Millions of years ago, a pair of exploding stars showered our planet with radioactive fallout. Had those supernovae popped off a bit closer to home, Earth's biosphere would have been toast. But even at a distance of 300 light years, the stellar events might have had an impact on the evolution of life here. A supernova remnant. Image: NASA
The idea that astrophysical phenomena, including black hole x-ray flares and supernovae, can shake up life on Earth enough to direct evolution has been around for a while. And when a crop of scientific studies published in Nature and Science this past April presented evidence for two nearby back-to-back supernovae toward the end of the Pliocene, scientists immediately began discussing potential impacts on Earth's climate and biology.
Now, the hypothesis that there were impacts has gotten a boost from a computer modelling study, which estimates how much additional radiation life on Earth was dosed with following the cosmic fireworks. To cut to the chase, the radiation load for terrestrial and shallow marine life would have roughly tripled for thousands of years after each event, thanks to a 20-fold increase in the number of high-energy muon particles striking the ground.
This was a bit of a surprise. "I was expecting there to be very little effect at all," said University of Kansas physicist Adrian Melott, who co-authored the study appearing this week in The Astrophysical Journal Letters.
Melott is not a biologist, but he doesn't seem to mind a bit of wild speculation where supernovae are concerned. Regarding the new finding, he says the extra radiation at ground level might have been enough to increase the rate of DNA mutation, which in turn could have briefly sped-up evolution. (Evolution can't occur unless DNA is mutating, a process that typically happens very, very slowly.)
The study further suggests that high-energy cosmic radiation could have increased the ionisation, or electric charge, of the troposphere, resulting in more cloud-to-ground lightning strikes. Whether tropospheric ionisation had an additional climatic or ecological impact remains an open question.
It's by no means a silver bullet for the argument that supernovae have impacted evolution — just another shred of evidence that the history of life on Earth was a rocky roller coaster. Really, it's a miracle we're here at all.