A bombardment of cosmic rays from ancient supernovae may have triggered a series of events that encouraged early human ancestors to walk upright, according to a new study published in the Journal of Geology. The scientists behind the research believe that the cosmic explosion could have helped trigger a shift in Earth's environment that forced humanity's fore bearers to adapt.

Supernovae are some of the most dramatic and energetic events known to take place in the visible universe. They can occur either when a white dwarf strips too much matter from a nearby companion star, causing it to become unstable, or when a stellar body many times the mass of our Sun runs out of material to fuel the nuclear fusion process raging in its core.

The end result of both scenarios is a violent explosion that seeds the surrounding environment with vast amounts of stellar material, and bombards worlds hundreds of light-years away with cosmic rays.

There is evidence to suggest that Earth's atmosphere has been interacting with cosmic energy thrown out by a series of supernovae explosions for the last 8 million years. The authors of a recently published study assert that, at the peak of the bombardment, roughly 2.6 million years ago, the radiation was partially responsible for reshaping the environment of northeast Africa, where humanity's ancestors where developing.

The research was conducted by Emeritus Professor Adrian L. Melott of the University of Kansas, and Professor Brian Thomas from Washburn University.

Based on an analysis of specific iron deposits discovered on seabeds across the world, astronomers are confident that one, or a number of supernovae exploded at a distance of between 50 – 100 parsecs (163 – 326 light-years) from Earth, roughly 2.6 million years ago. During this period our planet was transitioning from the Pliocene Epoch to the Ice Age.

When cosmic rays reach Earth, they ionize atoms and molecules in our planet's atmosphere, stripping them of their electrons. This process could lead to a dramatic uptake in the amount of lightning bolts.

The researchers set out to calculate the amount of atmospheric ionization that could result from cosmic rays emanating from a supernova that exploded 50 parsecs from Earth.

"We contend it would increase the ionization of the lower atmosphere by 50-fold," said Melott, lead author of the study. 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."

Under normal circumstances, voltage that builds up between the negatively charged lower part of a cloud and the positively charged ground below is unable to travel immediately in the form of a lightning bolt, as there are not enough electrons on hand to carry the current. Eventually, the charge in the cloud becomes high enough for electrons to start moving. They in turn knock other electrons off of atoms as they find the path of least resistance from the cloud to the positively-charged ground.

The overabundance of unbound electrons saturating the lower atmosphere in the wake of a supernova would allow this process to start faster, thus allowing for a greater number of cloud-to-surface lightning bolts.

The lightning strikes could have played a significant role in starting wildfires, that would have devastated the forest environment of northeast Africa, transforming the region into a savanna over time.

Carbon deposits discovered in soils dating back to the cosmic ray bombardment supports the theory that there were rampant wildfires at this time.

Prior to the upsurge in lightning strikes, humanity's early ancestors had already begun to walk on two legs. However, according to the authors of the study, this ability was mostly used to climb around in trees. The savanna-like environment that may have emerged in the wake of the forest fires would have made bipedal movement an even more valuable survival skill.

"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," commented Melott. "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."

Thankfully, Earth isn't likely to be bombarded again by supernova-induced lightning any time soon. The closest star to our Sun that is most likely to go supernova in the near future (in astronomical terms at least) is a red supergiant called Betelgeuse, which forms part of the famous Orion constellation.

Whilst astronomers know that Betelgeuse is nearing the end of its time as a magnificent red supergiant star, it is impossible to know exactly when it will explode. It could be tomorrow, or it could be a million years from now. Regardless of the time-frame there is little threat to life on Earth, according to the authors behind the new research.

"Betelgeuse is too far away to have effects anywhere near this strong," Melott said. "So, don't worry about this. Worry about solar proton events. That's the danger for us with our technology — a solar flare that knocks out electrical power. Just imagine months without electricity."

The paper has been published in the Journal of Geology.