Monday, August 01, 2011
Symbiosis, Deep Homology, and Planetary Osteoporosis
by Kevin Baldwin
Though Darwin is best known for his theory of natural selection (1859), another contribution he made to biology was his recognition of coevolution and symbiosis (living together). Nature was more than simply "red in tooth and claw" (Tennyson 1849): Organisms not only compete, but can cooperate with each other to achieve new capabilities. One of Darwin's most famous inferences was that a newly discovered African orchid with a foot long corolla (!) must have a moth pollinator with a similarly long tongue (Darwin 1862). This moth was soon discovered and given the subspecies name praedicta, to indicate its prophesied existence.
Life abounds with examples of cooperation despite our preoccupation with competition and predation. Eukaryotes are much larger and more complex than the bacteria from which they evolved. They originated when large bacteria engulfed smaller ones that provided sugars (derived from photosynthesis) or high energy phosphate compounds in return for shelter in the larger bodies of their hosts. This event, called the endosymbiotic origin of eukaryotes is one of the most important transitions in the history of life. Lichens are a symbiosis of a photosynthetic algae and a nutrient scavenging fungus, which by themselves would not be terribly successful, but together can live in some of the most inhospitable places imaginable. The coevolution of flowering plants and pollinators is well known (see above). Floral nectar and pollen are traded for pollination services by insects, birds, and bats.
Reef building corals are basically tiny sea anemones that harbor symbiotic photosynthetic algae that trade their sugar production for nitrogenous waste produced by the coral host. Sugar is a source of energy that is exchanged for Nitrogen, an important component of proteins. This cooperation enables corals to have the high rates of calcium carbonate deposition necessary for healthy, growing reefs.
Recent molecular analyses tell us that the gene that directs coral exoskeletal development is the same one that directs human skeletal development! This deep homology indicates that corals and humans share a common ancestor from over half a billion years ago. The expanses of time and evolutionary change that separate us may make corals seem remote, but to me it is amazing and humbling to contemplate that relationship.
Another profundity that emerges from recognizing this homology has to do with skeletons, the scaffolding that supports animal bodies. Long after the flesh has been eaten or decomposed, skeletons remain (Alas, poor Yorick! I knew him, Horatio); thus vertebrate bones and invertebrate shells make up much of the fossil record. Scaffolding also supports many of the structures we make and rely on: Think about buildings or airplanes without their skins. What would happen to them if the scaffolding dissolved? Imagine the Minneapolis I- 35W bridge collapse of 2007 occuring to most of our transportation infrastructure over a short time span.
Dismantling biological skeletons is no mean feat, yet we are accomplishing it on a global scale. Rising atmospheric CO2 levels are increasing both the temperature and acidity of the world's oceans, which is stressing living corals, causing them to expel their symbiotic algae, which results in coral bleaching. In some cases, bleaching can lead to the dissolving of their exoskeletons (Weis & Allemand 2009). In effect, we are causing osteoporosis in coral reefs around the world.
Reefs such as Australia's Great Barrier Reef are the world's largest living structures. They are like the tropical rainforests of the ocean because of their high productivity and species diversity. That we are killing and dissolving corals is one of the many consequences of global climate change. By dismantling the skeleton of the earth we risk dismantling the ecological scaffolding upon which we and many other species depend.
The deep homology that links our bones to coral reefs have been around essentially unchanged for the half last billion years or so. This is as close to eternity as I can imagine. A materialist eternity will likely only be on the earth that we bequeath to our descendants. It is up to us to determine if it will be a heaven or hell.
Darwin, Charles. On the Origin of Species. John Murray. 1859.
Darwin, Charles. On the Various Contrivances by which British and Foreign Orchids are Fertilised by Insects and on the Good Effects of Intercrossing. John Murray. 1862.
Tennyson, Alfred. In Memoriam A.H.H., Canto 56. 1849.
Weis, V. M. and D. Allemand. What determines coral health? Science 324:1153-1155, 2009.
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