in compiling extinction records through time, such as Jack Sepkoski and David Raup of the University of Chicago, point out that this particular event stands alone in severity compared to all other such events. More than 50% of marine families died out, and this figure is more than twice that for any other extinction. Estimates of the percent of species (belonging to various families) that went extinct in this event vary from nearly 80% to more than 90%. It is clear that the vast majority of animal and plant life on the Earth was extinguished. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p179] The end-Triassic mass extinction, 202 million years ago: The end of the Triassic period witnessed a significant mass extinction in which about 50% of genera were eliminated. There is still a very poor record for the fate of land life at this extinction boundary, but it is clear that marine life at this time was extensively and catastrophically affected. This mass extinction, like the K/T event described at the beginning of this chapter, was thought to have been brought about by the impact of a large extraterrestrial body, either comet or asteroid. The Manicouagan Crater in Quebec is 100 kilometers in diameter (compared to the approximately 200-kilometer diameter of the Chicxulub Crater associated with the K/T event). This crater has been dated at 214 million years in age, which is older than the age of the Triassic/ Jurassic boundary. Environmental changes other than impact have been associated with this extinction event as well, most notably oceanographic changes creating anoxia in many shallow-water environments at the end of the Triassic. It is difficult, however, to see how such changes could have affected land life, which also suffered significant extinction at this time. The probable cause remains unknown. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p180] The Cretaceous/Tertiary boundary event, 65 million years ago: The mass extinction of the dinosaurs, as well as 50% or more of the other species then on Earth, has been recognized for more than a century and a half as one of the most devastating periods of mass death in Earth’s long history.
Earth succeeded in acquiring its ark-load of animals and complex then keeping them—for more than half a billion years (so far) because it retained its oceans for more than 4 billion years. Moreover, if our analysis of the sedimentary record is correct, for the last 2 billion years it maintained the oceans at average temperatures less than 50°C. Also—at least for the last 2 billion years—the oceans have been maintained at a chemical composition conducive to the existence of complex animal life: at a salinity and pH favorable to the formation and maintenance of proteins. The oceans are clearly the cradle of animal life—not fresh water, not the land, but the saltwater oceans have spawned every animal phylum, every basic body plan that exists or has existed on our planet. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p261] Discovering how Earth acquired its supply of water is one of the most critical concerns of the new field of astrobiology. As we pointed out in an earlier chapter, water was not abundant in the inner regions of the solar system when the planets formed. There was far more water in the outer regions of the solar system than among the inner planets. Where did our water come from? Although where our oceanic water came from is still the subject of debate, everyone agrees that it must have arrived during planetary accretion, with perhaps significant volumes added during the period of heavy bombardment. Ironically, the volume of water eventually found on Earth may be related to the formation of Earth’s core. When the iron- and nickel-rich core formed, most of the water found in the coalescing planet was consumed in oxidation processes whereby oxygen bound up in water was used to make iron and nickel oxides. It is the residual water that makes up the oceans. Perhaps that residual quantity was significantly enhanced by water carried by comets after Earth’s initial formation, perhaps not. In either case, the oceans reached approximately their present volume by 3.8 billion years ago. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p261] is evidence of deep-water, inorganic limestone formation in very old