LECTURE 22 -- JULY 23, 1997

EVOLUTION AND THE GAIA HYPOTHESIS

So we concluded last time that the discontinuous nature of most of the fossil record occurred because most evolutionary events happened at the margins of populations where subgroups are small and consequently inbreeding is common. This makes the lineages discontinuous because most fossilization would be at the main area of taxon distribution and therefore, would miss the evolutionary action.

Further, in examining evolution, it is heavily random. This comes from several groups of observations. First, in island biography, there is a neat linear relationship on a log/log scale whereby there is a systematic increase in species with larger islands. That is, the number of species (genera would work about as well) birds, or reptiles, or insects etc . increase systematically and linearly with little noise as islands get larger. No one ever specifies the specific species, in other words, all species are treated the same.

Island biogeography has been taken further, Many islands, after new formation (Surtsey - new volcano near Iceland) or from various reconstruction after everything was killed (Krakatoa) have been studied as to the rate new species come back. Also small islands have been studied experimentally by killing everything on them and then watching the recovery. Anyway, in all cases, species return relatively rapidly at first and then the rate diminishes to a zero. At that time some species go extinct (on the island) and new ones arrive, but the idea is that species number reaches an are dependent equilibrium (with all species treated equal).

There is other evidence of this randomness. For example if one plots the number of species in a larger taxon (e.g., order) that exist at a certain time in the past and then plots all those that exist afterwards every few million years (or less) one will find that on a log/log scale the species (all treated the same) number diminishes linearly. In radioactive decay, we plotted such events (e.g., U₂₃₈) algebraically. Here we are treating species as if they are atoms. However, the point is species go extinct with the same mathematics as the decay of atoms. The line is linear regardless of the larger taxon chosen. Slopes are different for different larger taxa, but so are half-lives different for different radioactive isotopes. The point is that we are forced to treat the decay of atoms as a random happening and so since species resemble atomic decay in their extinctions, it follows that extinctions are (perhaps still worrying about the noise) random events at least at the level of the precision of our data.

There are other ways of looking at evolution that suggest randomness, but what's so far said is suggestive enough. Do keep in mind that the data is sloppy so randomness could be very important and still something else could exist that has not yet been discovered.

Evolution used to be interpreted (starting with Lamarck) as a directional force leading eventually to humans. Essentially, this was interpreted as a long climb up a ladder. In fact they used to talk about elan vital ( a vital force that was not present in inorganic events). Nowadays, we tend to think of evolution as mostly random walk. That is, imagine you are in the midst of a huge dark woods and have no compass and can't see the sun. Thus you walk and since you have no sense of direction you turn and turn. Eventually after a complex path you might end up 5 miles from your start, but those 5 miles are not in a direction. If you could be returned to the start and do it over you would end up somewhere else. That is random walk. Motion, but in no specific direction. For further explanation see Gould, S., This Wonderful life -- This is a readable account of the Burgess shale and its fossils and the evolutionary implications.

Another aspect of evolution is the end of a species, or extinction. There are extinctions throughout geologic history. In fact, by far, most species (say 99.9% or more) have gone extinct. Of this, there is background extinction, or the regular extinction of taxa through time, and there is catastrophic extinction. These are times when a large number of taxa seemingly go suddenly extinct.

It is hard for me to think the two kinds of extinction are truly different, although they may be. One has also to keep in mind that the concepts of instants and simultaneous are somewhat fuzzy in geology. For example a million year error in deep time isn't much; and simultaneous is hard, but not always impossible, to gauge in geology. Sometimes one can get simultaneous fairly close still with a million year error.

Anyway, with regard to mass extinctions, there are measurements by good mathematicians (Sepkowski and Raup) that suggest there is a 26,000,000 year cycle (measured back into the Paleozoic) of mass extinctions. Such a cycle suggests a single cause (if truly cyclic and not just repeating) of mass extinctions. A favorite, of some is a Death Star -- that is a star (undiscovered so far) with which our star is in orbit and the orbit is very uncircular bringing the Death Star and its particles close every 26,000,000 years.

There are two clearly recognized (unlike many important others that stand out less) times of extinction -- the end of the Paleozoic when many animals failed to enter the Triassic, and the end of the Mesozoic when dinosaurs and ammonoids and many others went extinct. The most studied of these extinctions is the end of the Cretaceous where Alvarez had the interesting hypothesis that a large meteorite impacted with earth. The idea is that the impact (like those shown by the craters of the moon) was explosive because the velocity was so high that on impact the meteorite gasified and blew particles far into the stratosphere. In the troposphere (below the stratosphere and where clouds are located), tiny particles get glued together by water and settle rapidly. However, in the stratosphere, such particles stay effectively small and may take years (2 or 3) to settle to the troposphere. This accounts for the continuing red sunsets after volcanic eruptions. However, the idea on large meteorite impact is that the amount of material in the stratosphere would be much greater than our experience and would change the climate and cause plants and animals to go extinct.

The K/T --Cretaceous /Tertiary boundary was studied. Often times there is an unconformity there that proves nothing. However, they found a number of continuous sections and it is revealed that Iridium is concentrated there (meteorites have more iridium than the Earth's crust). Also there are other data so it looks like a meteorite impacted at that time and may well have caused some taxa to go extinct.

I have one objection to the theory of meteorite impacts as the cause of catastrophic extinctions. First of all it looks like the 26,000,000 year cycle is soundly based. That is it is truly periodic (although I saw that denied in Science (a journal) in about 1994. If it is periodic, then the Cretaceous extinctions must be just like all the others. I have never seen any good evidence that meteorite impact evidence has been found at any other extinction time. Thus the meteorite may well have hit and may well have caused some taxa to become extinct, but it is not the driving cause. If other times of extinction show up meteorite evidence, I may well change my mind.

One can add here, about extinction, that the only other general extinction hypothesis postulated develops off the island biogeography model. That is, the areas available for species are related to species richness. Transgressive (advancing) seas make more epeiric sea bottom area and so encourage richness and may do the opposite for terrestrial taxa. Regressive seas encourage marine extinction. This model was applied especially to the Permian/Triassic boundary where a vast number of species and higher taxa were extinguished. Later analysis showed that if islands were about as numerous as nowadays the extinctions would not have been as severe as calculated suggesting that island biogeography considerations could not be the whole cause of Permian/Triassic boundary extinctions. Islands offshore area is little affected by transgression and regression..

Everything aside, the periodic evidence of extinction cries out for a single cause. Keep in mind that random events could aggravate any extinction in addition to the periodic cause.

 GAIA

The gaia hypothesis is most championed by James Lovelock in a book of that title. Briefly the first basis of Gaia is that everything on Earth plus the sun interact. That is the sun interacts with life (photosynthesis) and life interacts with rocks (especially in soil making) and the mantle sends nutrients to the surface (volcanoes -- especially at spreading ridges). The crust, at subduction zones, dives into the mantle and alters it with all the products of weathering delivered downward. These diving plates also make the andesite and granite associated with subduction. Radical Gaia fans even bring the core into this interaction through convective turnover and magnetic directions and reversals.

Another way to look at Gaia is to consider how much of present Earth is governed and caused by life. First of all, in level bottom communities, those with soft marine sediments (and that is most shallow marine communities) any that are deeper than wave base have a sedimentary character that is entirely caused by the biota. Sedimentology plays no role. Wherever waves affect things (sandstones and the like) sedimentology plays a role, but not in any fine grained sediment -- these in rocks are fine limestones and shales. Thus the structure of most marine rocks is biologically determined. Terrestrial rocks (e.g., the badlands of South Dakota) are a series of soils formed on overbank deposits and so, being soils, most of the structure is biologically determined. Channel deposits, of course don't count. So, in summary, most marine and terrestrial rocks derived their structure biologically. Thirdly, the atmosphere contains much free oxygen and little Carbon dioxide. Since life is responsible for both facts, life controls the atmosphere. Fourthly, atmosphere interacts with the ocean and since atmosphere is changed, so is ocean composition, acidity etc. Fifthly, plate tectonics works because the plates cool and can subduct. Before life diminished the CO2, it must have been quite warm with the resulting greenhouse effect. This would have suppressed subduction and plate tectonics. For example, Venus is very hot and has no plate tectonics because the plates can't cool and so can't be subducted.. In effect, plate tectonics is biologically caused. This has great ramifications. Degassing is especially prominent at spreading ridges and this would supply much nutriment. Most mountain building is caused by plate tectonics, so, in effect, life caused plate tectonics. A good partial test of this opinion is to notice if plate tectonics (in the real sense of plates) is ever discovered on Venus. Gaia says it won't be discovered.

The more radical Gaia people say Earth is alive. They note that life has adiopoeisis; that is that life, unlike all other chemical reactions, does things that keep the reaction going. Before dying, life reproduces (often enough) so the reaction is permanent. They see the same things on Earth -- to wit, plate tectonics, oxygenated atmosphere -- that keep the reaction going. Thus they say Earth is alive.

I, on the other hand, learned what is alive v what is not from louids Pasteur. To me life has to grow and reproduce. Earth does not and is therefore, not alive. The reactions observed by Lovelock etc. are merely equilibrium reactions, and such reactions are characteristic of science and not only life!