New work out of the Danish National Space Center (DNSC) suggests a startling connection between star-making in the Milky Way and the evolution of life on Earth. During a period of intense star-creation that began some 2.4 billion years ago, ocean-borne bacteria went through cycles of growth and decline of an intensity never since equalled. The Danish study links this variability with incoming cosmic rays that reach Earth from exploded stars. The star-making period in question was a time of numerous supernova explosions.

To reach these conclusions, the Space Center’s Henrik Svensmark studied the record of heavy carbon in sedimentary rocks. Growing bacteria and algae in ocean waters absorb carbon-12, leaving carbon-13 to enrich the sea; the latter begins to appear in the carbonate shells of sea creatures. By studying variations in carbon-13, Dr. Svensmark can see how much photosynthesis was going on when the shell-making species were alive.

And it turns out that the biggest fluctuations in productivity coincided with high star formation periods as well as cool periods in Earth’s climate. During a billion year period when star formation was slow, the Earth’s climate was warmer, cosmic rays were less intense, and productivity in the biosphere was all but unchanged. Says Svensmark: “The odds are 10,000 to 1 against this unexpected link between cosmic rays and the variable state of the biosphere being just a coincidence, and it offers a new perspective on the connection between the evolution of the Milky Way and the entire history of life over the last 4 billion years.”

Image: Cosmic radiation penetrating the atmosphere promotes the formation of clouds which have a cooling effect on Earth’s climate. Credit: Danish National Space Center.

How would cosmic rays affect biology on Earth? One possibility is a link between cosmic rays and cloud formation, the subject of recent experiments reported by the DNSC. Stronger winds during the cold periods caused by increased cloud cover would stir ocean waters and improve the supply of nutrients in surface water, creating greater fluctuations in biological activity. All this as the result of supernovae activity in nearby space.

The paper is Svensmark, “Imprint of Galactic dynamics on Earth’s climate,” Astronomische Nachrichten Vol. 327, Issue 9 (October 2006), pp. 866-870), with abstract here.