on Earth.”

The metapopulation model development of Cronus would be the next step — something Corey and I have been talking about how to do.


Perhaps this is flogging an analogy too far but the parallels I see between the Lake Eyre pelicans and humans is
LEp Discovered they could exploit a temporary lake full of food.
Humans Discovered they could exploit half a billion years of fossil fuel accumulation.
LEp Bred like crazy and depleted the resource.
Humans Bred like crazy and depleted the resource.
LEp Though weakened by lack of fuel a few made the effort to seek out new sources of energy.
Humans ditto
LEp A few members of the species survive to rebuild the colony.
Humans Not sure what happens, check back later.


Nice hypothesis. It seems a commonsense idea that life can have both a stabilising and de-stabilising effect on planetary conditions, depending on circumstances.

I’ve got no problem with Gaia theory in its strictly non-teleological form (it’s not the planet that does the optimising, the stable conditions are an emergent property of the system as a whole). Though like many, I really don’t like the name. You could argue that anybody who talks about an Earth ‘system’ is tacitly accepting the important idea in Lovelock’s thinking.

I don’t know much about Medea, but it seems to assume that all the major extinctions in Earth’s past were caused by life. I’d heard of the oxygen catastrophe (and the follow-on first snowball Earth event) being caused by the first photosynthesising microorganisms, but I’d understood that many (if not all) of the later extinctions had abiotic causes such as flood basalt eruptions/asteroid impacts etc. Is there any substantial evidence to suggest otherwise? If not, the Medea hypothesis rests on shaky foundations…

Of course there may be periods when unstable positive feedbacks develop, and Cronus is nice because it’s an overarching idea which links them to the stable periods, but in the case of the Earth these episodes clearly haven’t been overwhelming destructive and de-stabilising, or we wouldn’t be having this discussion now. Maybe these are the early coarse fluctuations which occur as a planetary biogeochemical system becomes established?

Perhaps we need to look at more planets to settle this. Roll on ESA’s Darwin mission!


Ok, let me see if I understand this. Take a population following a simple logistic model rP(1-P/K).
At small P, r isn’t a constant at all but has a probability distribution with, possibly, P as a parameter.
So, when finding a mate gets hard, r can vanish … allee takes over. Under the Cronus “mapping”, as
the number of species decline, does the rate of speciation go up or down? If Cronus is correct, speciation
should decline. Yes? Consider the extinctions driven by hundreds of millions of sheep and tens of
millions of cattle in Australia. I’m guessing that cattle carve swathes through an area and could
create reproductively isolated populations. e.g. graze along a valley and wipe out valley limited plants
and isolate plants on opposite facing hillsides. With more reproductively isolated populations, shouldn’t
speciation increase?


Geoff, I don’t think so, at least in your sheep/cattle example. Unless we’re talking about stable environmental conditions over geological timescales (or at the very least tens of thousands of years) there’ll be insufficient time for speciation, and the Allee effect is likely to be more important- certainly in species adapted to a higher population density (passenger pigeon would be a good example).

I’ve heard it said that it takes about 2 million years for species diversity to fully recover after a major extinction event, presumably this is based on fossil evidence.


The notion of a community of species as a population of selfish individuals retains the Darwinian view of contestation

In “The Selfish Gene” Richard Dawkins very convincingly makes the case that the pure Darwinian selfishness is an attribute of genes not individuals. And he puts it together with some very neat game theory and mathematics. In doing so he explains why individuals are frequently not selfish and the circumstances where altruism is most likely. I don’t think anybody should venture into this field without Richards book very high on their reading list.


Matt #14, thanks, that is my view also.

TerjeP #15, you are right that Dawkins focuses on the selfish gene rather than individual choices. The species in the Cronus model is, in many ways, more akin to a gene than an individual, because species don’t make ‘decisions’ (this would imply group selection, which is pretty well discredited).


You could also look at the ‘selfish gene’ component as species insofar as extended phenotypes are possible. Symbioses from parasitism right through to mutualism would give a spectrum of interspecific interactions ranging from pure exploitation right through to mutual benefit (cf. siblicide to kin rearing). There’s also the fuzzy idea of species in the first place and the gradient of gene complexes among them, so defining where and when a species becomes another is as difficult as drawing a definitive genetic line between individuals.


It would appear metaphors such as of the “Medea hypothesis” and “Cronus hypothesis” promote, or at least hint at, an inherent self-destructive nature of life.

A paradigm applicable to “H. Sapiens”.

On the other hand, the great mass extinctions of species were in the main triggered by external forcing, including endogenic volcanic eruptions and extraterrestrial asteroid/comet impacts.

The latter resulted in some instances in the release of CO2 and methane from target rocks such as carbonates and carbonaceous shale , where they accumulated as a result of biogenic activity.

Otherwise the great mass extinction events were essentially due to external forcing unrelated to notions of self-destruction, which renders terms such as “Medea” or “Cronus” less than consistent with the origin of these events.


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