I just stumbled upon a very interesting research result: “Fossil Records Show Biodiversity Comes and Goes“:

A detailed and extensive new analysis of the fossil records of marine animals over the past 542 million years has yielded a stunning surprise. Biodiversity appears to rise and fall in mysterious cycles of 62 million years for which science has no satisfactory explanation. The analysis, performed by researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley, has withstood thorough testing so that confidence in the results is above 99-percent.

And if I read the included graph correctly, the last mass extinction was about 60-some million years ago, so the current extinctions would fit in too. One of the researchers posits an astrophysical explanation:

“Comets could be perturbed from the Oort cloud by the periodic passage of the solar system through molecular clouds, Galactic arms, or some other structure with strong gravitational influence,” Muller said. “But there is no evidence even suggesting that such a structure exists.”

One of the books in my collection, Second Genesis by Donald Moffitt, also posited such a galactic feature and links it to mass extinctions; I think he even got the period about right. I really must dig the book up from my stack of less-frequently-reread books. I think it dates from the 1980’s.