The researchers believe that the avalanche of cosmic energy in our atmosphere may have led to an increase in lightning strikes that sparked wildfires, turning the tree-filled forests into open grasslands.
The strongest signal of supernovae, or exploding stars, is traced to a peak that happened 2.6 million years ago from about 163 light-years away, although Earth was getting hit with cosmic energy from supernovae as early as 8 million years ago.
This dump of cosmic energy into Earth's atmosphere would cause a wealth of electrons to pour into the lower atmosphere, charging it.
The researchers suggest that this would have triggered an increase in lightning, specifically cloud-to-ground lightning. Outside of humans, this type of lightning is the largest cause of wildfires. With this charged atmosphere, forest fires would have sparked around the world. Their study published Monday in the Journal of Geology.
"It is thought there was already some tendency for hominins to walk on two legs, even before this event," said Adrian Melott in a statement, study author and professor emeritus of physics and astronomy at the University of Kansas. "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. It's thought this conversion to savanna contributed to bipedalism as it became more and more dominant in human ancestors."
Evidence for these supernovae are embedded in the Earth as a layer of iron deposits across the ocean floor. The iron deposits allow for dating and determining the origin of the supernova.
With cosmic rays bombarding Earth, electrons would essentially provide pathways for lightning.
"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," Melott said.
And the excess of electrons pushing toward the ground made it easier for more lightning bolts to form.
"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," Melott said. 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. But with this ionization, that process can get started a lot more easily, so there would be a lot more lightning bolts."
There is also evidence for the worldwide spread of wildfires because carbon deposits were found in soils that can be dated to the same time as the supernovae.
"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's thought to be related to human evolution in northeast Africa. Specifically, in the Great Rift Valley where you get all these hominin fossils."
The supernovae and its potential effects would have happened at an interesting time during the transition from the Pliocene Epoch, between 5.3 and 2.5 million years go, to the Ice Age. This is also the time of Homo habilis, the upright human ancestor nicknamed "handy man" because of their ability to master stone tool technology.
Could this happen again? It's possible, but not anytime soon. Betelgeuse is the closest start that could explode into a supernova within the next million years and it's 652 light-years away. This distance is too far to have the same effects. But, Melott warns, solar flares pose a greater threat to knocking out our electrical grid.
Currently, the Parker Solar Probe is studying the sun in the hopes of helping predict solar flares.
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