Walter Alvarez William P. Elder Thor Hansen Erle G. Kauffman Gerta Keller Eugene Merle Shoemaker Paul R. Weissman Piet Hut 1987 <div id="Abs1-section" class="c-article-section"><div id="Abs1-content" class="c-article-section__content"><p>If at least some mass extinctions are caused by impacts, why do they extend over intervals of one to three million years and have a partly stepwise character? The solution may be provided by multiple cometary impacts. Astronomical, geological and palaeontological evidence is consistent with a causal connection between comet showers, clusters of impact events and stepwise mass extinctions, but it is too early to tell how pervasive this relationship may be.</p></div></div> application/pdf 10.1038/329118a0 en Nature Publishing Group Comet showers as a cause of mass extinctions article