Two teams of scientists are reporting new evidence that speeding objects from space struck the Earth 370 million and 65 million years ago, possibly disrupting the global climate and causing mass extinctions.
The report on the 370-million-year-old impact raises to three the number of celestial collisions linked to major extinctions of life on Earth.
”This is quite exciting,” said Dr. David Raup, a paleontologist at the University of Chicago who believes periodic strikes from space have shaped the evolution of life.
Scientific skeptics hold that natural causes, like volcanoes or slow climate change, are mainly responsible for the five mass extinctions revealed in the geologic record. At best, they say, the impact of large asteroids or comets may have contributed to mass exterminations already in progress by kicking up clouds of dust to block sunlight.
The two new reports on extraterrestrial linkages appeared in Friday`s issue of the journal Science. The first is by three geologists, Dr. Stanley Margolis and Dr. Philippe Claeys at the University of California at Davis, and Dr. Jean-Georges Casier of the Belgian Royal Institute of Natural Sciences, in Brussels.
It reports the discovery of tiny glass beads, called microtektites, in Belgium shale. They were deposited some 370 million years ago in the late Devonian period, when 70 percent of Earth`s marine species mysteriously vanished. Among the hardest hit species were corals and trilobites, primitive creatures with segmented bodies.
While some glassy beads found in nature are volcanic in origin, the authors report that these tektites have varying shapes and a lack of bubbles that attest to their formation during a catastrophic impact, when molten rock was thrown skyward and then cooled in a rain of glassy debris.
The beads, of which more than 400 were found in a rock layer 2 to 4 inches thick, are reported to have round, elongated, teardrop and dumbbell shapes. The authors said geochemical analysis strongly suggested that they were produced from a cosmic collision.
While strikes from space have previously been suggested as the cause of the late Devonian extinction, with one proposal made as early as 1970, searches for geological evidence until now have always come up short. The discovery, the Science article said, strengthens the case for extraterrestrial impacts as the cause of mass extinctions of marine life during the late Devonian era.
