To quote a famous fictional lion, the Earth is a beautiful circle of life — predators and prey, plants and animals keeping each other in perfect balance. This idea — of Earth's lifeforms as epitomizing a collective, life-cultivating system — has a long history: the ancient Greeks called Earth "Gaia," the mother of all life. In the 1970s, two scientists hypothesized that planet Earth is really one giant super-organism, with all animals, plants, fungi and other life actively influencing (and encouraging) one another's evolution; Lynn Margulis, an evolutionary biologist, and James Lovelock, a scientist, environmentalist and futurist, accordingly called their theory the Gaia hypothesis.
"Life clearly does more than adapt to the Earth," Lovelock told Salon in an interview from 2000. "It changes the Earth to its own purposes. Evolution is a tightly coupled dance, with life and the material environment as partners. From the dance emerges the entity Gaia."
It is a nice idea, to think that life begets life — or that life on Earth works towards a larger purpose of self-perpetuation. The only problem is that this isn't a universally accepted proposition among scientists. Indeed, some believe the opposite is true: life on Earth doesn't have a larger goal, but rather, could accidentally and at any moment evolve in a way that would kill off great masses of life — or even all life.
Evidence for this much more cynical theory about life on Earth includes, for one, the numerous mass extinctions that have occurred throughout Earth's 4.5 billion year history, some of which extinguished 99 percent of all life. Some of these extinctions were caused not by asteroids, but by life itself behaving in its own self-interest.
Thus, some scientists say that Gaia, the mother of life, is a poor choice of deity to serve as a metonym for Earth. A better one might be Medea, an enchantress in Greek myth who murdered her own children.
"This name thus seems appropriate for an interpretation of Earth life ... to be inherently selfish and ultimately biocidal."
In the late 2000s, Peter Douglas Ward, a paleontologist and professor at the University of Washington in the geology, biology and astronomy departments, coined the term "Medea hypothesis" to describe the ways that complex life eventually generate the circumstances to drive their own extinction. The term was meant to evoke the Gaia hypothesis, the Medea hypothesis being somewhat of an opposite.
We see Medea's handiwork all around us in the fossil record. The Great Oxidation Event, some 2.45 billion years ago, is a prime example. Back then, the planet was all but devoid of oxygen, and microbial life metabolized just fine without it. But then cyanobacteria hopped on the scene, which evolved photosynthesis. That ended up spitting out tons of oxygen over millions of years, which was toxic to most life on earth, causing widespread death that shows up in the fossil record.
We also see Medea in the present, as the Earth undergoes its Sixth Mass Extinction, this one caused in large part by human activity. It's why we see billions of Alaskan snow crabs suddenly disappearing or thousands of other examples of ecosystem collapse. It's why climate change is so threatening to our continued way of life and why humanity may even join the dinosaurs in the dustbin of natural history.
"Why I started thinking about Medea [is that] I really disliked the Gaia hypothesis. Gaia is not even a theory to me, it's just New Age nonsense."
"This name thus seems appropriate for an interpretation of Earth life, which collectively has shown itself through many past episodes in deep time to the recent past, as well as in current behavior, to be inherently selfish and ultimately biocidal," Ward wrote in his 2009 book, "The Medea Hypothesis." "A result of this bad mothering, I propose, will be a shortening of the time that life will exist on our planet. Life will do this to itself by unconsciously changing environmental conditions to a point where there can no longer be plant life or, ultimately, any kind of life."
The Medea Hypothesis might be epitomized by the story of photosynthetic life on Earth. As Ward explained in our interview, plant life on Earth has a repeated tendency to evolve in a way that it removes carbon dioxide from the atmosphere. Reducing the amount of carbon dioxide in the atmosphere then freezes the entire planet, killing the majority of life that depends on Earth being, well, not frozen. This has happened many times in the fossil record.
Salon spoke with Ward about the basis for this theory and what it means for life on Earth — but also the implications for finding life on other planets, including Mars.
This interview has been condensed and lightly edited for clarity.
What got you interested in extinction events?
Peter Ward: What really struck me and why I started thinking about Medea, is I really disliked the Gaia hypothesis. Gaia is not even a theory to me, it's just New Age nonsense. So much good came out of it. Lovelock was no fool. But this idea has been bastardized to the point of thinking that life knows that we humans are putzes and will clean up our mess.
One of the really driving definitions of life is that, given a chance, any species will do anything it can to have all the resources. There's no altruism in nature. Take everything you can, reproduce to the point that you control everything. This is what humans have done. Other species have tried it in the past, they've just never had the ability technologically. But where does that drive come from? I think it's so innate within the structure of DNA and how cells are constructed, this dominance principle. Altruism is nonsense.
When I first encountered this idea of the Gaia hypothesis, I found it very seductive. It's very cute. It's a nice little narrative and there's some evidence for it, I can sort of see that. If you zoom out and you look at the planet, it's like this shelled organism that has a system, just like a microscopic cell. Like, it has a bunch of different components in it, that are all working together, that all evolved independently and then assembled. So maybe you can apply that same sort of lens to the entire planet. But in the Medea hypothesis, what are some of the most pointed examples?
Carbon dioxide has to be the most powerful molecule in all biological systems. We're going from 360 parts-per-million CO2 molecules [in the atmosphere] among a million other molecules... all we have to do is go up to 450 or 500 [parts-per-million]... I mean, you're adding another 100 out of a million, and the world goes from benign to completely insane. I mean, completely ends of the world, adding just 100 more [per] million molecules. When you think about the power, that is unbelievable.
CO2 runs the world. And we're on such a knife's edge. Long-term climate stability is partly plate tectonics, but an awful lot of is the carbonate feedback system. When you have CO2, it gets warmer. When it's warmer, chemical weathering increases in rate and therefore it gets colder and colder till now, chemical weathering isn't working as fast. Then you have more volcanoes pumping out CO2. So the balance is up, down, up, down. Three billion years of balance, then along come plants. And photosynthesis kicks in and CO2 is ripped out of the atmosphere, more and more of these plants grow. And then boom, the whole world freezes. Well, the first time plants get roots, boom. Life makes these innovations into very narrowly controlled systems. And it all goes to hell. It's not like life's mean, it's just the bad puppy idea. It f**ks up and knocks things over. Gaia would never do that.
It's embarrassing to admit, but I used to not really believe in climate change, like over a decade ago, until I started looking into the science. And that statistic was one of the things I used to dismiss it. Like there's only 400 parts per million of CO2 in the atmosphere? How could this have any influence? But it's amazing how even that small change is so impactful.
It's scary. The nice thing about deep time is you really get to relive this stuff. You asked, why am I wrapped up in death? Well, it fascinates and horrifies me. But there's such powerful lessons that you can see. The fossil record doesn't just tell us about the past. It predicts the future.
And usually after a mass extinction, I don't know how long, probably a long time, but there's sometimes an explosion of life. The Cambrian Explosion came after a mass extinction, right?
Yeah. But I had this argument with the SETI [Search for Extraterrestrial Life] dudes, back in the day, Seth Shostak and I used to go back and forth. Someone said, "mass extinctions are good things. And in fact, if we hadn't had the critical number we did, we probably wouldn't have the diversity we do." It's equivalent to thinning out the garden to get your other stuff to grow. But I had to point out that following the Permian mass extinction, it took 2 million years to get anything close to what was there before. I personally don't think I want to wait that long. It's a long dead period.
"If there's life on Venus it's up in the clouds. But I've never liked that idea. I think there's a much better chance around Jupiter's moons."
When teaching my undergrad class, I started really looking at how bad ice ages are for planets. I've been to Antarctica four times. What struck me was how little life there was down there.
And if you go all the way back to the snowball Earth — we had one [event] at 2.1 billion years ago and we had one 700 to 600 million, where the planet froze over. You can see it in carbon isotopes. Carbon isotopes are really a meter of the quantity of life on the planet. And every time we have ice ages, the amount of life on this planet drops. Why do we have these cold periods? Why is it the longest Ice Age in Earth history followed [by] the first forests? Plants did it every time.
Every time roots figure out how to go deeper, they break up more rock below them. More minerals with silicate minerals in them get exposed to CO2, they turn into clay, they pull the CO2 out of the atmosphere. Boom, Earth becomes cold. It's life that does it every single time. Every one of the mass extinctions, it's life that kills itself off.
It's like a stupid puppy. It runs amok. It doesn't have evil intentions — that's not possible. This is not some guiding principle. I just have the sense that life is just this clumsy oaf.
What does the Medea hypothesis tell us about the search for life on exoplanets?
There's only a few ways you could make life. Life is probably going to always be affected by natural selection, it's going to have to metabolize, it's gonna have to reproduce. And this is what NASA says. The three parts, if that's how it works, then it is going to be this blunt instrument. Once you start introducing these little chemical factories — that's what a bacterium is, a tiny, little chemical factory — there's only three morphologies.
Bacteria are a little spheres, little rods or little spirals. That's all they can do. Three things. Not a lot of body plans. So when their environment gets f**ked up, they build new chemicals. I can't change this. I can't change myself physically. But I'll change where I'm living chemically. I will make it chemically different. I can make it chemically warmer, chemically colder, I can change the toxins. All I can do is build chemicals. That's profoundly different.
There's probably no life on Mars. But if there's bacteria — if Mars ever had life down deep in the rocks, you're probably gonna find it [there].
Could Mars life have killed itself off in the way that Medea tries to kill itself off here? Of course it could. Same principles are going to apply. Life does some blunderbuss thing on a system that's less forgiving than Earth was. Smaller planet, gas is thinner, you don't have many minerals. You've got a much smaller toolkit to build yourself with. So the margin of error is probably narrower. Surely, blundering life [on Mars] could easily knock itself off.
What about life on Venus or Jupiter's moon Europa?
Well, in the early Earth history, the sun was way less energetic. So Venus wouldn't have been as nasty as it is now. But Venus is the bad case history for too much CO2. It just got hotter and hotter. And then sooner or later, it lost its surface water. And once you do that, you've got a runaway greenhouse [effect].
If there's life on Venus it's up in the clouds. But I've never liked that idea. I think there's a much better chance around Jupiter's moons. The cool thing about Jupiter's moons is that you've got oceans down there. And if you look at Europa, you see all these craters in it. Well, those craters would punch right through. Any asteroid punching through is going to throw a whole bunch of seawater up into space. Those moons are being orbited by frozen water that was blasted out of them. If there was life there, you don't have to go [down] into it. You're gonna find it in orbit around it. So the joke among us is, let's just get a scoop and go fish up the fish frozen in orbit around Europa.
about the history of life on Earth