The Gaia Hypothesis

Somehow I stumbled upon the Gaia hypothesis a few days ago, a really interesting idea which I'd like to delve into a bit further. The theory goes like this -

The Earth itself is in a state of homeostasis where the biological systems are in symbiosis with its surroundings.

That's a little hard to understand stated so - let me start with the classical example. Imagine there is a planet with two different species of daisies, black and white. Over the course of time, the sun that the planet orbits around goes through standard temperature fluctuations: for a few thousand years the sun grows hotter, and for a few thousand years it grows cooler, and vice versa. On a planet without daisies, the temperature on the planet would fluctuate just as widely.

However, on a planet with daisies, the plants will be placed under a evolutionary selection process as the sun fluctuates. For instance, as the sun grows hotter, the temperature of the planet will grow higher until it begins to have a negative survival impact on the daisies. The daisies that survive will be the ones that absorb the least amount of heat - in this case, the white daisies, which are better at reflecting the suns rays back into space. The white daisies now are in a great majority, and theoretically at least, they have a planet-wide effect: the heat they reflect makes the planet itself cooler.

If the sun were to cool back down again, now the daisies that predominate are those that can absorb heat, the black daisies. They will tend to absorb more heat, and thus keep the planet warmer than it would have been. So we see that the daisies themselves are in a kind of equilibrium with the sun, and ensure that the temperature of the planet remains palatable to life.

This effect is possible with a variety of mechanisms - not just the reflectivity of the flower. For instance, as the temperature of the planet gets higher, trees will grow faster and with more leaves, converting carbon dioxide back into oxygen. Since CO2 is a greenhouse gas, as its percentage decreases, the temperature of the planet cools. Similarly, as it gets colder, trees grow slower and have sparser leaves, allowing the amount of CO2 in the atmosphere to come back up (as a consequence of respiration by animals like us), and thus warming the planet again.

Could this be the reason why trees lose their leaves in winter? Are they attempting to heat the earth back up again?

Probably not, according to most climate scientists. The current belief in the Gaia theory is that it is interesting, and might explain small effects, but the effect it is supposed to be driving, even a planet full of species, could not make significant change on a planet-wide scale.

Other skeptics, among them the famed evolutionary biologist Richard Dawkins, are against the theory because individual organisms have no reason to try to pursue an effect that would only be helpful to the group as a whole. That is to say - a species cannot be evolutionarily selected to benefit a planet.

I'm not as skeptical - the theory at least is quite interesting. I'd like to run a little climate model to examine how powerful the effects would have to be, and how many organisms would be needed to breathe to drive CO2 levels enough to make a difference.

The beauty of this system is that it would partly explain the anthropic principle. The anthropic principle is, in summary, that conditions on Earth are incredibly rare - it is unimaginably hospitable to life. Therefore, the only reason we are here to observe it, is that if we weren't here to observe it, no one else could observe it either. So no matter how rare the circumstances for life might be, since life is the only thing that could observe it, it will always look rare.

But a planet doesn't need to have ideal conditions for life. It just needs to have a little bit of biological activity (in our case, early prokaryotes that converted oxygen to CO2), and then trust that the growth of the lifeforms along with the feedback cycle would lead, inexorably, to life prospering more and more.

I think negative feedback mechanisms are deeply underestimated. They are all over biological systems, and so it might seem like they are understandable as relatively small effects that apply on a per organism (or even community of organisms) level. But I don't think it's unreasonable at all to think that that such feedback mechanisms could drive planetary evolution itself.

That's probably the deepest part of this whole theory. When organisms are selected for survival, it is in their ability to reproduce that the change is "measured". That is, if you have 10 different subtly different parrots, the one that can open the nuts and feed itself, and mate with other parrots best, is the one that propagates its genetics to the next generation.

But under Gaia, there isn't really a next generation at a planet level. If organisms decide to pursue a "runaway" effect that leads to global catastrophe, the planet doesn't get a do-over.


  1. Gaia Hypothesis.