Gaia Hypothesis and Daisy World

Gaia Hypothesis and Daisy World

The development of the Gaia Hypothesis is described with some emphasis on how the concept has evolved in response to other scientist's skepticism. The Gaia concept itself is described and discussed. A possible means for reconciliation of the holistic Gaia hypothesis with reductionist thinking is discussed. I conclude by summarizing what the paper has accomplished.

The Gaia Hypothesis was first published by James E. Lovelock in 1972 (Lovelock, par. 23). It was, at the time, the end product of a series of observations Lovelock had made about Earth, Mars and Venus (Margulis and Lovelock, p. 11(2)).

The premise underlying the Gaia hypothesis is that the entire planet evolves over time because of the interaction of living things with their surrounding environment. Because of the interdependence of the evolution of environment and biome, Lovelock likened the entire planet Earth to a living thing with complex interlocking planet-wide systems that maintain a condition of homeostasis conducive to Earth's long-term habitability.

While the existence of symbiotic relationships between some species has been long observed and accepted, the ultimately symbiotic relationship of every living thing on Earth proved too broad a claim for many scientists to accept. Consequently, the Gaia hypothesis has been generally lambasted by most mainstream scientists.

In the next section, I describe the hypothesis in some detail and provide background information. This will be followed by a discussion contrasting Lovelock's claims with those of his critics and proffers an approach toward reconciliation and my concluding comments.

Background.

In 1957 James E. Lovelock invented a device called the electron capture detector that can accurately measure minute traces of specific elements and chemical compounds. He used this in Antarctica to detect the presence of CFCs in the atmosphere as well as the presence of DDT in many samples from around the world, including human breast milk (Holden, p. 1977). In other words, James Lovelock is the person more responsible than any other for getting DDT and CFCs removed from the market, for it was his invention that first allowed accurate measurements of these pollutants.

In 1961 he was invited to join NASA and contribute toward America's exploration of Earth's moon (Lovelock, par. 16). Later, he found himself advising NASA on its Viking Project planned to land stationary probes on the Martian surface. Part of the mission goal was to establish whether life was present on the Red Planet.

Before the mission was launched, Lovelock advised NASA to cancel the biological research portion of the mission because, he surmised, there was not currently any significant life on Mars. His reasons had to do with his examination of the Martian atmosphere.

Mars' atmosphere consists primarily of carbon dioxide at very low atmospheric pressure. It is essentially chemically inert and thus in a state of chemical equilibrium. Lovelock reasoned that were there living things on Mars capable of photosynthesis, we would expect to find free oxygen in the Martian atmosphere. He drew a similar conclusion based on the absence of methane from Mars' atmosphere. Methane is a common byproduct of the decomposition of dead plants and animals. With neither methane nor oxygen in detectable quantities in Mars' atmosphere, there seemed ample reason to conclude that nothing lives there that could be detected by the Viking landers' (there were two of them) instruments.

He drew the same conclusion about the likelihood of finding life on Venus, based on similar evidence.

Earth's atmosphere is not in a state of chemical equilibrium. It is full of gases that have an affinity for each other. Oxygen, nitrogen, and methane exist in sizable quantities, and left to their own devices, would quickly interact to form a new atmosphere of inert gases, very much like Mars' atmosphere (Margulis and Lovelock, p. 13(2)). So the persistent presence of uncombined methane, nitrogen, and oxygen in the atmosphere is strong evidence that something is replenishing these gases as fast as they naturally interact.

The answer is life (Margulis and Lovelock, p. 88 (1)). Organisms that can perform photosynthesis continuously pump oxygen into the atmosphere faster than it can react with methane or nitrogen. Microbes break down nitrates in the soil and pump out nitrogen faster than it can react with atmospheric oxygen. Decaying flora and fauna release methane into the environment in the same way. Essentially, the current state of Earth's atmosphere is the inevitable result of the kind of things that live here.

This realization was the seed for Lovelock's Gaia hypothesis. Among other things, the Gaia hypothesis proposes that the complexity of Earth's ecosystem is analogical to the complexity of an organism. Thus, Earth, while not being actually alive, is like a living thing in many respects.

One of the key features of the hypothesis is that the Gaia principle operates to maintain a condition of environmental homeostasis, just as an organism maintains a condition of biological homeostasis. If a person gets overheated, his or her body automatically responds to restore the previous state by perspiring. Moisture moved to the surface of the skin is subject to the cooling process of evaporation.

Lovelock surmised that Earth has planet-wide systems operating in similar fashion to preserve environmental homeostasis. Consider the following scenario in which carbon dioxide is allowed to accumulate to high levels in Earth's atmosphere.

A surplus of carbon dioxide actually promotes the growth of vegetation two ways. First, vegetation requires carbon dioxide to perform photosynthesis, so more carbon dioxide leads to more vegetation because it facilitates this process (Laurence, p. 96). Second, carbon dioxide is a greenhouse gas, and if enough accumulates in the atmosphere there is a proportional increase in average global temperature. One implication of this is that the annual freeze lines in both hemispheres gradually move closer to the poles, freeing up more territory for plants to exploit.

With a larger percentage of Earth's surface covered by faster-growing vegetation, more carbon dioxide becomes impounded in the new biomass. The ratio of carbon dioxide to oxygen falls. The annual freeze lines move away from the poles as Earth cools, thus reducing the territory vegetation can exploit. The net effect is that long-term ratios of atmospheric gases (and consequently, global temperatures) vary within otherwise very stable parameters.

This feedback loop is fundamental to the Gaia hypothesis; life influences its environment, which in turn influences the proliferation of life.

Discussion.

In spite of what may seem innate plausibility, the Gaia hypothesis was not received by the scientific community with open arms. In fact, Lovelock himself states that for a long time none of the peer-reviewed journals would publish his papers (as quoted in Potts, p. 33).

Perhaps the most vocal of Lovelock's critics is Oxford University's Richard Dawkins. Dawkins has said the Gaia hypothesis is teleological, untestable and, further, that it is based upon assumptions about the nature of natural selection that are "profoundly erroneous" (Fairbairn, 1994, p. 1210). More specifically, Dawkins criticizes the Gaia concept in its conclusion that natural systems evolve symbiotically over time because this implies an element of altruism in the behavior of the organisms concerned.

The introduction of a selfish individual into a population formerly consisting only of altruistic individuals will give the selfish individual a tremendous advantage to exploit the others, who will not respond negatively since they are, by definition, altruistic. Since this selfish individual's biological "success" is assured, his offspring will inherit his selfish tendencies. The conclusion is inevitable; the descendents of the original selfish individual completely replace the altruists, who become extinct.

Starting from such a premise it is not difficult to understand the problem Dawkins has with Lovelock's hypothesis. How can mutually beneficial relationships evolve when natural selection favors selfishness?

Part of the problem may be semantics; what is meant when we say "beneficial" or "symbiotic?" There may be some connotation to these words that is inappropriate given the actual relationships that Lovelock is trying to describe. The symbiotic interdependence of living things is not necessarily planned nor arrived at consensually. It may be simply inadvertent.

Plants and microbes are not consuming carbon dioxide and producing oxygen to benefit animal life, and likewise, the animals aren't consuming oxygen and producing carbon dioxide to benefit the plants. Each benefits the other as a consequence of what each does in self-interest.

Partly in response to criticism by Dawkins and others, Lovelock developed the Daisy World computer model (Lenton, p. 441). In Daisy World, a planet is populated by an arbitrary population of two species of daisy, white and black. The amount of solar radiation the world receives increases with time, just as scientists claim has happened on the real Earth (Newman and Rood, p. 1035). Since the white daisies tend to reflect light, they have a cooling effect on Daisy World's environment; conversely for the black daisies.

When the simulation is run, the intensity of solar radiation received by Daisy World stimulates the growth of one or the other type of daisy until an equilibrium is reached. From that point, the daisies either proliferate or decline depending on the temperature, maintaining a more or less stable environment over time, though not forever.

While the black and white daisies do cooperate, the level of cooperation involved is implicit; there is no "committee" of organisms that decide how hard to work for the good of all. Each organism, by doing what it does best (exploit the environment to perpetuate its own survival) works for the greater good quite by accident.

Of course, the problem with any computer model is that it is based on rules. If Daisy World is programmed to behave according to rules that are not based in reality, then the simulation offers no actual evidence to support the hypothesis. Rules can be contrived that, run in a simulation, pretend to show whatever we want; GIGO -- garbage in, garbage out.

What Daisy World does show is that the Gaia hypothesis is at least a plausible explanation for the conditions we observe on Earth over time; it represents a system that maintains stability even given variance in multiple variables. What prevents runaway entropy is a set of very simple rules that are neither implausible alone nor implausible in how they interact. Their simplicity is what makes them plausible.

Perhaps the true value of Daisy World is that it shows that a natural system of self-regulation can evolve on a planet-wide scale without necessarily demanding purposeful intervention by an outside intelligence, thus obviating critics' objections that the Gaia hypothesis is teleological.

The very complexity of Earth's environment makes any investigation of the Gaia hypothesis a daunting task. If the feedback loops exist that are predicted by the hypothesis, they may be very subtle and involve mechanisms not easily observed. If the amount of cloud cover on Earth depends on the interactions of three kinds of microbes, and these are in turn influenced by the amount of cloud cover overhead, how would we go about discovering this? We might never discover the relationship among those species of microbes and the cloud cover.

Not only this, but if Lovelock is right, it follows that we can expect a great deal of redundancy in the feedback systems as well, so any such systems we discern may not be the only systems working to produce the observed effect. There may even be nested systems; smaller loops imbedded within larger ones (Lenton, p. 440).

"Absence of evidence is not evidence of absence" as the saying goes, so not finding key feedback loops does not mean that the hypothesis is untrue. Demonstrating that some suspected loops were false would not constitute evidence against the hypothesis. Proponents could always say that evidence supporting it has just not been discovered...yet. This is perhaps the biggest problem with the hypothesis; its cunning ability to defy refutation.

While the hypothesis does have that philosophical problem, proponents do not need to fall back upon the "absence of evidence" argument to defend their position. The inventory of possible Gaian feedback loops continues to grow (Lenton, pp. 445-446, Barlow, p. 1, van de Koppel and Rietkert p. 118).

Further, it could be argued that a powerful piece of evidence in favor of the Gaia hypothesis is the fact that, only on Earth do we find life squirming away everywhere we look. From the tops of mountains to the bottoms of oceans, from Antarctic glaciers to rocks from deep within Earth's crust (Wackett, p. 430), there seem to be virtually no environments that have not been exploited by living things, even if only microbial life.

If the Gaia hypothesis is right, we should expect such exploitation as a natural and inevitable consequence. Life makes the environment more habitable for itself, so the environment is home to more and more life.

Conversely, discovering a planet inhabited by only one form of very ancient (in geologic terms) life could be a potent argument against the verity of the hypothesis. If the Gaia hypothesis is correct, then wherever life exists it ultimately facilitates the development of new species. If some form of microbial life exists on Mars, for instance, is ubiquitous there, yet does not alter its environment (Mars' atmosphere is almost completely inert), then the Gaia principle cannot be a universal one. In that case, if it operates at all, it does so only on Earth.

It may be possible to more or less reconcile the hypothesis' proponents and opponents. Lovelock and others who generally support the hypothesis take what can be called a holistic approach to understanding the problem of terrestrial life, while Dawkins and others who generally object to the hypothesis take a reductionist approach. Dawkins believes that the individual gene is supreme and drives natural selection, whereas Lovelock believes, essentially, that all genes matter because they all belong to the complicated integrated system that is Gaia. Since Gaia is like a living thing, it evolves as the myriad species that comprise it evolve.

These two apparently disparate positions may simply be opposite sides of the coin, in which case, both Dawkins and Lovelock are correct.

Dawkins rightly identifies the gene as the driver of natural selection; without mutations at the gene-level all members of a species are equally fit for their environment and none has a selective advantage over any other. Even the mutation itself may be an unexploited one that does not give the organism a selective advantage until some new stress is introduced to the environment. The ability of insect pests to resist formerly potent pesticides is proof of this.