Ancient supernovae could have had a role to play in encouraging our hominin ancestors to walk upright, a team of researchers has suggested.
According to a paper published in the Journal of Geology, supernovae—cataclysmic explosions of dying stars—showered the Earth with cosmic rays between around 8 million and 2.6 million years ago.
When this space radiation entered the lower atmosphere, it triggered a process known as ionization which may have produced a huge increase in cloud-to-ground lightning strikes. This in turn could have led to an uptick in forest fires around the world.
The authors of the study suggest that these fires could have been one factor in the development of bipedalism—a characteristic that may have allowed our ancestors to better adapt to the savannas that replaced the burnt forests of northeast Africa.
"It is thought there was already some tendency for hominins to walk on two legs, even before this event," Adrian Melott, lead author of the study from the University of Kansas, said in a statement. "But they were mainly adapted for climbing around in trees."
"After this conversion to savanna, they would much more often have to walk from one tree to another across the grassland, and so they become better at walking upright," he said. "They could see over the tops of grass and watch for predators. It's thought this conversion to savanna contributed to bipedalism as it became more and more dominant in human ancestors."
In the study, the researchers identified a layer of iron-60—a variant of iron—in sea beds around the world. This indicates, they say, that supernovae occurred within 163 light-years of Earth during the transition from the Pliocene epoch to the last Ice Age
"We calculated the ionization of the atmosphere from cosmic rays which would come from a supernova about as far away as the iron-60 deposits indicate," Melott said. "It appears that this was the closest one in a much longer series."
"We contend it would increase the ionization of the lower atmosphere by 50-fold," he said. "Usually, you don't get lower-atmosphere ionization because cosmic rays don't penetrate that far, but the more energetic ones from supernovae come right down to the surface—so there would be a lot of electrons being knocked out of the atmosphere."
The team argue that ionization in the lower atmosphere would have led to an abundance of electrons, thus increasing the chances that lighting would form.
"The bottom mile or so of atmosphere gets affected in ways it normally never does," Melott said. "When high-energy cosmic rays hit atoms and molecules in the atmosphere, they knock electrons out of them—so these electrons are running around loose instead of bound to atoms. Ordinarily, in the lightning process, there's a buildup of voltage between clouds or the clouds and the ground—but current can't flow because not enough electrons are around to carry it."
"So, it has to build up high voltage before electrons start moving. Once they're moving, electrons knock more electrons out of more atoms, and it builds to a lightning bolt," he said. "But with this ionization, that process can get started a lot more easily, so there would be a lot more lightning bolts."
According to the study, there is a significant possibility that this increase in lightning strikes led to a spike in wildfires around the globe, as evidenced by carbon deposits which have been detected in soils that correspond to the time of the cosmic ray bombardment.
"The observation is that there's a lot more charcoal and soot in the world starting a few million years ago," Melott said. "It's all over the place, and nobody has any explanation for why it would have happened all over the world in different climate zones. This could be an explanation."
"That increase in fires is thought to have stimulated the transition from woodland to savanna in a lot of places—where you had forests, now you had mostly open grassland with shrubby things here and there. That's thought to be related to human evolution in northeast Africa. Specifically, in the Great Rift Valley where you get all these hominin fossils."
