The "Gaia hypothesis" is the brainchild of the "independent scientist" and interesting character James Lovelock, who in the early 1970s proposed that the entire Earth was alive.
According to him:
"During the time, 3.2 billion years, that life has been present on Earth, the physical and chemical conditions of most of the planetary surface has never varied from those most favorable to life".He claimed that this was because the living and nonliving components of the world cooperate, and together display a capacity for self-regulation that is (he said) analogous to homeostasis. Thus, the whole of the Earth, by extension, is analogous to a massive single organism. He named this "organism" after the ancient Greek goddess of the Earth, "Gaia" (This NOVA documentary does a great job of thoroughly explaining the reasoning behind the idea.).
Many people see this idea as the scientific embodiment of "Mother Nature", and though to some the existence of Gaia seems incredibly far-fetched, others see it as being rather self-evident and undeniable; where some see an insightful revelation and appealing world perspective (especially when considering phenomenon such as climate change), others see a misguided logic, or even an ideologically-inspired pseudoscientific concept. But despite the controversy, those who actually understand Gaia (at least as Lovelock conceives it) are few and far between.
Why is an understanding of Gaia so elusive? And is it a valid scientific concept?
For starters, the very name, "Gaia", is highly misleading, and often provokes an automatic bias one way or the other. Lovelock admits it has given him trouble in terms of scientific acceptance, although I have not heard him lament it's effect on popular acceptance (or book sales). In any case, he hasn't exactly done a great job of compensating for this effect with his language (by referring to Gaia as "she", for example).
Moreover, Lovelock himself now has a vastly different conception of Gaia than he did forty years ago. He says this is because he has come to "better understand Gaia theory" over the years, but it might be more accurately stated that it is because he has had to disown many of his original claims: he has now abandoned the idea (expressed in the quote above) that there is such as thing as a particular set of conditions that are "most favorable to life"; and he no longer insists that Gaia is consciously trying to self-regulate itself. I personally think it is to his credit that he has recognized these blatant errors, and has attempted to compensate for them. But it is important to remember that these were not small details: they were some of his main points!
One inevitable consequence of this back-peddling has been that Lovelock's Gaia has been made a bit mundane- it's implications are certainly distinctly less profound than they were forty years ago. Another consequence is that many different interpretations of Gaia now exist. This huge range of views on the subject makes it almost impossible to determine what, exactly, someone actually means when they use the term "Gaia hypothesis", which makes even more difficult to properly understand and scrutinize.
"Ok, so people dont agree on what it is. But is Gaia real?"
It is worthwhile noting that the Earth is not really analogous to an organism at all. In fact, the ability to perform homeostasis is but one of life's defining traits. For example: the Earth doesn't reproduce; and neither did it undergo evolution by natural selection. And yet every other single thing that we describe as being "alive" has met these conditions. By saying "the Earth is alive" what you are, in effect, doing is redefining the word "life"; Lovelocks great analogy is nowhere close to perfect. Gaia isn't an organism: "she" is just a system (I doubt, however, that "the System hypothesis" is a title that would have moved quite as many of Lovelock's books).
It is also interesting to ponder how humans fit into all this. Are we a part of Gaia or not? This is not a petty distinction: if we are then Gaia is in trouble: humans aren't exactly helping to regulate the climate at the moment. On the other hand, how can it be that humans are distinct from Gaia? Are we not also of the Earth?
In this interview with the CBC's David Cayley, Lovelock explains that the greatest opposition to this theory has historically come from biologists, who (according to him) correctly perceive it as threat to Darwinian orthodoxy: after all, how could Gaia ever arise from a simple struggle between organisms for "the survival of the fittest"? He even suggests that the theory of evolution should be modified to account for the existence of Gaia.
But I think Lovelock missed the point: after all, the idea of an ecosystem is not foreign to Darwinism, so why should the idea of a global ecosystem constitute a threat to it? I think the bottom line is that where Lovelock sees Gaia, biologists see an equilibrium - and they are distincly unsurprised by this because equilibriums are the natural state of any system. Gaia does not threaten Darwin: it is simply that a Darwinian outlook does not require Gaia to explain anything.
"So what does Gaia explain?"
The quote I used at the beginning of this post suggests that Lovelock's original intent was to explain why "the planetary surface has never varied to those most favorable to life". But as we have seen, even he now admits that this is based on a false assumption. So I find myself at a bit of a loss as to how to answer this question.
And it occurs to me that an idea that is so grand to deserve the name "Gaia" - and one that increasingly referred to as a "theory" - should be able to do a lot better than this. Maybe I just don't get it, but as I currently see things, Gaia - however appealing - is just a highly misleading and confusing concept, that also happens to be entirely devoid of any practical value. Don't get me wrong: James Lovelock is no crackpot by any means. He has made undeniably valuable contributions to science. But I find myself currently of the opinion that the Gaia hypothesis is just not one of them.