More from THE LIFE AND DEATH OF PLANET EARTH, mit comments by M. E.
. . . [Earthly life] may have reached a steady state of diversity some time ago! Scientists also learned that biological productivity - the total amount of living plant and animal tissue on Earth - appears to have been higher 200 million to 300 million years ago, when the planet was warmer and richer in atmospheric carbon dioxide than it is today. . . . This is a seminal discovery. It would mean that irrespective of the number of varieties of life, there would be more life in the past - a great volume of living tissue on the planet.
It may be, then, that diversity peaked early in the history of animals and, in contrast to all views since the time of Phillips [that diversity increased markedly in the Mesozoic, and has continued to do so to the present day], has remained in an approximate steady state since. While the colonization of land led to many new species, the proliferation ended by the late Paleozoic, perhaps 300 million to 250 million years ago. Since then the number of species on the planet has been approximately constant, or perhaps even dropping. The planet may have been a richer place at the time of the dinosaurs, when carbon dioxide levels that plants require were higher and temperatures were generally warmer, but not too warm: a tropical world that may have exhibited the lushness of today’s tropics. If true, the implication of this for our thesis is important: perhaps our planet, rather than still growing in biodiversity totals, has already peaked.
What about the recent but staggeringly important role of humans in dictating levels of biodiversity? These models have been constructed looking at the deep past, and theorize a world where evolution runs according to the “old,” i.e., prehuman rules. But it is naïve to believe the processes acting in the long period of prehumanity will take place in the same fashion in our world where humanity has dominion over so much happening on the planet.
What is the future of biodiversity on the future Earth? Here the presence of humanity clouds our crystal ball, and astrobiological models are of little use. Humans are a wild card. We promote biodiversity in some areas and curtail it in others. The only certainty is that the biota making up our world will be different. Even in the near future the makeup of species and their distribution, relative numbers, and relationships between one another will have changed, and by the far future the accumulated changes may be breathtaking, for there can be no doubt that the evolutionary forces - perhaps highly affected by humans - will create new species and varieties, resulting in a global biotic inventory of species on Earth different from that of today. However, in this Age of Humans, the old rules resulting in the biodiversity of the current world are unquestionably changed by the presence of humanity.
It is an unambiguous fact that very early on our species learned to manipulate the forces of evolution to suit its own purposes, creating varieties of animals and plants that would never have appeared on Earth in the absence of our will. Large-scale bioengineering was under way well before the invention of written language. We call this process “domestication,” but it was nothing less than efficient and ruthless bioengineering of food stocks - and the elimination of those species posing a threat to the food stock. Once the new breeds of domestic animals and plants became necessary for our species’ survival, wholesale efforts toward the eradication of the predators of these new animals were undertaken.
The modern efforts at biological engineering are but an extension of the earlier efforts of domestication. Until the end of the twentieth century the natural world had never evolved a square tomato, or any of the numerous other genetically altered plants and even animals now quite common in agricultural fields and scientific laboratories. Just as physicists are bringing previously unseen elements into existence in the natural world through technological processes, so too has our species invented new ways of bringing forth varieties of plants and animals that would never have graced the planet but for the hand of man. The new genes created and spliced into existing organisms to create new varieties of life will have a very long half-life; some may exist until life is ultimately snuffed out by an expanding Sun some billions of years in the future.
Humans have thus profoundly altered the biotic makeup of the Earth. We have done it in ways both subtle and blunt. We have set fire to entire continents, resulting in the presence of fire-resistant plants in landscapes where such species existed only in small numbers prior to the arrival or evolution of fire-branding humans. We have wiped out entire species and decimated countless more either to suit our needs for food or security or simply as an accidental byproduct of changing the landscape to favor our new agricultural endeavors. We have changed the role of natural selection by favoring some species that could never otherwise survive in a cruel Darwinian world over others of estimably greater fitness. [On the other hand, that “cruel Darwinian world” would never have come up with such evolutionary abortions as bulldogs with muzzles so impacted that breathing is almost impossible for them, Hairless Rex cats with immune systems virtually nonexistent from conception on, and other egregious biological travesties. Kinder ain’t necessarily better when it comes to evolution; such “kindness” as we have used to breed such poor monsters is anything but, whereas “cruel Darwinian” evolution gets the whole business of monsters over with in a hurry by killing them off quickly, leaving the race to the swift and strong and healthy. - Monty Eisenstein] We have created new types of organisms first with animal and plant husbandry, and later with sophisticated manipulation and splicing of the genetic codes of various organisms of interest to us. The presence of humanity has begun a radical revision of the diversity of life on Earth - the number of species present, and their abundance relative to one another. We have created not only new ways of producing animals and plants through brutal unnatural selection, but we have also manipulated the most potent force of evolutionary change - the phenomenon of mass extinction. Humanity has even created a new mass extinction that is different from any that has ever affected the planet.
Extinction is the ultimate fate of every species; just as an individual is born, lives out a time on Earth, and then dies, so too does a species come into existence through a speciation process, exist for a given number of years (usually counted in the millions), and then eventually becomes extinct. The fossil record has tabulated random extinctions taking place throughout time. But the rate at which these “random” extinctions have taken place through geologic time turns out to be remarkably low. In order to account for this, Chicago paleontologist David Raup introduced the term “background extinction rates.” Raup has calculated that the background extinction rate during the past 500 million years has been about one species every four to five years. In contrast, conservationist Norman Myers of Oxford has estimated that four species per day have been going extinct in Brazil alone for the past thirty-five years. Biologist Paul Ehrlich has suggested that by the end of the twentieth century, the extinction rates were measurable in species per hour. These rates of extinction exceed anything known in the deep past. If continued for an appreciable period they will reduce the world’s biodiversity to levels under those of anytime during the past 100 million or even 200 million years, and will certainly contribute to the planetary reduction in biodiversity that will ultimately occur through natural affects [sic] of heating and carbon dioxide reduction - but millions of years sooner than would occur if humans were not on this planet.
- Peter D. Ward and Donald Brownlee, The Life and Death of Planet Earth: How the New Science of Astrobiology charts the Ultimate Fate of Our World (Times Books, 2002; ISBN 978-0805075120; http://www.amazon.com/Life-Death-Planet-Earth-Astrobiology/dp/0805075127, pp. 42-45