Using the revised timescales and Fourier analysis, Rohde and Muller looked for a periodic signal in the history of biodiversity. They were looking for evidence of a 26-million-year extinction cycle that had been hinted at in the 1980s; the strong peak in their power spectrum indicating a 62-million-year cycle was a surprise. We found evidence of the same cycle in three more data sets.

Nothing known in the motion of the Earth itself can make a 62-million-year cycle. Further, the laws of celestial mechanics rule out any object orbiting the Sun with such a long period; it would be so distant that the gravity of other stars would pull it away. But other astronomical cycles are still in play.

It takes about 200 million years for the Sun to complete one orbit around the center of our Milky Way galaxy. Moreover, the galaxy is a thin disk, and there is also a motion along a vertical direction. As our solar system slowly orbits the Milky Way's center, it oscillates through the galactic plane with a period of around 65 million years. When we move up in the disk, we are pulled back down by gravity, coasting past the midpoint, then rising back up again, akin to a weight bobbing up and down on a spring.

Was this the missing mechanism? In fact, Rohde and Muller had considered this and dismissed it, for the same reason almost anyone would: One would think that any effect would occur when we passed through the disk of the galaxy, or perhaps when we got very far away from it. But that would happen twice per cycle, every 30 million years or so, which doesn't explain the 62-million-year signal.

It turns out that the biodiversity minima of the 62-million-year cycle happens when the Sun is "bobbed up" on only one side of the galaxy, when the solar system is on the disk's upper, "north" side. The galaxy's north side lies toward the constellation Virgo, as well as the largest concentration of mass in our neighborhood, the Local Supercluster some 60 million light-years away. This supercluster is so massive that its gravity pulls our galaxy toward it at a velocity of about 200 kilometers per second.

As our galaxy falls into the Local Supercluster, it should disturb this gas and create a shock wave, like the bow shock of a jet plane. Shocks in hot gas at such high speeds generate cascades of high-energy subatomic particles and radiation called "cosmic rays." These should be showering the north side of the galaxy's disk. We are protected by the galactic magnetic field, much as the Earth's magnetic field protects our planet. When we rise to the north side, we are less protected -- and the ensuing flux of cosmic rays contains particles of such energy that they can reach the Earth's surface.