Some scientific ideas are far-fetched. This one is way out there. About 163 light-years, to be specific.
A spate of exploding stars is thought to have bathed Earth in excess cosmic rays that peaked about 2.6 million years ago. That much is known. Here’s where things get speculative: The outburst created “an avalanche of electrons in the lower atmosphere” that sparked “an enormous upsurge in cloud-to-ground lightning strikes that ignited forest fires around the globe,” researchers postulate May 28 in the online version of the Journal of Geology.
The fires turned northeast Africa’s forests into savannas, the thinking goes. Forced to walk farther to find the next tree to swing from, early human ancestors may have perfected their bipedalism.
“It is thought there was already some tendency for hominins to walk on two legs, even before this event,” says the study’s lead author, Adrian Melott, 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. 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.”
Other research indicates human ancestors became bipedal at least 4.2 million years ago, but getting good at it would have required lots of evolutionary changes to anatomy, “and in the meantime you will be slow, clumsy and unstable,” Chris Stringer of the Natural History Museum in London said in a 2012 article in New Scientist.
The evidence for the massive stellar explosions, called supernovae, was established in 2016 when scientists found radioactive iron-60 in sediment and crust samples below oceans around the globe. The iron-60 was embedded in layers put down between 3.2 million and 1.7 million years ago. Given the apparent intensity of the events, researchers figure the supernovae were around 163 light-years away, which is in our relative cosmic backyard. The Milky Way Galaxy is about 100,000 light-years wide. A light-year is the distance light travels in one year.
Here’s why this evidence matters:
“We were very surprised that there was debris clearly spread across 1.5 million years,” said Anton Wallner, a researcher at the Australian National University who led that 2016 study. “It suggests there were a series of supernovae, one after another. It’s an interesting coincidence that they correspond with when the Earth cooled and moved from the Pliocene into the Pleistocene period.”
One of our earliest ancestors, Homo habilis, evolved in the late Pliocene. The climate then underwent dramatic change. After the fires, the Great Ice Age developed, forcing both plants and animals to migrate great distances to survive. Toward the end of the Pleistocene epoch, as Earth warmed, many large mammals went extinct.
But it’s the other effects of the supernovae that interested Melott and colleague, Brian Thomas of Washburn University.
“We contend it would increase the ionization of the lower atmosphere by 50-fold,” he explains. “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.”
Electrons would’ve formed pathways for lightning to strike the surface, the researchers figure
“The bottom mile or so of atmosphere gets affected in ways it normally never does,” Melott says. “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.”
That idea is supported by an abundance of soot and charcoal in layers of the Earth dated to a few million years ago, the scientists say.
It should be noted, however, that even scientists who study lightning for a living don’t really know how lightning begins or why it strikes the surface.
If Melott is correct—and it’s all speculation for now—this would solve the mystery of why fires ravaged much of the planet back then.
“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,” he says. “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.”