Periodically, every fifty to one hundred million years or so, the earth has experienced mass extinctions, the relatively rapid, large-scale disappearance of many if not most living creatures. The world may be experiencing this phenomenon now at a far more rapid pace than ever before.

Of the five largest mass extinctions, the oldest occurred during the lower Ordovician period some 435 million years ago, when approximately one-quarter of all ocean families, and half of ocean genera, disappeared. (Creatures living in the sea are often used to measure the severity of mass extinctions, because the fossil record of marine sediments is more complete than that for terrestrial animals.) Among the creatures lost during this time were many types of trilobites, cephalopods, and crinoids, all of which are thought to have died out in shallow tropical waters because of sudden changes in ocean levels.

During the late Devonian period...