About 2.6 million years ago, when small-bodied and small-brained human ancestors were first chipping flakes off rocks, they may have noticed a startling bright light arriving in the sky – and worried that the shining body, a supernova, about 150 light years from Earth, heralded their destruction.

Maybe it did, especially if they spent much time in the water. Because it appears that radiation from this exploding star killed off the sea monsters – including the great Megalodon, a shark said to be the size of a school bus.

Megalodon’s jaws could have swallowed people two at a time. Photo: Getty

They weren’t boiled alive in the ocean, as some tabloid reports suggest, but rather died from cancer, caused by a massive and prolonged shower of particles called muons.

This is the argument laid out in a paper published in the journal Astrobiology, which brings together evidence of multiple supernovae that occurred relatively close to Earth, and the long mysterious vanishing of marine megafauna at the end of the Pliocene period.

Evidence mounts up

The extinction is known to have been concentrated in coastal waters, where larger organisms would catch a greater radiation dose from the muons.

“I’ve been doing research like this for about 15 years, and always in the past it’s been based on what we know generally about the universe – that these supernovae should have affected Earth at some time or another,” said lead author Adrian Melott, professor emeritus of physics & astronomy at the University of Kansas, in a prepared statement.

A supernova is brighter than the galaxies and puts out more energy than our sun in its lifetime. Photo: NASA

“This time, it’s different. We have evidence of nearby events at a specific time. We know about how far away they were, so we can actually compute how that would have affected the Earth and compare it to what we know about what happened at that time – it’s much more specific.”

By the time the radiation sickness took hold, the supernova – a body so bright that it briefly outshines the galaxies, and radiates more energy than the sun in its lifetime – had died and vanished from the night sky by some hundreds of years.

What remained was a “tsunami of cosmic energy” that, according to Professor Melott and his team could have reached our planet and pummelled the atmosphere, touching off climate change and triggering mass extinctions.

Radiation on the seabed had fallen from space

Professor Melott said recent papers revealing ancient seabed deposits of iron-60 isotopes provided the “slam dunk” evidence of the timing and distance of supernovae.

The argument goes like this: 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 from the remains of exploding stars.

“There’s some debate about whether there was only one supernova really nearby or a whole chain of them,”Professor Melott said.

Professor Adrian Melott has spent years looking for evidence of supernova carnage. Photo: University of Kansas

“I kind of favour 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.”

Professor Melott said that the supernova energy that spread layers of iron-60 all over the world also caused penetrating particles called muons – much like very heavy electrons – to shower Earth, causing cancers and mutations, especially to larger animals.

We have muons passing harmlessly (mostly) through our bodies all the time. “But when this wave of cosmic rays hits, multiply those muons by a few hundred,” Professor Melott said.

“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.”

The scientists estimated the cancer rate would go up about 50 per cent for an animal the size of a human.

“The bigger you are, the worse it is,”Professor Melott said. “For an elephant or a whale, the radiation dose goes way up.”