Two stellar explosions could have made life interesting for early humans.
Roughly 2 million years ago, two supernovae exploded so close to Earth that they showered our pale blue dot with debris, leaving behind traces of radioactive iron-60 found buried in the sea floor across the globe and even mixed within the dust layers on the moon.
Those supernovae were several hundred light-years from Earth, far enough away that their radiation shouldn鈥檛 have led to a mass extinction, but close enough that the blast could have affected our ancestors. At the time, the human ancestor Homo erectus was descending from the trees.
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Now, 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鈥� behavioural patterns, and potentially increase cancer rates.
The first radiation to bombard Earth would have simply been visible light. Supernovae can be so bright that they briefly outshine all the stars in their host galaxy 鈥� an effect that wouldn鈥檛 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, Thomas says, as we know from studies of the effect of artificial lights on wildlife.
鈥淐ertain species use light from the moon to navigate,鈥� he says. 鈥淭hey 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.鈥�
Sleep hormone
Additionally, recent evidence suggests that increased light at night can affect hormone production in people. Take melatonin for example: it doesn鈥檛 just put us to sleep, it also mediates some of the repair mechanisms in our bodies.
鈥淲e鈥檙e not talking about wiping out species here, but there may be some impact on one or two generations,鈥� says Thomas.
But visible light isn鈥檛 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 radiation felt across the globe would have been three times higher than the background levels typical today. They speculate that our ancestors could have faced an increased cancer risk as a result.
But they might not have had much to worry about, says at Colorado State University.
That鈥檚 because the average isn鈥檛 very telling 鈥� there鈥檚 a considerable range across the globe. As an example, Weil says he often sends his students to do field work at the site of the Fukushima nuclear disaster in Japan.
鈥淭hey receive less of a dose at Fukushima while they鈥檙e doing their field work then they do when they鈥檙e studying in classes at Colorado state,鈥� he says. That鈥檚 because there is a large amount of granite in Colorado, which means a fair amount of uranium in the soil and thus a larger radiation dose.
Radioactive rock
Colorado isn鈥檛 even the most radioactive place on Earth: there鈥檚 a radiation hotspot in Kerala, India, where the .
鈥淧eople have struggled to show an increase in cancer rates in those areas, and they haven鈥檛 managed to do it,鈥� Weil says. 鈥淪o it鈥檚 really, really hard to spot any biological effects from tripling the average background radiation levels, which is what this [supernova] would do.鈥�
Astronauts receive 30 times the average background radiation, so just three times would probably not be a problem, says , the principal investigator of the radiation assessment detector on the Mars rover Curiosity.
The numbers aren鈥檛 worrisome until they are 1000 times higher than Earth鈥檚 average, says Hassler. 鈥淭hat鈥檚 the canonical number that agencies use as a lifetime limit,鈥� he says. And it corresponds to a 3 per cent increase in the risk of developing a fatal cancer. 鈥淕iven that an average American may have a 20 to 25 per cent chance of developing fatal cancer in their lifetime, this number is still relatively small,鈥� he says.
That said, the radioactive particles that stream from supernovae are mostly muons 鈥� unstable subatomic particles 鈥� that are thought to be extremely penetrating. This places them in a different camp to the predominate source of background radiation, radon, which has to be inhaled or ingested in order to be damaging.
So instead of thinking about background radiation, it might make more sense to think about the supernovae in terms of diagnostic radiation. Thomas and his colleagues calculated that the increased dose is equivalent to getting one CT scan per year. have suggested that less than one scan a year in children leads to an increase in leukaemia and brain tumours.
Any damage caused by the supernovae is hard to quantify. Luckily, the chance that there will be another nearby supernova any time soon is small, says , who works at the NASA Space Radiation Laboratory. He puts that number at roughly one nearby supernovae every billion years. 鈥淵ou wouldn鈥檛 have to take out a life insurance policy on it,鈥� he says.
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