Millions of years ago, the Earth’s oceans were dominated by one of the biggest and most fearsome predators to have ever existed: the Megalodon. An 18-meter-long (59 foot) monster, it chowed down on sperm whales, could outswim Jaws, and once faced down Jason Statham armed with torpedoes. It would take something monumental to kill off this beast.
And yet, killed off it was. 2.6 million years ago, the megalodon, along with more than a third of the large marine creatures that lived alongside it, was wiped out by a mass extinction event.
Ever since researchers uncovered the first evidence of this biological catastrophe, they’ve been trying to figure out what caused it. Perhaps it was climate change that killed the Meg, says one theory, or maybe, in a worrying metaphor for our own future, its own greed destroyed the ecosystem that was keeping it alive.
But a new paper published last month in the journal Astrobiology offers an astronomical explanation: a collection of dying stars, going supernova just 150 light-years away from Earth.
Normally on Earth, the constant barrage of cosmic rays fired at us by distant exploding stars aren’t a big problem. But the researchers believe the amount of energy released by this chain of supernovae would have been enough to break through the atmosphere, showering the prehistoric megafauna with lethal amounts of radiation.
“We have the Local Bubble in the interstellar medium,” lead author Adrian Mellott said in a statement. “We're right on its edge … When we do calculations, they're based on the idea that one supernova that goes off, and its energy sweeps by Earth, and it's over. But with the Local Bubble, the cosmic rays kind of bounce off the sides, and the cosmic-ray bath would last 10,000 to 100,000 years. This way, you could imagine a whole series of these things feeding more and more cosmic rays into the Local Bubble and giving us cosmic rays for millions of years.”
“About one-fifth of our radiation dose comes by muons [particles that are created by cosmic rays colliding with the Earth’s atmosphere],” explained Millett. “Only a small fraction of them will interact in any way, but when the number is so large and their energy so high, you get increased mutations and cancer – these would be the main biological effects.”
And for a beast like megalodon, the effects would be extra deadly.
“We estimated the cancer rate would go up about 50 percent for something the size of a human – and the bigger you are, the worse it is,” he added. “For an elephant or a whale, the radiation dose goes way up.”
Although the idea that a cosmic explosion one and a half quadrillion kilometers away could trigger the deaths of millions of animals on Earth might seem farfetched, research last year by the same team put the “kill zone” for a supernova at just one-third that distance. At 150 light-years from Earth, the cosmic rays from supernovae wouldn’t cause something like the Great Dying, which saw up to 90 percent of all life wiped off the face of the planet – but an extinction event like the one that took the megalodon would be feasible, says Melott.
“There really hasn't been any good explanation for the marine megafaunal extinction,” he explained. “This could be one. It's this paradigm change – we know something happened and when it happened, so for the first time we can really dig in and look for things in a definite way. We now can get really definite about what the effects of radiation would be in a way that wasn't possible before.”