History of Life - Mass Extinction
Western Governor’s University
QXT2 ID Biological Science
March 13, 2015
History of Life - Mass Extinction
More than 99 percent of all species that have ever lived are now extinct. Usually, extinctions happen gradually, but several times in Earth’s history mass extinctions wiped out entire ecosystems. During these events, some biologists propose, “many species became extinct because their environment was collapsing around them, rather than lack of adaptations to a slowly changing ecosystem.” (Donovan, 61) Under sudden, extreme or violent environmental pressures, extinction is not necessarily related to ordinary natural selection.
One such event was the increase in the amount of oxygen in the ancient atmosphere. Ancient photosynthetic organisms produced a rise in oxygen in Earth’s atmosphere. These single-celled prokaryotic organisms (much like modern bacteria) evolved in the absence of oxygen over 3 billion years ago. Over time, they became abundant in shallow seas of the Precambrian era and were producing oxygen at a high rate as a by-product of photosynthesis. The oxygen gas began accumulating in the atmosphere causing methane and hydrogen sulfide levels to decrease and the emergence of the ozone layer. Biologists hypothesize that the increase in this highly reactive gas “drove some life forms to extinction, while other life forms evolved new, more efficient metabolic pathways that used oxygen for respiration.” (Miller & Levine, 426) This forced most anaerobic organisms into extinction or into the few airless habitats where their anaerobic descendants remain today. The few that survived evolved ways of either using oxygen for respiration and were precursors to modern eukaryotes (the Endosymbiotic Theory).
Another mechanism that has been suggested to have caused mass extinction is the effects of meteorite impacts on the early Earth (several specific records from Cambrian period through most recent Tertiary period). While the meteorite impact theory of mass extinctions has become accepted by many scientists for particular extinction events, there is still considerable controversy. To highlight one such event:
“The mass extinction at the end of the Mesozoic Era, that is the Cretaceous - Tertiary boundary (often called the K-T boundary) 65 million years ago, shows much evidence that it was related to an impact with an extraterrestrial object. This event resulted in the extinction of over 50% of the species living at the time, including the dinosaurs.” (Meteorites, Impacts, & Mass Extinction, n.d.) Through geological research in the late 1970’s, a layer of Iridium laden clay deposits was found in a large worldwide distribution suggesting the fallout from a massive meteorite impact. Scaled effects of such an event would include (depending upon location of impact): massive magnitude 13 earthquake, wildfires triggered by the fireball from the atmospheric emergence of the meteorite, acidification of surface waters due to the nitrogen oxides produced combining with water to produce nitric acid rain, and finally the “large quantities of dust put into the atmosphere would block incoming solar radiation.” (Meteorites, Impacts, & Mass Extinction, n.d.) The dust would significantly lower the ability of photosynthetic organisms, such as plants, to photosynthesize. These producers are the base for all food chains and this lack of an entire trophic level would violently disrupt ecosystem stability.