It was the evolutionary leap that defined the species: while other apes ambled around on all fours, the ancestors of humans rose up on two legs and, from that lofty position, went on to conquer the world.
The benefits of standing tall in the African savannah are broadly nailed down, but what prompted our distant forebears to walk upright is far from clear. Now, in a radical proposal, US scientists point to a cosmic intervention: protohumans had a helping hand from a flurry of exploding stars, they say.
According to the researchers, a series of stars in our corner of the Milky Way exploded in a cosmic riot that began about 7m years ago and continued for millions of years more. The supernovae blasted powerful cosmic rays in all directions. On Earth, the radiation arriving from the cataclysmic explosions peaked about 2.6m years ago.
Q&AWhat is a supernova?
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Supernovae are exploding stars. The violent events are the largest explosions in space and can shine for months and even years with the brightness of 100m suns. A star can explode in one of two ways. The first type of supernova happens when a white dwarf star made from carbon and oxygen attracts material from another star that it orbits. The white dwarf grows steadily until the density of its core becomes so high that the carbon and oxygen fuse in a runaway reaction that drives the explosion. The second, more common, type of supernova happens when stars burn up their nuclear fuel and cannot withstand their own gravity. The core of the star collapses but triggers a massive explosion. Many common elements are forged inside stars and are scattered through space by supernovae, where they go on to form new stars and planets.
The surge of radiation triggered a chain of events, the scientists argue. As cosmic rays battered the planet, they ionised the atmosphere and made it more conductive. This could have ramped up the frequency of lightning strikes, sending wildfires raging through African forests, and making way for grasslands, they write in the Journal of Geology. With fewer trees at hand in the aftermath, our ancient ancestors adapted, and those who walked upright thrived.
That, at least, is the thinking. In the history of human evolution, walking upright dates back at least 6m years to Sahelanthropus, an ancient species with both ape and human features discovered from fossil remains found in Chad. One prominent theory is that climate change transformed the landscape, leaving savannah where trees once stood.
One of the study’s authors, Adrian Melott of the University of Kansas, said ancient human relatives were already dabbling with standing upright before the effects of any supernovae took hold. But he believes the violent explosions still played a role. “Bipedalism had already gotten started, but we think this may have given it a strong shot in the arm,” he said.
“Lightning has long been thought to be the primary cause of fires before humans had a role, and with a lot of fires you get the destruction of a lot of habitat,” Melott said. “When the forests are replaced with grasslands, it then becomes an advantage to stand upright, so you can walk from tree to tree, and see over the tall grass for predators.”
The cosmic rays from one star known to have exploded about 164 light years from Earth would have increased the ionisation of the atmosphere 50-fold, the scientists calculate. Cosmic rays ionise the atmosphere when they knock electrons out of the atoms and molecules they slam into in the air. Cosmic rays normally only ionise the upper reaches of the atmosphere, but powerful ones from nearby supernovae can penetrate the entire depth of the atmosphere, ionising it all the way to the ground. “We are sure this would have increased lightning strikes, but lightning initiation is not well understood, so we cannot put a number on it,” Melott said.
If the scientists are right, future supernovae could potentially trigger more wildfires on Earth. But the planet appears safe for the moment. The nearest star on course to explode in the next billion years is Betelgeuse, one of the brightest in the constellation of Orion, which lies a safe 642 light years away.
The researchers concede that more research is needed to understand if cosmic rays really do drive lightning. “If the lightning–cosmic ray connection turns out to be incorrect, this whole thing falls apart,” said Melott.
