Were Muons to Blame for a Mass Extinction More Than Two Million Years Ago?

Supernova explosions could have far-reaching consequences for life on Earth, but probably not in this case.

The supernova that formed the Crab Nebula (portrait by the Hubble Space Telescope) was fairly recent, but not as deadly, perhaps, as explosions in the distant past.

In a recent paper published in the journal Astrobiology, Adrian Melott from the University of Kansas and two co-authors from Brazil suggest that supernova explosions that occurred about 2.6 million years ago were the culprits behind one of Earth’s major extinction events. They propose that cosmic rays from the supernovae collided with particles in the Earth’s atmosphere, creating muon showers that exposed living things to radiation dosages about 14 times higher than the average dose we experience on Earth’s surface today

Muons are elementary particles with a half-life of a small fraction of a second. Because they move at near-light velocity and are produced mostly by collisions in Earth’s atmosphere, they could, in principle, be devastating to life on our planet. Melott used modeling to explain that muons would penetrate deep into the oceans, and would increase the lifetime risk of developing cancer in large animals by 16 to 40 percent. A supernova, they suggested, could thus be responsible for a mass extinction 2.6 million years ago that wiped out 36 percent of the genera of large marine animals.

That event marks the boundary between the Pliocene time period and the Pleistocene era, when we see a notable change in flora and fauna in the fossil record. The traditional explanation for this particular extinction event is that it happened due to extreme climatic changes associated with the advance and retreat of glaciers.

In spite of the new study by Melott and colleagues, that traditional explanation still seems to make the most sense. For one thing, the timing doesn’t fit. In a 2016 paper by Dieter Breitschwerdt from the Technical University Berlin and colleagues, two nearby supernova explosions were identified based on iron isotope signatures in the geologic record—one 2.2 million years ago and another 1.5 million years ago. There were no major extinction events at these times.

Even if the times did match up (and it’s difficult to pinpoint the exact timing of these events), the supernova radiation signal would have had less effect on life than the climatic shifts going on in that tumultuous period of Earth’s history. Furthermore, an increase in cancer rate alone would not trigger a mass extinction. Most of the cancers would arise late in life, after animals would have reproduced already. Higher radiation dosages could have altered evolutionary trajectories due to higher mutation rates, but it’s difficult to see a link to extinction.

There is one fascinating aspect of the supernovae that occurred about two million years ago, though. At that point our own hominid ancestors were already around. One can only imagine what these early humans would have thought of bright lights appearing suddenly in the night sky.

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