TORONTO -- More than 350 million years ago, Earth was ruled by fish, some up to 10 metres long.
But a mass extinction stretching across millions of years killed up to 80 per cent of all species that existed at that time, bringing an end to the Devonian Period.
Scientists have come up with numerous theories over the years for why this extinction might have occurred, such as volcanic activity, meteorites or rapid global warming.
Now, new research is bringing a new possibility to the forefront: what if a supernova was responsible?
A in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) details the brand new theory, and how it could feasibly be proven -- or ruled out.
The Devonian Period occurred from roughly 416 million to 358 million years ago. The world looked vastly different then, characterized by two supercontinents, Gondwana and Laurussia, which would eventually combine to form Pangaea.
Part of the Paleozoic Era, the Devonian Period is also called The Age of Fishes, as biodiversity exploded within Earth鈥檚 oceans during this time. The ancestors of sharks had their beginning in the Devonian, and a fossilized creature from the Devonian period found in the Canadian Arctic in 2004, called a tiktaalik, is thought to be a 鈥渧ital link between fish and the first vertebrates to walk on land,鈥�
During the Late Devonian period, there was a huge loss of biodiversity that occurred over millions of years. Two extinction pulses, the Kellwasser event and, around 10 million years later, the Hangenberg event, are thought to have finished off the Devonian period for good, leading the planet into the Carboniferous Period.
The Hangenberg Crisis refers to a confluence of events that had a catastrophic effect on the living things of that time. There was a widespread issue with the oceans losing a high percentage of oxygen, called an ocean anoxic event, creating massive dead zones within the seas. There was also a dramatic fall in the sea level around the same time.
According to Tuesday鈥檚 paper, recent evidence has suggested that the Hangenberg event at the end of the Devonian was also associated with a depletion of the stratospheric ozone -- the layer that filters out dangerous ultraviolet radiation from the Sun.
The new research theorizes that a supernova millions of miles away could have bombarded the planet with ionizing radiation, causing the depletion of the ozone.
A supernova is when a dying star of massive proportions explodes, creating either a neutron star or a black hole in its place, and firing a shock wave of elements, gas and charged particles out into the galaxy.
The researchers believe that if a supernova is responsible, it would have been within around 20 parsecs, or 66 lightyears, of Earth, 鈥渟omewhat beyond the 鈥渒ill distance鈥� that would have precipitated a full mass extinction.鈥�
Numerous massive stars capable of producing supernovas live in the Milky Way. A supernova within the distance that researchers have posited would have sent cosmic rays washing over the Earth for around 100,000 years.
鈥淭he cosmic ray intensity would be high enough to deplete the ozone layer and induce UV-B damage for thousands of years,鈥� the researchers wrote.
They pointed out that while ozone depletion caused by enhanced convection -- one of the other theories surrounding the extinction -- is generally geographically limited and episodic, ozone depletion caused by a supernova would be 鈥渓ong lived and global and is therefore much more likely to lead to an extinction event.鈥�
Although ionizing radiation from space is known to be a possible cause of ozone depletion, the research says, this theory has never been applied to this particular mass extinction before.
But is there a way to prove this theory? Researchers say there is, if we inspect the distinct layer of rock in the Earth鈥檚 crust that corresponds to the Devonian Period, where fossils and preserved material can allow us to peer into the extinction itself.
If a supernova caused by the core collapse of a massive star was close enough to cause this mass extinction, it would also have peppered supernova 鈥渄ebris鈥� over the Earth as 鈥渕icron or submicron-sized particles created early after the explosion.鈥�
This would鈥檝e left radioactive isotopes on Earth -- distinct versions of chemical elements that are unstable and emit radiation as they decay.
Different radioactive isotopes have different lifespans, meaning 鈥渢hose with lifetimes comparable to the time since the event would provide suitable signatures,鈥� if found within fossils or rock from the Devonian Period.
Researchers speculate that two of the long-lived radioisotopes that could鈥檝e been deposited on Earth -- and would be still detectable today -- could be samarium-146, and plutonium-244.
The end of the Devonian Period, spurred on by numerous extinction events that severely cut down the level of biodiversity in Earth鈥檚 oceans, is still a mystery right now. But if scientists can find these radioisotopes, it may mean that supernovas have played a greater role in our planet鈥檚 history and evolution than we ever knew.
