A Series of Fortunate Events

Dumb luck, happenstance, accident, call it what you will. McFarland's late arrival to the airport was purely an accident, albeit an accident with enormous personal consequences. It is sobering to think what a thin line there can be between victim and survivor, between life and death. What a difference just 30 minutes can make. It's a thin line in nature as well, not just for individual creatures—think about animal prey—or even for species, but of whole worlds. Drive almost anywhere outside of a city, and the road is likely cut in places through a rock bed. Chances are most of us just ignore the pages of history staring us in the face. But those stacks of often-colorful stone tablets tell stories, if you know how to read them. Strada Reginale 298 winds through a limestone gorge just outside Gubbio, a charming medieval town in the Umbria region of Italy. In the mid-1970s, geologist Walter Alvarez saw an interesting pattern in a column of rock very close to the road. He noticed that in one section of the many layers of limestone, there was a switch in color from white below to red above. When Alvarez looked closer, he saw that there was a peculiar layer of grayish clay separating the two colors of rock. Alvarez's decryption of that one-centimeter-thin line would lead to one of the most stunning and revolutionary scientific discoveries of the 20th century, and begin to tell the story of the most important day on Earth in the last 100 million years. A day that was very, very unlucky for most everything alive, but would eventually turn out to be extremely fortunate for us. And on that day, a long, long time ago, 30 minutes would make all the difference. A Dividing Line Between Two Worlds One way that geologists characterize rocks is by the fossils they contain. The Gubbio rock formation was once part of an ancient seabed, so it contained the fossilized shells of tiny creatures called foraminifera, or forams for short. These abundant, single-celled organisms are part of the ocean's plankton community and food web. When forams die, their shells settle in ocean sediments and form parts of limestones. Different foram species, which have different sizes and shapes of shells, have existed throughout time, and so can be used to assign rocks to particular time periods. When Alvarez looked at the forams from the rock cut outside Gubbio, he saw that the white layer of rocks contained a diverse array of large, fossil forams, but the reddish layer of rock just above it lacked those species and contained only a few, much smaller species of forams. And the thin layer of clay in between the two colors of rock appeared to lack fossils altogether. Alvarez realized something dramatic had happened in the ocean that had driven many foram species extinct in a short period of time.