Between 8 and 2 million years ago, cosmic-ray energy from one or more nearby supernovae reached Earth and pummeled the planet’s atmosphere, initiating an avalanche of electrons and setting off a chain of events that ended with bipedal hominins, according to a radical study published in the Journal of Geology.
Evidence point to one or more moderately nearby supernovae, with the strongest signal at 2.6 million years ago. Image credit: José A. Peñas / SINC.
“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,” said University of Kansas Professor Adrian Melott.
“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.”
Based on a ‘telltale’ layer of ancient seabed deposits of the radioactive isotope iron-60, Professor Melott and his colleague, Dr. Brian Thomas of Washburn University, have high confidence one or a series of supernovae exploded in Earth’s immediate cosmic neighborhood 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,” Professor Melott said.
“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.”
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. 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,” Professor Melott said.
“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.”
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,” Professor 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 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.”
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Adrian L. Melott & Brian C. Thomas. From Cosmic Explosions to Terrestrial Fires? Journal of Geology, published online May 28, 2019; doi: 10.1086/703418
