TWO stellar blasts may have made life interesting for early humans.
Roughly 2 million years ago, two supernovae exploded so close to Earth that they showered the planet with debris, leaving behind traces of radioactive iron-60 now buried in the sea floor.
Those supernovae were several hundred light-years from Earth, far enough away that their radiation didn’t lead to a mass extinction, but close enough to have affected our ancestor Homo erectus, which was descending from the trees at the time.
Now, Brian Thomas at Washburn University in Topeka, Kansas, and his colleagues posit that the two supernovae could have hurled enough radiation at Earth to affect our ancestors’ behavioural patterns, and potentially increase cancer rates.
The first radiation to hit Earth would have simply been visible light. Supernovae can be so bright that they briefly outshine all the stars in their host galaxy – which wouldn’t go unnoticed on Earth.
In fact, such a close supernova would have been as bright as a full moon every night for up to a year after the initial explosion. The added light pollution could have had some biological impact.
“Certain species use light from the moon to navigate,” Thomas says. “They also use that cue for mating, reproduction, laying eggs, things like that. Even just foraging for food. This can screw with their usual behavioural processes.”
Increased light at night can also affect hormone production in people. Take melatonin for example: it doesn’t just put us to sleep, it also mediates some of the repair mechanisms in our bodies.
“We’re not talking about wiping out species here, but there may be some impact on one or two generations,” says Thomas.
But visible light isn’t the only radiation that would have burst from these stellar explosions. Roughly 500 years after the supernova faded, its radioactive particles would have pelted Earth.
Thomas and his colleagues calculated that the average global radiation would have been three times the background levels typical today. They speculate our ancestors may have faced an increased cancer risk as a result (arxiv.org/abs/1605.04926).
But they might not have had much to worry about, says Michael Weil at Colorado State University, as the average isn’t very telling – there’s a considerable range across the globe. For example, there’s a radiation hotspot in Kerala, India, where the radiation can be 20 times the global average. “People have struggled to show an increase in cancer rates in those areas, and they haven’t managed to do it,” Weil says.
Thomas and his colleagues calculated that the increased dose is equivalent to getting one CT scan per year, which recent studies have shown can increase the risk of leukaemia and brain tumours in children (The Lancet, doi.org/f2fmfz).
Any damage caused by the supernovae is hard to quantify. Luckily, nearby blasts only happen once every billion years, says Michael Sivertz at the NASA Space Radiation Laboratory. “You wouldn’t have to take out a life insurance policy on it,” he says.
This article appeared in print under the headline “Early humans felt supernovae blasts”