Exploding stars — or ‘supernovae’ — a mere 65 light years from Earth may have triggered the devastating mass extinction event around 359 million years ago.
The Late Devonian extinction saw vanish 50 per cent of all genera — groups of species — and 19 per cent of all families, the next largest grouping on the tree of life.
Among the groups to be worst impacted were the jaw-less fish, reef-building ‘rugose’ corals and the trilobites — iconic marine arthropods that look like woodlice.
This episode mass dying has previously been blamed on asteroid impacts, climate change, sea level changes and large-scale volcanic activity.
Researchers from the US, however, examined the boundary in the rock record between the Devonian and the next period, the Carboniferous.
Here lie hundreds of thousands of generations of plant spores that were sunburnt by ultraviolet light — suggesting a long-term weakening of the ozone layer.
This may have been caused by the impact of one or more nearby supernovae explosions on the solar system — and would have been catastrophic for life.
‘We propose that one or more supernova explosions, about 65 light-years away from Earth, could have been responsible for the protracted loss of ozone, said paper author and astronomer Brian Fields of the University of Illinois at Urbana-Champaign.
‘Earth-based catastrophes such as large-scale volcanism and global warming can destroy the ozone layer too, but evidence for those is inconclusive for the time interval in question.’
Professor Fields and his colleagues also ruled out other space-based events that could have potentially caused ozone depletion — such as deadly gamma-ray bursts, meteorite impacts and solar eruptions.
‘These events end quickly and are unlikely to cause the long-lasting ozone depletion that happened at the end of the Devonian period, said paper author and astronomer Jesse Miller, also of the University of Illinois.
In contrast, the researchers explained, a supernova has the potential to have a more lasting impact — especially given how they can deliver a ‘one-two punch’.
Should a nearby star have gone nova, the Earth would shortly have been bathed in damaging ultra violet, X-ray and gamma ray radiation.
This would have be followed up by the debris blast from the explosion slamming into the solar system and subjecting our planet to irradiation from the cosmic rays accelerated by the supernova.
Together, the damage to the Earth’s ozone layer would have likely lasted up to some 100,000 years, the researchers said.
Although this is not as long as the 300,000-year-spanning decline in biodiversity that led up to the end of the Devonian period, the supernova could have occurred in tandem with other catastrophes — or even multiple nova explosions.
‘This is entirely possible. Massive stars usually occur in clusters with other massive stars, and other supernovae are likely to occur soon after the first explosion,’ explained Mr Miller.
Proof for the supernovae hypothesis, however, would need to come from finding evidence of the radioactive isotopes plutonium-244 and samarium-146 within the rocks and fossils deposited during the time of the mass extinction.
‘Neither of these isotopes occurs naturally on Earth today and the only way they can get here is via cosmic explosions,’ said paper author and astronomer Zhenghai Liu.
Or, as Professor Fields explains it, they are like green bananas.
‘When you see green bananas [here] in Illinois, you know they are fresh, and you know they did not grow here.’
‘Like bananas, Pu-244 and Sm-146 decay over time. So if we find these radioisotopes on Earth today, we know they are fresh and not from here — the green bananas of the isotope world — and thus the smoking guns of a nearby supernova.’
The search for these metaphorical bananas, however, will be a task for the future, the researchers explained.
‘The overarching message of our study is that life on Earth does not exist in isolation,’ Professor Fields concluded.
‘We are citizens of a larger cosmos, and the cosmos intervenes in our lives — often imperceptibly, but sometimes ferociously.’
The full findings of the study were published in the journal Proceedings of the National Academy of Sciences.