Cosmic energy bombarded by supernovae eight million years ago may have triggered a chain of events that eventually led to ancient humans walk on two legs, a study claims.
The research, published in the Journal of Geology, suggests that the supernovae initiated an avalanche of electrons in the lower atmosphere.
This atmospheric ionization probably triggered an enormous upsurge in cloud-to-ground lightning strikes that ignited forest fires around the globe.
These infernos could be one reason ancestors of Homo sapiens developed bipedalism—to adapt in savannas that replaced torched forests in northeast Africa.
"It is thought there was already some tendency for hominins to walk on two legs, even before this event," said Adrian Melott, a professor at the University of Kansas in the US.
"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," said Melott.
They could see over the tops of grass and watch for predators," he added.
It is thought this conversion to savanna contributed to bipedalism as it became more and more dominant in human ancestors."
Based on a "telltale" layer of iron-60 deposits lining the world's sea beds, astronomers have high confidence supernovae exploded in Earth's immediate cosmic neighbourhood—between 100 and only 50 parsecs (163 light years) away—during the transition from the Pliocene Epoch to the 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 ionisation of the lower atmosphere by 50-fold," he said.
"Usually, you don't get lower-atmosphere ionisation 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," he added.
Ionisation in the lower atmosphere meant an abundance of electrons would form more pathways for lightning strikes.
The researchers said that the probability that this lightning spike touched off a worldwide upsurge in wildfires is supported by the discovery of carbon deposits found in soils that correspond with the timing 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," he said.
Melott said no such event is likely to occur again anytime soon. The nearest star capable of exploding into a supernova in the next million years is Betelgeuse, some 652 light years from Earth.