For millions of years, the apex predator of the oceans was a bus-sized shark known as the Megalodon. But around 2.6 million years ago they just disappeared, and nobody really knows why. So what could wipe out a 50-ft (15-m) shark? According to a new study, the culprit wasn't Jason Statham but a series of supernovae exploding fairly close to Earth.

The mighty Megalodon is often considered a victim of climate change. As the Pliocene period gave way to the Pleistocene about 2.6 million years ago, there was a significant extinction event in marine megafauna. One of the leading theories is that the world cooled and ice began to gather at the poles, causing sea levels to drop and cutting off access to the sharks' nursery sites in shallow seas. Relegated to the deeper, colder oceans, Megalodon couldn't cut it and died out.

It's a convenient story, but it's not entirely supported by the evidence. Some studies showed that Megalodon was too widespread across the planet for that to have been the final straw. Other ideas suggest that a drop in the diversity of their prey was responsible.

But in a new study, researchers from the University of Kansas, Federal University of São Carlos and Federal University of ABC put forward another idea that they say is backed up by more direct evidence. According to them, the fault may literally lie in the stars.

When a big enough star dies it explodes as a supernova, throwing off most of its matter and sending shock waves of energy into the cosmos. If these explosions occurred close enough to Earth, the planet would be showered in high-energy particles and radiation, which can have devastating effects on the animals that call it home.

While Earth-affecting events are relatively rare, recent studies have found evidence for one that coincides with the Pliocene-Pleistocene boundary. Ancient seabed deposits have a high concentration of iron-60 isotopes, which are a dead giveaway for nearby supernovae.

"It's a telltale because there's no other way for it to get to Earth but from a supernova," says Adrian Melott, lead author of the new study. "Because iron-60 is radioactive, if it was formed with the Earth it would be long gone by now. So, it had to have been rained down on us. There's some debate about whether there was only one supernova really nearby or a whole chain of them. I kind of favor a combo of the two — a big chain with one that was unusually powerful and close. If you look at iron-60 residue, there's a huge spike 2.6 million years ago, but there's excess scattered clear back 10 million years."

The iron-60 may be the smoking gun, but it wasn't the bullet itself. While those isotopes are harmless, they would have arrived alongside particles called muons that can be downright dangerous in large numbers. They're passing through us all the time, but occasionally will interact with matter, causing mutations and risk of cancer in living cells.

A nearby supernova could drastically increase the amount of muons pummeling the Earth, which in turn increases the damage to living creatures. And since it's such a numbers game, bigger creatures would have an even higher chance of those mutations, based on the simple fact that they have more cells for the muons to interact with.

"When this wave of cosmic rays hits, multiply those muons by a few hundred," says Melott. "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. 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."

That would have made the Megalodon particularly vulnerable to this exposure. Its huge size made it a bigger target, and its tendency to live in relatively shallow seas put it more in the line of fire.

The evidence for this extinction event isn't just limited to the Earth either – it's written in the huge structures in our local corner of the universe. The Local Bubble is a void that stretches 300 light-years across and encompasses the solar system and surrounding regions of interstellar space. It appears to have formed from a series of supernova explosions, and might even explain how Earth was bathed in radiation long enough to do real damage to its lifeforms.

"The best way to manufacture a bubble like that is a whole bunch of supernovae blows it bigger and bigger, and that seems to fit well with idea of a chain," says Melott. "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."

The research was published in the journal Astrobiology.