SCIENTISTS who study the evolution of Earth's life forms now look to what they call the interplanetary ``shooting gallery'' to help explain what has happened. The once heretical notion that the crash of an asteroid, or perhaps of a comet, could profoundly affect organic life has entered the scientific mainstream. It is a major change in scientific perspective.
The latest evidence driving this change was published this month in Nature by Meixun Zhao and Jeffrey L. Bada of the Scripps Institution of Oceanography. They found extraterrestrial chemicals in sediments that mark the so-called K/T boundary between the Cretaceous and Tertiary periods. That's the time, 65 million years ago, when the dinosaurs died out.
These chemicals - the amino acids alpha-aminoisobutyric acid and isovaline - are rare on Earth but common in meteorites. Their presence at the K/T boundary supports the notion that the impact of a meteorite (or maybe a comet) did in the dinosaurs and caused the mass extinction of many other organisms.
Richard A. Muller of the University of California at Berkeley, who champions this concept, told a session of American Association for the Advancement of Science annual meeting in January: ``This discovery of what did in the dinosaurs has revolutionized the way we think about evolution. It may well be that mass extinctions were the key driving force in evolution. ... The creatures that survived were the ones that could adapt to catastrophe.''
The late Luis Alvarez (Nobel prize-winning physicist) and his son Walter (geologist), also at UC Berkeley, first suggested the impact hypothesis in 1980. They found a relatively high abundance of the element iridium in K/T boundary-layer clay at Gubbio, Italy. Iridium is rare at Earth's surface but abundant in meteorites. The Alvarezes and other investigators subsequently found the iridium meteorite ``signature'' in K/T bounday-layer clays in other places around the planet.
Many other scientists - especially traditional geologists and paleontologists - greeted the Alvarez hypothesis with great skepticism, sometimes with open derision. But evidence has accumulated to strongly support, if not definitively prove, it. And some of the most impressive evidence - including the amino acid finding - has been published this year.
Some investigators have found forms of quartz that form only under pressure of massive impacts or explosions. In March John F. McHone and colleagues at Arizona State University reported in Science the presence at the K/T boundary of a form of shocked quartz called stishovite. Dr. McHone noted that ``stishovite is known only from experiments, nuclear blasts, and known meteor craters.''
McHone reported this discovery informally last October at a meeting in Snowbird, Utah, that was held to review the entire subject of meteor/comet impacts. In a later report of the conference, Clark R. Chapman of the Planetary Science Institute in Tucson, Ariz., noted that ``such high pressure silicates ... are widely regarded as proof of impact.''
Commenting on the conference as a whole, he observed that ``it seems more plausible than ever before that similar impacts have played a major role in the evolution of life throughout Earth's history.''
John R. Cronin of Arizona State University, in reviewing the recent amino-acid discovery for Nature, says it ``supports the idea that impact events could [even] have supplied the Earth during a much earlier period with the raw materials for organic chemical evolution.''