What do theories hypothesize about the conditions of earths early atmosphere?

Summary

Lovelock's Gaia hypothesis conceptualized biodiversity and mutualism in their most advanced and elegant integration. In mycorrhizae, diversity and mutualistic functioning unite successive systems into networks and complex systems. In order to show that complexity has increased overall, it is sufficient to show, that – all other things being equal – connections have increased in at least one dimension. What is lacking is the ability to make predictions about how complexity in mycorrhizal communities will change their function as the systems and/or their environment is altered by human impacts and global change. Combining metagenomics, transcriptomics, molecular, metabolic, and biochemical data with nonlinear mathematical models might provide the foundations and rules for understanding mycorrhizal complexity. The limitations and utility of any data, however, remain in developing data-mining and complexity-modeling tools and techniques to utilize effectively the information from a local and global perspective, because data are gathered on scales from molecules to genomes, organelles, cells, tissues, and organs. Bioinformatics is the acquisition of knowledge by means of computational tools for the organization, management, and mining of genetic biological data. These analytical tools are being increasingly applied to the oceans of data collected by metagenomics studies. A more appropriate term for mycorrhizal systems may be ‘ecoinformatics’ or the accumulation of ecologically based data sets appropriate to mycorrhizae in situ, followed by data integration. In doing so, it will then be appropriate to say that diversity and mutualism provide ecosystem function and what that functioning may be.

2.4 Gaia hypothesis

The Gaia Hypothesis proposed by James Lovelock (1972) suggests that living organisms on the planet interact with their surrounding inorganic environment to form a synergetic and self-regulating system that created, and now maintains, the climate and biochemical conditions that make life on Earth possible. Gaia bases this postulate on the fact that the biosphere, and the evolution or organisms, affects the stability of global temperature, salinity of seawater, and other environmental variables. For instance, even though the luminosity of the sun, the Earth's heat source, has increased about 30% since life began almost four billion years ago, the living system has reacted as a whole to maintain temperatures at a level suitable for life. Cloud formation over the open ocean is almost entirely a function of oceanic algae that emit sulfur molecules as waste metabolites which become condensation nuclei for rain. Clouds, in turn, help regulate surface temperatures.

Lovelock compared the atmospheres of Mars and Earth, and noted that the Earth's high levels of oxygen and nitrogen were abnormal and thermodynamically in disequilibrium. The 21% oxygen content of the atmosphere is an obvious consequence of living organisms, and the levels of other gases, NH3 and CH4, are higher than would be expected for an oxygen-rich atmosphere. Biological activity also explains why the atmosphere is not mainly CO2 and why the oceans are not more saline. Gaia postulates that conditions on Earth are so unusual that they could only result from the activity of the biosphere (Lovelock and Margulis, 1974).

18.1.2 The Gaia Hypothesis

The Gaia hypothesis (Lovelock, 1972; Lovelock and Margulis, 1973) postulates: “the climate and chemical composition of the Earth’s surface environment is, and has been, regulated in a state tolerable for the biota” (Lovelock, 1989, p. 215). The Gaia hypothesis (Figure 18.1) – named after the Greek goddess of Earth by the author William Golding (Lovelock, 2009) – was criticized for not fully factoring in evolution by natural selection and, in particular, competition between organisms (e.g., Dawkins, 1982). In response, Lovelock contended that, “In no way is this [Gaia] theory a contradiction of Darwin’s great vision. It is an extension to it to include the largest living organism in the Solar System, the Earth itself” (Lovelock, 1986, p. 25). The Daisyworld model (Figure 18.2) (Lovelock, 1983; Watson and Lovelock, 1983) was developed to illustrate how Gaia may work (Kump and Lovelock, 1995). It also provides an initial ‘mathematical framework’ for understanding self-regulation (Lenton, 1998).

Daisyworld is an imaginary planet, similar in many respects to Earth, on which grow only daisies. The daisies have an abundance of nutrients and water. Their ability to spread across the planetary surface depends only on temperature, and the relationship is parabolic, with minimum, optimum, and maximum temperatures for growth. The climate system is correspondingly simple. There are no clouds, and no greenhouse gases [GHGs]. The planetary energy balance is a function only of solar insolation, albedo and surface temperature, and planetary albedo depends on the areal coverage of the soil (which is grey) by black and white daisies.

(Kump and Lovelock, 1995, p. 539)

(Reprinted from Kump and Lovelock, 1995; with permission from Elsevier.)

The Gaia hypothesis has evolved over time, generating further research to test its robustness and advance the notion of a holistic ES. For example, “When introduced, this [Gaia] hypothesis was contrary to conventional wisdom that life adapted to planetary conditions as it and they evolved in their separate ways. We now know that the hypothesis as originally stated was wrong because it is not life alone but the whole ES that does the regulating” (Lovelock, 2009, p. 166). In a paper presented at the United Nations University in Tokyo on 25 September 1992, Lovelock explained that, although contentious, the Gaia hypothesis has generated many experiments (Lovelock, 1993). This section describes how – through these experiments – researchers have attempted to elucidate how the ES works by using climate as an illustrative example of how processes and feedbacks can operate homeostasis. Indicative of this thinking is the investigation of the role of algae in the ocean and its control of Earth’s climate through the dimethyl sulfide (DMS) process.

18.1.2.2 Vegetation and Climate Interactions

When large changes were recognized as occurring in tropical rainforests (e.g., Salati and Vose, 1984), tests were conducted to try to determine their climatic impact (e.g., Henderson-Sellers and Gornitz, 1984). Fundamental aspects of this research included the use of stable water isotopes to track hydrological changes (e.g., Salati et al., 1979; McGuffie and Henderson-Sellers, 2004) and model simulations of tropical deforestation that helped elucidate the importance of an accurate representation of vegetation in global climate modelling (e.g., Dickinson and Henderson-Sellers, 1988; Henderson-Sellers et al., 2008).

Tropical deforestation simulations indicated a “sensitivity of the local climate to the removal of tropical forest…. Moreover, the scale of moisture convergence changes, and possibly also cloud and convection changes, is such that there is a possibility that nonlocal climatic impacts may also occur” (Zhang et al., 1996, p. 1516). Further studies (e.g., Zhang et al., 2001) found that tropical deforestation can impact large-scale atmospheric circulation. This supported previous Global Climate Model (GCM) studies (e.g., Sud et al., 1988) and suggested that land-use change (e.g., tropical deforestation) may affect projections of future climate (cf. Pitman and de Noblet-Ducoudré, 2012, this volume). However, research in Amazonia had yet to be studied in an interdisciplinary manner (Dickinson, 1987), a central tenet of an ESs approach.

Although it was not clear how deforestation might threaten interdependent (homeostatic) systems because “our scientific framework is yet inadequate to make such judgments” (Dickinson, 1987, p. 1), and well before detailed disciplinary research of the 1990s–2000s, research scientists joined an international conference on ‘Climatic, Biotic, and Human Interactions in the Humid Tropics with Emphasis on the Vegetation and Climatic Interactions in Amazonia’ in Brazil in 1985. This meeting brought together some of the world’s top scientists to examine critical processes linking climate and vegetation in the tropics. The humid tropics were chosen as the focus because they were deemed of fundamental importance to the global climate. The urgent need to carefully analyse land-use change and climate in the humid tropics was combined with a desire to communicate research findings clearly (Figure 18.3).

FIGURE 18.3. Forest moisture recycling increases precipitation in the Amazon, that is, why removing trees reduces rainfall. A Cathy Wilcox cartoon (first published on 4 March 2005 on the front page of The Sydney Morning Herald, Australia) illustrating a geophysiological discovery made by tracking and modelling stable water isotopes.

(Source: Reproduced by permission of Cathy Wilcox, SMH.)

Tropical forests are vulnerable to anthropogenic climate change through disturbances in precipitation and temperature (e.g., Lewis et al., 2011) and the compounding effects of tropical deforestation and greenhouse warming on climate have been investigated for some time (e.g., Zhang et al., 2001; Fearnside, 2011). There are many synergies operating among local people’s survival, climate, vegetation, and land-use change in the humid tropics. For example, as Fearnside (2011) notes, “Because half of the dry weight of the trees in a tropical forest is carbon, either deforestation or forest die-off releases this carbon in the form of greenhouse gases such as carbon dioxide (CO2) and methane (CH4), whether the trees are burned or simply left to rot” (Fearnside, 2011, p. 1283).

Gradually, as tropical forests became a key part of climate change research and policy debate, simulations became more like ‘Gaian-type experiments’ in which researchers attempted to describe how the ES works by using disturbances to the tropical forests’ climate as an exemplar (e.g., Henderson-Sellers et al., 1988). An integrated systems approach (big picture perspective) evolved through the lens of ESS. This understanding prompted the concept of teleconnections and tipping points resulting from tropical deforestation in Amazonia, Africa, and South East Asia, as discussed by Lenton (2012, this volume). Nobre (2011, personal communication) made the following comments:

Prompted by a need to create a scientific framework to better understand these complex processes, the workshop on Vegetation and Climatic Interactions in Amazonia in 1985 helped advance an integrated Earth systems approach. The Conference recommendations evolved into central Large-Scale Biosphere Atmosphere Experiment in Amazonia (LBA) themes of understanding the Amazon as a regional entity of the Earth system and of studying how climate and land cover changes can alter its physical, chemical and biological functioning.

C. Nobre, personal communication, 2011

Lovelock’s Gaia hypothesis advanced understanding that a planet with abundant life will have an atmosphere with ‘thermodynamic disequilibrium’ and that “Earth is habitable because of complex linkages and feedbacks between the atmosphere, oceans, land, and biosphere”, which helped shape ESS (Lawton, 2001, p. 1965). The remainder of this section focuses on the genesis and evolution of ESS.

Earth System Science and the ‘Gaia’ Hypothesis

Since the 1970s James Lovelock developed the Gaia hypothesis, named after the ancient Greek goddess of the Earth (See GAIA). As originally conceived the ‘Gaia’ concept envisages the Earth as a super-organism that operates to regulate its own environment, principally temperature, to keep it habitable for the biosphere. Lovelock has never argued that the biosphere consciously anticipates environmental change, but only that it automatically responds to it. Nonetheless some sections of the public have construed it that way, and in the popular mind Gaia gained a quasi-mystical connotation, enhanced by its name. The great value of the Gaia hypothesis is that it presents the interdependence of the constituents of the geosphere in a media-friendly way. Earth system science also involves a holistic approach to the geosphere, but without the ‘ghost in the machine’. Nonetheless Amazon, the internet book shop, still classifies books on Earth system science under ‘Religion and Spirituality > New Age > Earth-Based Religions > Gaia’.

Self-Organization in the Biosphere

Arguably the most ambitious ecological theory based on self-organization is the Gaia hypothesis, which postulates that the biosphere itself evolves to a homeostatic state. Lovelock suggested the Daisyworld model as an illustration of how this process might occur. On the hypothetical Daisyworld, black and white daisies compete for space. Although both kinds of daisies grow best at the same temperature, black daisies absorb more heat than white daisies. When the Sun shines more brightly, heating the planet, white daisies spread, and the planet cools again. When the Sun dims, the black daisies spread, warming the planet. In this way, competitive interactions between daisies provide a homeostatic mechanism for the planet as a whole.

The idea behind Gaia is that ecosystems will survive and spread more effectively if they promote the abiotic conditions required for their own persistence. If so, ecosystems might gradually evolve to be increasingly robust, and if this happened on a global scale, then the biosphere itself might behave as a self-regulating system. However, evidence for Gaian processes in real ecosystems remains tenuous and their theoretical plausibility is disputed.

4.4 Natural Selection and the Symbiome

Some of the things I am going to say in the following section are going to sound like a defence of the Gaia hypothesis against the sort of selectionist critique that (as noted above) seems to have given pause even to Lovelock himself. However, it is not the purpose of this paper to fully explicate the Gaia hypothesis of James Lovelock and Lynn Margulis [Lovelock, 1988; Lovelock and Margulis, 1974; Margulis, 1998]. (Thinking of Gaia as a mutualistic symbiome rather than as a single living organism may make the concept more palatable for some.) The major aim of this section is to explicate how the evolution of symbiotic associations of organisms (whether Gaia or something on a less grand scale) could be favoured by natural selection. Some of what I say here takes advantage of an analysis by Timothy Lenton [1998].

I will take as my nominal target a token of Dawkins’ influential critique of the Gaia hypothesis:

I don’t think Lovelock was clear—in his first book, at least—on the kind of natural-selection process that was supposed to put together the adaptive unit, which in his case was the whole world. If you’re going to talk about a unit at any level in the hierarchy of life as being adaptive, then there has to be some sort of selection going on among self-replicating information. And we have to ask, What is the equivalent of DNA? What are the units of code? What are the units of copyme code which are being replicated? … I don’t think for a moment that it occurred to Lovelock to ask himself that question. And so I’m skeptical of the rhetoric of the Gaia hypothesis, when it comes down to particular applications of it, like explaining the amount of methane there is in the atmosphere, or saying there will be some gas produced by bacteria which is good for the world at large and so the bacteria go to the trouble of producing it, for the good of the world. That can’t happen in a Darwinian world, as long as we think that natural selection is going on at the level of individual bacterial genes. Because those individual bacteria who don’t put themselves to the trouble of manufacturing this gas for the good of the world will do better. Of course, if the individual bacteria who manufacture the gas are really doing themselves better by doing so, and the gas is just an incidental consequence, obviously I have no problem with that, but in that case you don’t need a Gaia hypothesis to explain it. You explain it at the level of what's good for the individual bacteria and their genes. [Dawkins, 1995]

In fairness to Dawkins, these remarks were apparently made ex tempore at a conference. However, they illustrate a lack of clarity about symbiosis that is endemic to the thinking of evolutionary biologists.

The first thing to clear out of the way is to remind ourselves that we need to take care to avoid teleological language which is applicable only to conscious organisms such as humans who can plan ahead on the basis of imaginative representations of goals. Dawkins, who should know better, gets sloppy this way when he suggests that his hypothetical bacteria might produce a gas “for the good of the world”. No bacteria produce gases or anything else for the sake of anything, even themselves, while humans do all sorts of things for the sake of goals and purposes. (It would probably be better as well if biologists were to avoid the term “altruism” for the self-sacrificial behavior that sometimes occurs in mutualistic functioning, since that word is most accurately applied to certain human motivations.)

In a mutualistic system a species of bacteria may well have the function of producing a certain gas that facilitates the operation of the system as a whole; functional language is perfectly appropriate for coordinated living systems from protozoans to ecosystems [Allen, 2004]. But the fact that a system has evolved in such a way that some of its components have recognizable functions in the economy of the whole does not mean that they have purposes in the sense that things done intentionally by humans have a purpose, nor that they have their function for the sake of the whole. (This was expressed clearly by Simpson; see [Simpson, 1953, p. 181].) To say that (for instance) the cells in my kidneys cooperate in a certain way is to say that they happen to function in concert in a certain way, not that they cooperate in the sense that humans can (on selected occasions) choose to cooperate. My kidneys have the function of eliminating excess water and certain toxins from my body, but they do these things because these activities are supported by a complex network of feedback loops; they do not do them for my sake or even for their own. This is an important part of the answer to Paley [Paley, 1802] and other champions of “intelligent design”: the fact that parts of a complex system have recognizable functions does not by itself imply that they were products of intentionality.

A much tougher question is to say what constitutes a replicator. Dawkins thinks that it does not make sense to say that Gaia has a genome. But of course Gaia has a genome; the genome of Gaia and any other sort of symbiotic complex is comprised of the combined DNA and RNA of all of the myriad organisms of which it is composed. A distributed genome is very common at the eukaryotic cellular level. By now there is no controversy about the fact that there is cytoplasmic DNA, namely the DNA belonging to organelles of endosymbiotic origin such as the mitochondria and plastids. The genome of virtually all metazoan cell lines consists not only of nuclear genes but of the genetic heritage of an often bewilderingly complex suite of endosymbiotes. The genome of an organism does not have to be concentrated in one spot within the organism, and it rarely is.

A good illustration of this fact is the protozoan (or more properly protist) Mixotricha paradoxa, an extraordinarily beautiful organism often cited by Margulis (e.g., in [Margulis, 1998; Margulis and Sagan, 2001]) as an exemplar of symbiogenesis. M. paradoxa lives in the gut of certain termites, and apparently serves its hosts by digesting cellulose and lignin. But it is a symbiote built out of symbiotes: as well as its own nucleus, each M. paradoxa contains several hundred thousand individuals of at least four other species of bacteria [Margulis and Sagan, 2001]. (Curiously, the one type of symbiotic organelle it does not contain is the mitochondrion, probably because the termite gut is anoxic.) Each individual M. paradoxa is a populous community, cooperating as a mutualistic whole. So what, in such a case, is the unit of selection?

Dawkins is right that any chunk of genetic code that in effect says “make more of me” can be a replicator and will succeed in being replicated if it says this in just the right way to resonate with the demands of its environment. However, networks of cooperative behaviors can and often are sufficiently successful that they are amplified by natural selection into a coherent, reproducing whole. This can occur not only in the cases of endosymbiosis studied by Margulis; complex associations of metazoa can form such symbiotic networks as well, some of which may be more tightly coupled (that is, causally interactive) than others. To further complicate the story, it is increasingly evident that complex metazoa such as mammals are host to a rich array of microbial symbiotes, so much so that microbiologists are beginning to describe multicellular organisms as metagenomic [Grice et al., 2008; Ley et al., 2008]. If a symbiotic network is sufficiently coherent and coordinated that it reproduces as a whole, then its entire genetic code is a replicator. So the question of the unit of selection, the question of what is “seen” by natural selection, is not simple; it is not just the gene (whatever that is) unless by “gene” one means simply any replicator. A sufficiently well coordinated symbiotic association can itself become a unit of selection.

Most of Dawkins’ objections to Gaia apply to Mixotricha paradoxa as well, and if he were right, there ought to be no such thing. In fact, the way that M. paradoxa reproduces can give us some insight into the sense in which a hypothetical planetary-scale symbiotic unit could evolve. In symbiotic protists like M. paradoxa the orchestration of reproduction is complex and not yet well-understood. However, there is no reason to suppose that all the component symbionts of such organisms reproduce in perfect concert, even though the host cell is capable of division as a unit. Endobacterial symbionts within a larger complex could well run through many reproductive cycles of their own during one reproductive cycle of the larger complex. Their survival would depend upon adapting to the constraints within the larger organism, just as all organisms on Earth have to adapt to the often-inorganic but sometimes organic constraints of the larger world. (An important example of such a constraint is climate, which might best be described as an organically-mediated inorganic constraint. Clearly when one is speaking of an environmental parameter such as temperature, which is partially controlled by solar input and partially controlled by carbon dioxide concentration, the dividing line between the organic and the inorganic is often fuzzy.) To a single bacterium within M. paradoxa, one cell generation of its host is an entire cosmological cycle which defines a world to which the bacterium must adapt like any other organism in nature. Such symbionts within an organism such as M. paradoxa would often be subject to natural selection that would tend to favour their ability to contribute to the economy of the whole organism. Complex symbiotic associations like M. paradoxa therefore also can evolve piecemeal in response to internal constraints as well as all at once in the usually understood fashion, in which the composite organism evolves as a whole in response to external constraints. One can therefore distinguish between external evolution (which is well-studied) and internal evolution—evolution of the components of a complex symbiotic association in response to survival challenges and opportunities acting internally to the association.

A key difference between Gaia (as hypothesized by Lovelock and Margulis) and the kinds of organisms to which the usual model of natural selection applies is, therefore, that Gaia does not reproduce as a unit as do its component organisms, including M. paradoxa. Rather, Gaia evolves because evolution occurs within it, just as it does within M. paradoxa. Gaia reproduces gradually, part by part, in a process of growth, regeneration, adaptation, and decay, almost like an organic version of Neurath's boat of knowledge which is rebuilt piece by piece as it floats along. Gaia as a whole adapts to its external environment over millions of years in a piecemeal, not-perfectly-coordinated way as its component organisms adapt to the constraints of the external environment and the internal constraints imposed on them by the other organisms in the system. In a remarkably English manner, Gaia muddles through and remains tough and resilient despite its jury-rigged nature. Although the details must be very complex and may never be fully elucidated, there is no reason why we cannot suppose that Gaia (viewed as something like a planetary-scale M. paradoxa) cannot be supposed to evolve in the piecemeal way that a complex symbiotic association like M. paradoxa can evolve, even though it neither has a nucleus which partially coordinates its activities, nor reproduces as a unit the way a protist can.

Now, Dawkins suggests that we imagine that some mutant bacteria happen to start producing a certain gas that is beneficial to the symbiotic complex as a whole. He makes a very odd claim: “those individual bacteria who don’t put themselves to the trouble of manufacturing this gas for the good of the world will do better.” (This is more or less Garrett Hardin's tragedy of the commons at the cellular level.) But it should be clear that this is not necessarily the case; an organism manufacturing some component that increases the overall suitability of the environment for that organism could very well increase the reproductive success of that organism even if the manufacturing process has costs and risks associated with it. There is no guarantee that this would happen in all cases, but there is no a priori reason that it would not, either.

Some parasitical “free-riders” can be tolerated so long as the functionality of the system is maintained; indeed, some parasitism may benefit the system in indirect ways if it maintains variability. But if all organisms in an ecosystem are parasitical in the sense that they do not put themselves to the trouble of contributing something to the system, they certainly will not do better since the whole system will ultimately degrade.

Perhaps the notion of a cost-benefit analysis would be helpful here. Any conceivable activity by an organism has a cost. This need not be only in terms of energy and materials; adaptation to any particular environment also exposes an organism to the hazards typical of that environment, such as the predators peculiar to it. There are also opportunity costs: if an organism becomes adapted to the Arctic cold, for instance, then it may have given up survival options suitable to warmer weather. It is elementary that cooperative behaviour carries costs and risks precisely as Hardin indicated; for instance, if the organism shares some of its resources with others it will have less for itself, and it opens itself up to the risk that it may be out-reproduced or otherwise out-competed by others of its species or other species who are less inclined to share the goods. However, an action can be advantageous even if it has a cost, so long as its benefits outweigh its cost, while failure to cooperate may have costs as well, which could include (as in Hardin's tragic scenario) subversion of the very environmental conditions that made life possible for that organism in the first place. Again, at the risk of repetition, the existence of a co-operative symbiotic modality does not imply intentionality (as with co-operation between humans) but rather coherence of functionality.

As Lenton observes,

Organisms possess environment-altering traits because the benefit that these traits confer (to the fitness of the organisms) outweighs the cost in energy [emphasis added] to the individual [Lenton, 1998, p. 440].

This remark suggests a clarification of the sense in which benefit flows back to a symbiont. The most general sense of “benefit” to an organism is the availability of free energy; this can translate into reproductive opportunities or simply an increased survival probability for the individual (since more free energy allows for a wider repertoire of survival strategies and modalities). We see here again an instance in which thermodynamics can illuminate the workings of evolution.

If Hardin's scenario were the normal pattern—that is, if life typically subverts the conditions for its existence—how could there be life on Earth at all? Earthly life has proved remarkably resilient for over 3.5 billion years, despite celestial impacts, episodes of massive volcanism (and the occasional runaway greenhouse catastrophes possibly consequent upon them [Ward, 2007]), and steadily increasing solar output. This could only be possible if the persistence of complex life is somehow probabilistically favoured within the broad range of physical conditions that have been available on Earth for about the past four billion years, and that is only possible if life (despite the constant recurrence of endemic parasitism at all scales from the viruses to human society) has had (so far at least) a net tendency to co-operate in order to maintain the conditions necessary for its continuance. This is especially clear if we understand parasitism from the biophysical (thermodynamic) point of view as something that results in the physical degradation of the host; if life on Earth in net degraded its habitats then it would have destroyed itself long ago. Furthermore, if life in net were balanced on the knife-edge of commensalism, it is hard to understand how such a precarious state could have persisted for so long. A planetary-scale, rough-and-ready mutualism seems to be the only possibility, and this observation could be thought of as a minimal Gaia hypothesis.

Suppose that the cost of a new trait is that it requires self-sacrificial behavior for some members of the species. If a strain of mutant organisms simply commits suicide en masse then its evolutionary story is over. However, if the self-sacrificial behavior greatly facilitates the reproduction of the survivors, even if there are rather few of them, then it will tend to be amplified by natural selection. The importance of mechanisms of this sort has been emphasized by Bonner who has described, for example, the self-sacrificial behavior of slime mold amoeba (in vast numbers) in the formation of a slime mold fruiting body [Bonner, 1998]. There is nothing unusual about this sort of thing; it occurs throughout nature from the bacterial level on upward. Again, the fact that cooperative behaviour has costs and risks does not imply that it puts its possessor at a selective disadvantage, so long as there is a sufficient reward for the behaviour as well.

Apparently-altruistic behavior need not be explained merely as “kin selection”; an organism need not be in a mutualism merely with its cousins. It could be in mutualism with any other conceivable form of life at all, so long as the net effect is to provide a modality of survival for the organism. Here, by the way is the basis for so-called group selection, which is nothing more than selection in favour of mutualistic symbiosis. This is a point that even the most sympathetic and well-informed apologists for group selection do not bring out as clearly as they could [Sober and Wilson, 1998]. (Whether species selection can be understood in symbiotic terms is a different and difficult question since it is not clear that a species can always be thought of as a symbiome; I will not address this question further here. [Stanley, 1979].)

Dawkins’ distressingly sloppy argument is above all a crashing non sequitur— for from the fact that cooperation must inevitably have costs and incur risks it does not follow that it cannot have benefits as well, and indeed net benefits. What really matters is the timing of those benefits: the feedback from the environment has to return to the organism soon enough to make a difference to its reproductive success or probability of survival. Therein lies the real tragedy of Hardin's medieval commons: a social pathology that prevented sufficient rewards for cooperative behaviour from flowing back to the beleaguered peasants soon enough for those rewards to make a difference to their well-being.

Natural selection can be understood as a process involving feedbacks. If a trait increases reproductive success that process can be described as the amplification of the trait by positive feedback from the environment. On the other hand, if a trait triggers a chain of events that decreases the probability of its own recurrence then it will be damped out by that negative feedback from the environment. In order for the effect of the altered trait on the environment to make any difference to reproductive success, it has to feed back to the organism in time to affect its reproduction; it does not have to feed back within just one reproductive cycle, but the feedback cannot take forever or be so attenuated that it makes no difference to the reproductive or survival probability of the organism. (As in so many endeavors, timing is almost everything.) Such feedbacks can certainly reward cooperative as well as competitive behaviour. And once again, by “cooperative” behavior we do not mean activity that is motivated by warm feelings of fellowship, but coherence of functionality.

14.1.2 Gaia Hypothesis

In the early 1970s, James Lovelock theorized that Earth behaves like a superorganism, and this concept developed into what is now known as the Gaia hypothesis. To quote Lovelock (1995): “Living organisms and their material environment are tightly coupled. The coupled system is a superorganism, and as it evolves there emerges a new property, the ability to self-regulate climate and chemistry.” The basic tenet of this hypothesis is that Earth's physicochemical properties are self-regulated so that they are maintained in a favorable range for life. As evidence for this, consider that the sun has heated up by 30% during the past 4–5 billion years. Given Earth's original carbon dioxide–rich atmosphere, the average surface temperature of a lifeless Earth today would be approximately 290°C (Table 14.2). In fact, when one compares Earth's present-day atmosphere with the atmospheres found on our nearest neighbors Venus and Mars, one can see that something has drastically affected the development of Earth's atmosphere. According to the Gaia hypothesis, this is the development and continued presence of life. Microbial activity, and later the appearance of plants, have changed the original heat-trapping carbon dioxide–rich atmosphere to the present oxidizing, carbon dioxide–poor atmosphere. This has allowed Earth to maintain an average surface temperature of 13°C, which is favorable to the life that exists on Earth.

TABLE 14.2. Atmosphere and Temperatures Found on Venus, Mars, and Earth Plane

GasVenusMarsEarth without lifeEarth with lifeCarbon dioxide96.5%95%98%0.03%Nitrogen3.5%2.7%1.9%9%OxygenTrace0.13%0.021%Argon70ppm1.6%0.1%1%Methane0.00.00.01.7ppmSurface temperature (°C)459−53290 ± 5013

Adapted from Lovelock, 1995.

Copyright © 1995

How do biogeochemical activities relate to the Gaia hypothesis? These biological activities have driven the response to the slow warming of the sun, resulting in the major atmospheric changes that have occurred over the last 4–5 billion years. When Earth was formed 4–5 billion years ago, a reducing (anaerobic) atmosphere existed. The initial reactions that mediated the formation of organic carbon were abiotic, driven by large influxes of ultraviolet (UV) light. The resulting reservoir of organic matter was utilized by early anaerobic heterotrophic organisms. This was followed by the development of the ability of microbes to fix carbon dioxide photosynthetically. Evidence from stromatolite fossils suggests that the ability to photosynthesize was developed at least 3.5 billion years ago. Stromatolites are fossilized laminated structures that have been found in Africa and Australia (Fig. 14.1). Although the topic is hotly debated, there is evidence that these structures were formed by photosynthetic microorganisms (first anaerobic, then cyanobacterial) that grew in mats and entrapped or precipitated inorganic material as they grew (Bosak et al., 2007).

The evolution of photosynthetic organisms tapped into an unlimited source of energy, the sun, and provided a mechanism for carbon recycling, that is, the first carbon cycle (Fig. 14.2). This first carbon cycle was maintained for approximately 1.5 billion years. Geologic evidence then suggests that approximately 2 billion years ago, photosynthetic microorganisms developed the ability to produce oxygen. This allowed oxygen to accumulate in the atmosphere, resulting, in time, in a change from reducing to oxidizing conditions. Further, oxygen accumulation in the atmosphere created an ozone layer, which reduced the influx of harmful UV radiation, allowing the development of higher forms of life to begin.

At the same time that the carbon cycle evolved, the nitrogen cycle emerged because nitrogen was a limiting element for microbial growth. Although molecular nitrogen was abundant in the atmosphere, microbial cells could not directly utilize nitrogen as N2 gas. Cells require organic nitrogen compounds or reduced inorganic forms of nitrogen for growth. Therefore, under the reducing conditions found on early Earth, some organisms developed a mechanism for fixing nitrogen using the enzyme nitrogenase. Nitrogen fixation remains an important microbiological process, and to this day, the majority of nitrogenase enzymes are totally inhibited in the presence of oxygen.

When considered over this geologic time scale of several billion years, it is apparent that biogeochemical activities have been unidirectional. This means that the predominant microbial activities on earth have evolved over this long period of time to produce changes and to respond to changes that have occurred in the atmosphere, namely, the appearance of oxygen and the decrease in carbon dioxide content. Presumably these changes will continue to occur, but they occur so slowly that we do not have the capacity to observe them.

One can also consider biogeochemical activities on a more contemporary time scale, that of tens to hundreds of years. On this much shorter time scale, biogeochemical activities are regular and cyclic in nature, and it is these activities that are addressed in this chapter. On the one hand, the presumption that Earth is a superorganism and can respond to drastic environmental changes is heartening when one considers that human activity is effecting unexpected changes in the atmosphere, such as ozone depletion and buildup of carbon dioxide. However, it is important to point out that the response of a superorganism is necessarily slow (thousands to millions of years), and as residents of Earth we must be sure not to overtax Earth's ability to respond to change by artificially changing the environment in a much shorter time frame.

What were the conditions of the Earth's early atmosphere?

What theory is given for the Earth's early atmosphere?

Why have theories about Earth's early atmosphere developed and changed over time?