Massive supernova could have made humans walk upright, study says

Researchers claim cosmic particles bombarded Earth’s surface at such high levels forests turned to savannah

Exploding stars 2.6 million years ago may have caused ancient humans to walk upright, a new scientific paper claims.

Cosmic particles from these supernovae bombarded Earth’s surface at such high levels that they caused global forest fires, researchers from the University of Kansas suggest.

This led to the creation of large swathes of savannahs in places that would previously have been forested. Early hominins in northeast Africa had to learn to walk on two legs to cross these vast areas, according to the study, published in the Journal of Geology.

The theory was laid out by a team led by physicist Professor Adrian Melott in an attempt to join together different strands of research.

“It is thought there was already some tendency for hominins to walk on two legs, even before this event. But they were mainly adapted for climbing around in trees,” he said.

“After this conversion to savannah, they would much more often have to walk from one tree to another across the grassland, and so they become better at walking upright. They could see over the tops of grass and watch for predators.”

Professor Melott arrived at this hypothesis by drawing on research about historic supernovae and evidence for the impact they had on Earth. Ancient seabed deposits of iron-60 isotopes – radioactive forms of iron – provided a crucial clue.

Professor Melott said these materials must have arrived on Earth from a supernova, which would have exploded 163 light years away during the transition from the Pliocene Epoch to the Ice Age.

“We calculated the ionisation of the atmosphere from cosmic rays which would come from a supernova about as far away as the iron-60 deposits indicate,” he said.

“We contend it would increase the ionisation of the lower atmosphere by 50-fold. 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.”

They suggest ionisation in the lower atmosphere meant there were more pathways for lightning strikes which led to widespread fires.

Professor Melott also said he believed his theory was supported by the discovery of carbon deposits in soils at around the same time as this cosmic-ray bombardment was happening. 

“The observation is that there’s a lot more charcoal and soot in the world starting a few million years ago. 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.

“That increase in fires is thought to have stimulated the transition from woodland to savannah in a lot of places – where you had forests, now you had mostly open grassland with shrubby things here and there.”

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Professor Melott said no similar event was expected to happen any time soon. The nearest star capable of exploding into a supernova in the next million years is Betelgeuse, which is 652 light years from Earth.

“Betelgeuse is too far away to have effects anywhere near this strong,” he said.

At the end of last year Professor Melott led another study that found supernovae 2.6 million years ago could have swept Earth’s prehistoric oceans wiping out creatures like the giant shark known as the Megalodon.

His research suggested that particles from these exploding stars could have caused cancers in large marine mammals to spike.

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