Science

Did a perfect supernova storm wipe out the Megalodon?

An artist's impression of Megalodon, a giant prehistoric species of shark that went extinct about 2.6 million years ago
An artist's impression of Megalodon, a giant prehistoric species of shark that went extinct about 2.6 million years ago
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An artist's impression of Megalodon, a giant prehistoric species of shark that went extinct about 2.6 million years ago
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An artist's impression of Megalodon, a giant prehistoric species of shark that went extinct about 2.6 million years ago

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.

Source: University of Kansas

9 comments
VincentWolf
It might explain how man mutated from the apes too...the timing is about right.. if so were a product of chance muons blasting our descendants until a smarter ape thrived.
Rumata
Well, it might explain how homo mutated from apes, but still doesn't explain the extinction of megalodons. Megalodons might be more exposed to cancer from radiations, but might not be more exposed for mutations, because mutations hapen at DNS scale, so the size of megalodons might not matter at all.
piperTom
Muons are not heavy, the mass is only 105.7 MeV, which is about one ninth that of the neutron. Also muons have a mean lifetime of 2.2 μs -- how are they supposed to get here from lightyears away? The referenced press release from U.K. also says muon. But it still doesn't pass the smell test.
Brian M
Possibly, but of course at the egg/sperm and embryo stage Megalodons are no different in size to other creatures. Yes its possible when fully grown they are more at risk, but for this to be significant any mutations/cancer have to kill the individual Megalodon before it can reproduce. At what age could the Megalodon reproduce? Interesting theory none the less!
WilliamSager
Supernova's nearby leave quit a trace. So which Nova are we talking about?
Nik
Maybe the effect on the megalodons was just superficial, but the effect on the food chain was significant. It could have affected the plankton, where hits would have been devastating, on such a small creature, which would have had a snowball effect on the whole food chain, reducing the food available to a massive creature such as the megalodon, so that they just starved to death.
Lsaguy
So how did blue whales survive? Sperm whales? Very fishy!
dete
@ piper Tom. Cosmic rays arrive from supernovas. As the cosmic rays hit the Earth's atmosphere they form pions. The pions decay into muons. Yet muons are still short lived. It is because of Einstien's time dilation at the relativistic speed of muons that secondary muons with time dilation can reach the Earth's surface and penetrate matter.
Chris74
Water is a great natural barrier to radiation so this does not pass the smell test at all. Also, ocean levels were hundreds of feet lower thousands of years ago so I wonder if this was taken into account when sample were taken.