Thirty Million Years
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In the essay on the Permian-Triassic mass extinction I noted that paleontologists familiar with that time claim that life on Earth took about thirty million years to recover fully from the Great Dying. On first impression that statement sounds absurd.
Forests that have been destroyed by fire can regrow themselves within a century or two. Animal populations, devastated but not extinguished, can recover in even less time. So what took thirty million years?
In the Great Dying some ninety percent of life on Earth got wiped out. It wasnít only a diminution of the quantity of life: the mass extinction also reduced the diversity of life. Whole families of the Linnaean classification went extinct. When life grew back in the opening centuries of the Triassic period it came back simpler than it was before the Permian-Triassic disaster; it came back with fewer species of plants and animals.
Consider the difference between a natural forest and a tree farm or an orchard. In the artificial woods we find one species of tree, no shrubbery or underbrush, and few species of animals. In terms of biological diversity it is sparse, much like forests at the beginning of the Triassic period. On the other hand, in a natural forest of the present era we find at least half a dozen species of trees overshadowing scrub and bushes and harboring numerous species of birds, rodents, larger browsers/herbivores (such as deer, moose, and elk), and predators (usually snakes, canids, cats, and raptors). Why does that difference exist?
Biologists call it radiation and we can think of it as something akin to the law of entropy. Just as particles in a gas spread out to fill their container (and thereby increase the entropy of the gas), so too does life spread out to fill all available ecological niches, creating new niches as it does so. In the case of life the spread comes about through the interplay between mutation and natural selection. Thus, for example, when plants emerged from the water and began growing on land, they created an opportunity for certain arthropods (creatures related to crabs and shrimp) to come out of the water and evolve into insects. The insects, in their turn, created a niche that certain lobe-finned fish could fill by coming out of the water to eat them.
In the Great Dying large numbers of ecological niches became biological ghost towns and, as a consequence, many more ceased to exist. In order to recreate a viable biological economy, evolution had to repopulate those ghost towns and create new niches to replace the vanished ones. But while we conceive those empty niches as beckoning to life, there was actually no phenomenon that urged life to fill them: they existed only as opportunities and Nature remained indifferent to their being filled or not. If some organism began to exploit one of those opportunities, effectively taking up residence in the niche it represented, it was a matter of pure happenstance.
First, some members of the population of organisms had to possess the mutations that would let them enter the niche. The organism could then enter the niche as a predator or as a symbiont (plants are either predators of or symbionts with the soil, depending upon whether they only exploit the soil or exploit and enrich it). Natural selection weeded out the organisms that fit the niche least well and promoted those who fitted it best, however imperfect that fit was. Over time, as the new population accumulated mutations, natural selection refined the fit until the organisms had become a new species more or less perfectly fitted to their environment.
But the world of the early Triassic period didnít see the evolution of a new species here and there, now and then, as in the normal world. Like the particles of a gas expanding to fill a container that is itself expanding, life evolved to fill the available niches, creating new niches as it did so and then filling those. It takes thousands of generations for a new species to evolve from a founding population. What appears on paper as a frenzy of evolution actually took millions of years. The burst of evolutionary creativity at the beginning of the Triassic continued until life achieved evolutionary equilibrium, in which old species went extinct at the same rate as new species came into being. Thatís what took thirty million years.
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