The theory goes like this: cosmic energy from supernovae hit the earth millions of years ago, increasing electrical activity in the lower atmosphere (lightning), which led to increased forest fires, which forced ancient humans out of forests and into the savannas.
Though human ancestors spent most of their lives climbing trees, they are believed to have had the ability to walk upright between trees though it wouldn't have been their primary mode of travel. The ability to walk upright in savannas would allow humans to see over the grasses and scan the horizon for predators.
The forest-fire theory is supported by increased carbon deposits in corresponding soil. Scientists determined the supernova would have been between 50-100 light years away and it would have increased electrical activity up to 50x greater than normal.
Don’t worry about any of this happening again though. “The nearest star capable of exploding into a supernova in the next million years is Betelgeuse, some 200 parsecs (652 light years) from Earth.”
