(This is Part 5 of a series. Go back to Part 4.)
Two hundred and fifty million years ago 95% of all the life in the oceans was obliterated and a similar percentage perished on land. It was the largest extinction of life in the history of the earth, though we notice that dinosaurs, mammals and then humans rose up afterwards.
But that is only one of five major extinctions of species that have occured on earth. They occurred 440, 365, 250, 210 and 65 million years ago respectively. In addition, the geologic record shows that there have been countless smaller extinctions.
Jack Sepkoski, a paleontologist at the University of Chicago, wanted to see if he could quantify all of this. Drawing together countless studies in the field, Sepkoski in 1993 published a gigantic database compiling the fossil records of some 40,000 genera, or families of species.
For the first time science now had a comprehensive record, over the last 600 million years, of the flourishing and perishing of life on earth. The physicists Ricard Sole and Susanna Manrubia decided to analyze this data and published their results in 1996, just eight years ago.
What they found is that the record of extinctions follows a power law, in fact the same one that earthquakes follow. That is, if you double the size of an extinction it occurs four times less frequently—and this relationship holds across the whole spectrum of extinction sizes.
The first implication of this finding is that there need be no special reason for the large extinctions. Rather, large extinctions are normal and something to be expected from time to time.
The second implication is that life itself exists at the
critical state, where "avalanches"—extinctions—of all sizes can and will happen. How do we know? Because inverse power laws are a symptom of, they arise from, systems that are operating at the critical state.
The physicists Per Bak and Kim Sneppen wanted to see if they could model the interactions of species on a computer. So they set up artificial "species" and assigned each one a random "fitness level"—how well it's fitting in to its environment.
Then, in the simplest way possible, they made it possible for the different computer "species" to interact randomly and affect each other's population growth and decay. Notice that this crude model did not contain any of the specifics of species in real life. These were simply random computer species, programmed to interact randomly.
They set the computer to run the simulation and sat back to see what would happen. And what they found was amazing: They found that even in this simplistic model the system evolved until it reached a critical state. What does this mean?
It means that even this crude model was able to self-organize itself to a point where it existed in a state far from equilibrium, a state where the computer "life" would grow and flourish and yet be punctuated periodically by "extinctions" of various sizes.
Furthermore, they found that these computer "extinctions" followed an inverse power law just like the real extinctions in evolution—numerous small extinctions, a fair number of medium ones and a few catastrophic ones now and then.
They found that, when the system reached the critical state, sometimes the extinction of a single species could set off an extinction "avalanche" that would cascade through the system and cause thousands or millions of other species to go extinct as well—with no outside
cause. Just a feature of the system.
We've now seen examples—in scientific revolutions, in wars, in the movements between tectonic plates, in the interaction of grains of sand, in the rise and fall of species—how various systems tend to self-organize to the critical state and then stay there as they evolve.
Each of these systems is essentially a network. The interacting nodes in the network can be grains of sand, tectonic plates, species of plants and animals, scientists, nation-states and so on.
In each case the network self-organizes into a critical state—far from equilibrium—where stability and growth are "punctuated" from time to time by cataclisms, extinctions, revolutions that ripple through the system.
These cataclisms, while painful, also serve to eliminate distortions that have arisen—so that the system can renew and regenerate itself and embark on a new period of stability and growth.
Moreover, a common self-organizing thread seems to be weaving itself through areas of human discovery that today seem as widely separated as physics, geology, biology, sociology and geo-politics.
The astonishing view that non-equilibrium physics is beginning to discern is that there may be a continuous self-organizing principle that operates at all levels and dimensions of existence.
And the Gaia hypothesis—the hypothesis that the earth itself may be a single organism existing and evolving at the critical state—is strongly supported by the emerging theory of self-organized criticality.
(This is the end of Part 5. Go to Part 6.)
—jim sloman, 2.21.04 for Nov 4
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