Space Invader

A comet or an asteroid strike about 65 million years ago is the favored explanation for why dinosaurs vanished from the earth so abruptly after having dominated the planet for the preceding 70 million years. The tremendous plume of smoke and dust thrown into the atmosphere by the space intruder's impact, equivalent to perhaps a million hydrogen bombs, would have blocked out sunlight for months. The globe would have gone into a dark, deep freeze, killing first the plants and then the giant lizards that fed on them, directly or indirectly -- paving the way for mammals, and eventually humans, to take over.

Now a report in Science suggests that 370 million years ago, during the Late Devonian period, a comet or asteroid caused an even greater catastrophe, one that wiped out fully 70% of all marine species on earth. American and Belgian scientists have found tiny glass beads just .3 mm (.004 in.) across, embedded in underground sediments in Belgium. The beads, called microtektites, are thought to be caused when silicon and other minerals melt and then cool following either a volcanic eruption or a high-speed impact. The chemical composition of these beads seems to point much more convincingly to an impact, and the theory is supported by the existence of at least two giant impact craters (one in Sweden and one in Quebec), also about 370 million years old. It also strengthens the idea that evolution owes much to giant rocks falling from space.