It was the evolutionary leap that defined the species: while other monkeys wandered on all fours, humans' ancestors rose on two legs and conquered the world from this high position.
The benefits of standing up in the African savanna are largely nailed down, but what prompted our distant ancestors to walk is far from clear. In a radical proposal, US scientists point to a cosmic intervention: Protohumans have been helped by a flood of exploding stars, they say.
According to the researchers, a series of stars in our corner of the Milky Way skyrocketed in a cosmic upheaval that began about 7 million years ago and continued for another million years. The supernovae emitted powerful cosmic rays in all directions. On Earth, the radiation of cataclysmic explosions peaked about 2.6 million years ago.
The radiation burst triggered a chain of events, the scientists argue. As cosmic rays hit the planet, they ionized the atmosphere and made it more conductive. This could have increased the frequency of lightning strikes, hunting forest fires through the African forests and giving way to the meadows, they write in the Journal of Geology. In the aftermath, fewer trees were available, and our ancestors adapted, and those who lived upright flourished.
At least that's thinking. In the history of human evolution, upright walking goes back at least 6 million years to the Sahelanthropus, an ancient species that has both monkeys and human traits and comes from fossil remains found in Chad. A prominent theory is that climate change is changing the landscape and leaving the savannah where trees once stood.
One of the authors of the study, Adrian Melott of the University of Kansas, said that old human relatives have already tried to uphold the effects of any supernovae. But he believes that the violent explosions still played a role. "Bipedalism had already started, but we believe that gave him a strong shot," he said.
"It has long been believed that lightning is the main cause of fires before humans play a role and with them." Many fires destroy many habitats, "Melott said. "When the forests are replaced by grasslands, it is beneficial to stand up so you can go from tree to tree and look for predators over the tall grass."
The cosmic rays of a star are known to have exploded About 164 light-years away from Earth, atmospheric ionization would have increased 50-fold, according to scientists. Cosmic rays ionize the atmosphere as they eject electrons from the atoms and molecules into which they leap in the air. Cosmic rays normally only ionize the upper part of the atmosphere, but powerful rays from nearby supernovae can penetrate the entire depth of the atmosphere and ionize them to the ground. "We are sure that this would have led to an increase in lightning strikes, but the initiation of lightning is not well understood, so we can not put a number on it," Melott said.
If scientists are right, future supernovae could potentially cause more forest fires on Earth. But the planet seems safe for the moment. The next star on the way to the explosion in the next billion years is Betelgeuse, one of the brightest in the constellation Orion, which is 642 light-years away.
The researchers admitted that further research is needed to understand the cosmic rays. Drive really lightning. "If it turns out that the connection between lightning and cosmic rays is wrong, it's falling apart," Melott said.