Did our ancestors evolve to walk upright because of exploding stars?
Cosmic rays from supernovae might have destroyed forests and forced us up onto two legs
Humans might be able to thank exploding stars in our cosmic neighborhood for triggering our evolution from a knuckle-walking ape into fully bipedal creatures.
A team of American astronomers is suggesting that between about eight and 2.5 million years ago, nearby supernovae bathed our planet in cosmic rays. This, they think, led to a chain of events that drove a shift from forested to grassland landscapes. These new grasslands favoured the evolution of upright walking in our ancestors.
What the astronomers are proposing is a trigger for an evolutionary transition that anthropologists have long recognized in our history. Our last common ancestor with chimpanzees was probably more like a knuckle-walking chimp than an erect modern human.
"There was an evolutionary push … to go from climbing around in trees to walking through the grasslands," Brian Thomas, an astronomer at Washburn University in Kansas, told Bob McDonald of CBC Radio's Quirks & Quarks.
From ape to man
Starting about six million years, ago the fossil record starts to show the evolution of apes that are more and more adapted to bipedal walking. This includes a range of changes to the pelvis, hip and foot structure, and eventually led to taller, fully upright creatures. These modified apes would be better able to walk long distances and stand tall to see both prey and predators on the new plains of Africa.
An upright posture would also have freed their hands to be able to hold, carry and use tools, indirectly paving the way for more complex behaviours and the evolution of larger brains.
It was the shift in landscape from forest to grassland that led to this adaptation that Thomas thinks he and his team can now explain as the the result of supernova explosions in our cosmic neighborhood.
Their argument is that over hundreds of thousands to millions of years, one or more supernovae triggered an increase in the rate of wildfires that wiped out forests and replaced them with grasslands.
The chain of events goes something like this: exploding stars would have produced vast quantities of cosmic rays which rained down on the Earth's atmosphere after crossing the vast gulf of space. The evidence to support this part of the story lies in other particles produced from the supernova explosions.
A cosmic trigger
"When you have a supernova explosion, it produces a lot of stuff and one of the things it makes is an isotope of iron called iron 60," said Thomas.
Traces of this distinct form of radioactive iron have been found in marine sediments at higher levels starting about seven million years ago and peaking at about 2.5 million years ago.
This, the team says, is evidence of at least one and probably several supernovae at distances of 150 to 300 light years, which would have sent showers of particles into space, some of which would have struck out planet.
The next critical part of the story is that these cosmic rays could have facilitated an increase in lightning strikes that would have started fires.
Thomas said that it's well known that lightning occurs when an electrical charge difference between clouds and the ground causes current to flow as a massive spark through the atmosphere.
"Something has to get that going," Thomas said. "There has to be some kind of cascade that causes the charge to flow to cause this current. And one explanation for what gets it going is cosmic rays."
Lightning strikes and wildfires
Cosmic rays are mostly protons — hydrogen atoms whose electron has been ripped free by the force of the supernova explosion. These particles enter our atmosphere at tremendous speed and hit gas molecules in the atmosphere, freeing their electrons.
These free electrons increase the electrical conductivity of the atmosphere, acting a little like electrical wires to bridge the gap between cloud and ground, and increasing the chance of a lightning strike.
"We increase our cosmic ray background by about 50 times in the lower atmosphere, and so we would expect pretty simplistically maybe a 50 times increase in lightning," said Thomas. "More lightning produces more wildfires and more wildfires is going to cause this kind of transition from more tree cover to more grasslands."
The geological record provides some evidence for this part of the story as well, Thomas explained. "We do see that there was an increase in lightning — or increase in fire rates, rather — around eight million years ago and that continues up through past even two and a half million years ago."
Evidence for these fires can be seen in sediments around the world as layers of soot and charcoal.
Thomas says there's not much chance of history repeating itself with nearby supernovas creating conditions for global wildfires in the future. There simply aren't any stars near enough in our stellar neighbourhood that will be exploding any time soon.
"The closest one is a star called Betelgeuse, I believe in the constellation Orion, and it's several hundred parsecs away," Thomas said. "So it's too far to be really much of a threat."