  The terrestrial fossil record shows that the exponential rise in biodiversity
since the Precambrian period has been punctuated by large extinctions, at
intervals of 40 to 140 Myr. These mass extinctions represent extremes over a
background of smaller events and the natural process of species extinction. We
point out that the non-terrestrial phenomena proposed to explain these events,
such as boloidal impacts (a candidate for the end-Cretaceous extinction), and
nearby supernovae, are collectively far more effective during the solar
system's traversal of spiral arms. Using the best available data on the
location and kinematics of the Galactic spiral structure (including distance
scale and kinematic uncertainties), we present evidence that arm crossings
provide a viable explanation for the timing of the large extinctions.
