Day 13
Mother Earth
Come, where is this young gallant that is so desirous to lie with his mother earth?
As You Like It, Act I. Scene II.
William Shakespeare (1564–1616)
The Oxford Shakespeare, 1914
To Shakespeare, “mother earth” was poetry. Bruno, an Animist, placed a living spirit in the Earth as well as in many normally inanimate objects. There is a modern scientific middle ground, the Gaia hypothesis of James Lovelock and Lynn Margolis. Does the Earth act as a single large biological organism? That is, does life on Earth act as a consortium of organisms for the benefit of all?
The hypothesis smells of the Natural Theology of William Paley on one end. That is, each organism has a role assigned by the Creator for the good of all. On the other end, the weak form of the hypothesis, that terrestrial life has not yet produced instability that led to its undoing, is a restatement statement of the weak anthropic principle. Our ancestors had to survive to breed for us to be here to observe. Yet the strong form of the Gaia hypothesis as proposed by Lovelock and Margolis is science and not mysticism.
Working together. Real consortia of organisms exist including right within our cells. Margolis, in her day job, studies such consortia. There are more microbes in our gut than people on the Earth. Most of them are beneficial. We would starve without them. The benefit is two way. The microbes extract energy from the food and break it down to a form we can use in the process. Margolis has shown that termites need microbes in their guts to digest wood. Cows have microbes in their guts that eat grass. The cow in turn digests some of the microbes while providing a home for the rest.
Similar mutual benefit, called symbiosis, exists with large organisms. The squirrel buries acorns to eat later. The few that do not get eaten are unintentionally planted away from the parent tree. They germinate often in open soil where it is easy for the squirrel to dig. This is group selection in that the genes in the eaten acorns benefit from the traits that make them attractive to squirrels for planting. Jays do the same with acorns in California where there are no native tree squirrels near sealevel in the Bay Area. You have probably seen a lot of examples on TV, like tickbirds eating parasites off a rhinoceros’ hide to the benefit of both species (Figure 1).
Figure 1: Two tick birds rest of the shoulders of a water buffalo. Both species benefit. The water buffalo has fewer ticks after the birds have eaten. Photo by George Thompson.
Margolis found a more intriguing form called endosymbiosis (Greek for inside together life) within the Eukaryote cell. The descendents of once free-living Bacteria make their homes as organelles within the cell walls. There was endosymbiosis early in eukaryotic evolution. Vestiges of once free-living spirochetes (one of their relatives causes syphilis) aid in our cell division. These organelles are also the flagella that propel Eukaryote microbes. Margolis likes to point out the close link between flagellation and sex in her lectures.
The mitochondria and the chloroplasts in Eukaryote cells are more easily traced to the free-living Bacteria relatives. Both types of organelles are very important to the Eukaryote hosts. Mitochondria produce useful energy from oxygen and food. Chloroplasts convert sunlight into useful energy.
The capture of mitochondria and chloroplasts occurred long enough ago that the origin is unclear. They may have been parasites that evolved to be less virulent and finally useful, they may have been captured and put to use in the cell, or they may have been organisms that evolved a way to avoid being digested once they were engulfed by another microbe. The endosymbiosis occurred so long ago that mitochondria and chloroplasts cannot live outside their host cells. The fate of mitochondria in a rat is the fate of the rat. Natural selection acts both on the host and the mitochondria to make the association more fit. Faulty mitochondria cause countless human diseases. http://www.neuro.wustl.edu/neuromuscular/mitosyn.html
The organelles benefit relative to their free-living relatives. Take, for example, chloroplasts in an oak leaf. They must go wherever the oak puts its leaf, but the oak puts them in a good place where there is lots of light. It supplies them with nutrients and water. It keeps some of them alive in the winter (in cool climates) to make more when it sets leaves in the spring. The free-living cyanobacteria, relatives of the chloroplasts, have to make do without this help on the ground.
Close symbiosis also occurs in microbial mats. The film on unbrushed teeth is an example. The mats self-organize so that one microbe can use the waste products of another. The symbiosis is complex enough that, like the mitochondria and us, many of the mat organisms could not survive without it.
Feedback and control. Surely symbiotic consortia exist, but the Gaia hypothesis implies that the whole biosphere acts as one. On a micro-ecological level, organisms do act to their own benefit and the detriment of other organisms. Nature is not a song in the Lion King; it is a struggle of existence. The law of the jungle does not mean a utopia. The rhinoceros gets cleared of ticks; the tickbird gets lunch, but the tick is lunch. The bird does not try to improve the ticks; it eats both sick and healthy ones. It will starve if there is a dearth of ticks. The rhinoceros will starve if there is a dearth of grass. There is feedback including natural selection, but it is to the demise of individual organisms. Neither was feedback in the end a boon to the countless species that have become extinct. The early worm got caught.
Gaia is limited; in its scientific form, local consortia only provide an analogy to get Margolis and Lovelock thinking. They imply that biology keeps the Earth as a whole clement and otherwise habitable. There are limits to this. Lush conditions do not now exist on Mars and Venus. The recent mass extinction on Earth resulted from evolution becoming unstable, with our own species overly fit. We attribute the Cretaceous-Tertiary extinction to an asteroid impact. But some of the other mass extinctions may owe to climate and/or evolution becoming unstable.
It has been surprisingly hard to document global Gaian feedbacks, as opposed to contrived examples. Kelvin Zahnle of Ames NASA shared this one with me. It is like a supply and demand graph in beginning economics. [see Primer on Economics] On the Archean Earth, methane was a significant greenhouse gas. At a given level of carbon dioxide, the (globally averaged) temperature is a function of the amount of methane in the air. Figure 2 is somewhat more complicated than a supply and demand diagram because temperature is a function of methane concentration while methane production (supply) is a function of temperature. At low methane concentrations, the temperature (climate curve) increases with methane concentration. It reaches a maximum temperature (C) at some methane concentration. Above this concentration, methane forms a haze in the air. This, like dust from volcanic eruptions, cools the climate. The biological production of methane and the concentration in the air in equilibrium with it (supply curve) increase with temperature at low temperatures. There is a maximum methane production at temperature-concentration point E. Methane production goes to zero at very high temperatures where no life can survive.
The temperature and methane concentration at any given time lie on the climate curve (by assumption to maintain simplicity). If we start the climate between the crossing point A and point 0, there is less methane production than is needed to maintain the climate. The temperature decreases toward point 0 where the frozen surface produces no methane. If we start between crossing points A and B the methane production is more than is needed to maintain the climate. Methane builds up and the temperature increases to point B. At point B, methane production and climate are in equilibrium. If we start between points D and B, there is too little methane production to maintain the climate. Methane decreases increasing the temperature until stable point B is reached. From the diagram, we see that both stable and unstable equilibrium exist. The equilibrium at B is stable. If we start anywhere between D and A, we end up at that point. The equilibrium at A is unstable. If we start even slightly off it we end up at either 0 or B.
Figure 2: The temperature and the methane concentration track the climate curve. The equilibrium at point A is unstable. The equilibrium at point B is stable.
In analogy with economics, several stable and unstable equilibria exist (Figure 3). We can make things exceedingly complicated. (There is a Nobel Prize for economics so the field is not simple.) We could have the curves intersect four (or even more) times rather than twice. Then we have two stable and two unstable equilibria. We could add more axes, like one for CO2 concentration in the air. In that case, we can have four crossings for some values of CO2 and 2 crossing for others. At the transition value, the equilibrium jumps across the diagram.
Paleontologists make use of this property with evolution. Punctuated equilibria exist for the form of an organism. Typically an organism evolves slowly. It is already fit for its niche. Natural selection works to maintain stasis. Sometimes it evolves rapidly jumping from one equilibrium niche to another.
Figure 3: The previous figure at a lower CO2 concentration provides simple jumps between equilibria. The climate curve intersects the supply curve at four points. Points A and B are the unstable and stable points on the previous figure. Point Q is unstable while point P is stable. A large perturbation might cause the system to jump between the stable points. An gradual increase in CO2 will bring the climate curve to the one in the previous figure dashed line. The stable point P will no longer exist and the system at P will jump to B.
The gist. The Gaia concept is useful shorthand for saying that stable equilibria exist on the Earth. However, we do not understand the biology and physics of any real global example. The concept is not yet exportable to astrobiology beyond saying that a long-term stable environment is possible, as on the Earth.
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