One argument against the Gaia Hypothesis is that Gaia does not reproduce. Maybe Gaia is simply not yet mature. And maybe she is maturing even now.
Disclaimer: I am not an expert on this subject. Further, as an intelligent, educated layman, I deplore the approach to science so often seen in the popular media that oversimplifies scientific ideas, often to the point of absurdity, and perpetuates ignorance rather than promoting genuine understanding of modern science. For examples of this absurd oversimplification, read almost any article in the popular media about the debate between the proponents of the "Out of Africa Hypothesis" and the proponents of the "Multi-Regional Hypothesis." (Mind you, I'm not saying either of these competing hypotheses is absurd, I'm saying that the presentation of these hypotheses in popular media is often absurd.) Nevertheless, I am diving into the controversial "Gaia Hypothesis" with incomplete knowledge and possibly flawed understanding of the thought behind this hypothesis. The idea I am presenting here is perhaps best discussed late in the evening around the campfire with a mild buzz.
The Gaia Hypothesis, first formulated by Dr. James Lovelock in his book, Gaia: A new look at life on Earth, holds that the entire biomass of the Earth, interacting with the atmosphere, crust, and other abiotic features, may be regarded as a single organism, and one which creates and perpetuates conditions on Earth that favor its own development. You can read more about the hypothesis and its history at http://en.wikipedia.org/wiki/Gaia_hypothesis. Or read a more updated presentation of the hypothesis in The Quest for Gaia by Kit Pedler.
A classic supporting idea of this hypothesis involves the interaction between the tectonic carbon cycle and living organisms that draw carbon from the atmosphere. (This tectonic carbon cycle is not the "carbon cycle" that describes the flow of energy through the food web, but the larger, slower exchange of carbon between the atmosphere and the lithosphere of the Earth.) As carbon dioxide accumulates in the atmosphere, Earth's climate becomes warmer, and carbon-sinking organisms can live at higher latitudes, farther from the equator. That is, when the atmosphere is rich in carbon, the reef-building organisms and diatoms and such expand their range and population, drawing more carbon out of the atmosphere and locking it into the sea-floor. This tends to decrease the concentration of atmospheric carbon dioxide, causing the climate to become cooler, and carbon-sinking organisms retreat to lower latitudes again. The carbon locked in the sea-floor is returned to the atmosphere in volcanic emissions after the sea-floor has been subducted into the Earth's crust as a result of tectonic activity. There is a constant exchange of carbon between the atmosphere and the sea-floor, and this exchange is moderated by the activity of reef-building and shell-forming organisms. The moderation by living organisms tends to keep the climate relatively stable despite periodic changes in solar warming of the Earth and of volcanic activity. Thus, the action of living organisms tends to promote conditions favorable to life.
Within the Gaia Hypothesis camp are arguments about what Gaia "wants." Does the interaction of the biota with the Earth tend to promote maximum biomass of life, or maximum diversity? Or something else?
Never mind all that. Some opponents of the Gaia Hypothesis dismiss it out of hand, calling it the result of flawed thinking, perhaps with chemical impediment to thinking, and more of pagan mysticism than of science. Other opponents take it seriously enough to offer serious, scientifically based arguments against it. One such argument is that Gaia does not reproduce. One of the defining characteristics of living things is that they are capable of reproducing themselves, and while Gaia may be seen to perpetuate itself, it does not reproduce itself.
Well, what about that?
A caterpillar is not capable of reproduction. It has no reproductive organs at all. All it can do is breathe, walk, eat, and grow. If you were an investigator from another planet studying caterpillars, you might be totally mystified as to where baby caterpillars come from. Examining the caterpillar itself, you could not possibly imagine a butterfly. Why would you think that this fat eating machine might metamorphose into a delicate, beautiful flying thing that is capable of reproducing? You'd never imagine it until you saw it.
Maybe Gaia is like a caterpillar. Maybe it is immature, and it will some day change into something that we can not imagine, something that is capable of reproducing itself.
Maybe it's happening now.
Take another look at that caterpillar, other metamorphosing insects, and many other complex animals. When young, they eat and eat and grow and grow, and not much else. When they become reproductively mature, they seek mates, produce offspring, and die. Many insects in their adult form are not even capable of eating. A Pacific salmon that reaches reproductive maturity is doomed.
Even among birds and mammals, even humans, the onset of reproductive maturity is the beginning of decline of the individual. From our late teens, the plaque is accumulating on our arterial walls. Our immune systems and our healing abilities are not what they were when we were children.
Often, when living things become capable of reproduction, they begin to deteriorate. It happens faster in some species than in others, but in nearly all animals, maturity and decline come together.
Look what has happened to Gaia in the last millennium or so. As human technology advances, we are destroying the Earth at an ever-increasing pace. Some would say we have already passed a point of no return, and Earth will become incapable of supporting us despite any effort we can take to save it.
Others, wide-eyed optimists, say that we can move somewhere else. Indeed, many say that we can and must and will move to other planets regardless of whether the Earth continues or not.
But where will we go? Even the widest-eyed optimists concede that there is no life-sustaining planet within human reach, and our reach is not likely to expand to include any "new Earth" for many centuries.
Undaunted, they speak of "terraforming." We don't have the technology in hand yet, but we can realistically envision a day, not too far into the future, when we can modify the atmosphere and environment of another planet in such a way that it becomes suitable for life. Some say we might be able to do this with Mars or Venus or some of the larger moons within the solar system. Perhaps more realistically, we might terraform one or more planets in other not-too-distant star systems which are more Earth-like than any of our planetary neighbors.
Now, let's take another look at where this rambling discussion has brought us.
Human beings are sort-of realistically talking about producing "other Gaias" out there somewhere. And the emergence of this technology is exactly what has brought about the decline of the original Gaia.
Perhaps the emergence of advanced technology on Earth is the maturation of Gaia. As Gaia becomes capable of reproduction, she also begins to deteriorate.
Perhaps we humans are the reproductive organ of Gaia.
Epilogue 1: Incidentally, I'm at least a little skeptical of this "terraforming" thing. Never mind ethical considerations about our right to interfere with nature on a planetary scale (and never mind that we already have), I don't think it's technically feasible. I don't think it will be technically feasible even for millennia to come. The problem is that we don't understand the full complexity of what it takes for a planet to support advanced life, and when we do come to understand it, we will find it so complex that we simply will not be able to reproduce it. For an inkling of how complex this is, read Rare Earth: Why Complex Life Is Uncommon in the Universe by Peter Ward and David Brownlee.
Epilogue 2: Having read a little further, I have found that this compound idea that humans are the reproductive organs of Gaia, and that the present decline of Gaia is related to maturation, is not new. Well, I did think of it without influence from other sources, even though I was not the first to have or to express the idea.