How exploding stars may have led to humans walking upright
Scientists are describing a celestial phenomenon that may have ultimately caused humans to walk upright. According to a study from the University of Kansas, the Earth was bombarded by cosmic energy from supernovae over millions of years. At its peak 2.6 million years ago, this influx of energy initiated an avalanche of electrons in the lower atmosphere and triggered a series of events that led to bipedal movement.
The researchers theorize that atmospheric ionization instigated a large increase in cloud-to-ground lightning strikes that lit forests on fire all over the world. This likely means that homo sapiens mastered the ability to walk upright in response to the transformation of forests into savannas.
“It is thought there was already some tendency for hominins to walk on two legs, even before this event,” said study lead author Professor Adrian Melott. “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. 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.”
The researchers have found evidence in seafloor deposits that supernovae exploded around 163 light years away from the Earth 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,” said Professor Melott. “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. 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.”
Professor Melott and study co-author Brian Thomas explained that ionization in the lower atmosphere meant an abundance of electrons would form more pathways for lightning strikes.
“The bottom mile or so of atmosphere gets affected in ways it normally never does,” said Professor Melott. “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. But with this ionization, that process can get started a lot more easily, so there would be a lot more lightning bolts.”
According to Professor Melott, ancient carbon deposits support the idea of widespread fires because they correspond with the timing of the Earth’s bombardment.
“The observation is that there’s a lot more charcoal and soot in the world starting a few million years ago,” said Professor Melott. “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.”
Professor Melott pointed out that there will not be a cosmic event like this anytime soon. In the next million years, the nearest star capable of exploding is 652 light years away.
“Betelgeuse is too far away to have effects anywhere near this strong,” said Professor Melott. “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 study is published in The Journal of Geology.
Image Credit: NASA