Theories of Terrestrial Mass Extinction by Extraterrestrial Objects

Abstract

Theories of mass extinction by extraterrestrial objects have been intermittently proposed by European and North American scientists for at least 250 years. Until the late twentieth century, such theories usually treated biological extinction as a single aspect of global cataclysm. Proposed ultimate causes of mass extinction have included the passage of planetesimals close to Earth, impacts of comets or asteroids, and transient increases in cosmic radiation. These theories have consistently extrapolated from astronomical observations to suggest that extraterrestrial objects had affected Earth at some time in the past. However, until the late twentieth century, none of these theories proposed definitive tests of temporal coincidences between mass extinctions and and the terrestrial effects of extraterrestrial objects. Harold Urey's (1973) cometary impact hypothesis was the first to propose such a test. The impact-extinction theory of Luis Alvarez and his colleagues (1980) was the first to provide direct evidence of such a coincidence. Such tests were probably not possible before the middle of the twentieth century. The failure of earlier theories to generate sustained scientific interest can be most simply attributed to their lack of such evidence. Theories of extraterrestrially caused mass extinction have almost always combined hypotheses of ultimate and proximate causes of extinction. The stability of these combinations has varied greatly from category to category and age to age. In general, these combinations evolved much more rapidly when they were presented and discussed in mainstream scientific literature. Despite their ubiquity, hypotheses of proximate causes have served a limited role in the reception of these theories. Scientific critics of these theories have consistently required hypotheses of proximate causes to appear plausible. However, the recent history of impact theories indicates that an ultimate cause of mass extinction may be widely accepted without definitive proof of the proximate cause of extinction.