Scientists say surge of radiation led to lightning causing forest fires, making adaptation vital

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.