Getting around on our hind legs was one of the most important developments in human evolution, leaving our hands free for expanded tool use. A new theory attributes this seminal shift to successive supernovae over a period of more than 5 million years.
Your first guess of how this might have worked is probably wrong. Nearby supernovae would certainly have been a spectacular sight, and it’s possible our ancestors stood more upright to observe these temporary second suns. However, no one thinks this was sufficient to induce a long-term change in posture.
The theory outlined in the Journal of Geology is much more indirect. Professor Adrian Melott of the University of Kansas claims stars exploding in our corner of the galaxy peppered the Earth with cosmic rays, increasing atmospheric ionization. This, in turn, increased lightning activity.
More lightning means more forest fires, which would have changed ecosystems planet-wide, including eastern Africa where humans seem to have done most of our evolving. The replacement of forests with open savannah left tree-dwellers with two choices; retreat to the diminishing remnant woods, or adapt. Although fully quadrupedal animals thrive in open grasslands, they're less well suited to the crouching walk of our nearest relatives.
"It is thought there was already some tendency for hominins to walk on two legs, even before this event," said Melott in a statement. "But they were mainly adapted for climbing around in trees. After this conversion to savanna, they would much more often have to walk from one tree to another across the grassland, and so they became better at walking upright. They could see over the tops of grass and watch for predators.”
Every step of Melott’s case is plausible. We know a nearby supernova exploded around 8 million years ago, and there was at least one follow-up, possibly contributing to the start of the ice ages. It is likely these were the result of a cluster of large stars that formed together and died not long apart, leaving a legacy of deposits of radioactive iron-60 on the ocean floor.
Melott calculated the cosmic rays associated with this much iron-60 would have increased the ionization of the atmosphere by a factor of 50. Some of the rays would have been energetic enough to produce these effects in the lower atmosphere, where greater ionization should allow lightning strikes to occur with lower atmospheric voltages.
Melott offers the extra electrical activity as an explanation for the previously unexplained retreat of forests across many climatic regions around the same time.
Nevertheless, there are a lot of steps in the chain, any one of which could be wrong, so far more work on each is required for the claim to gain acceptance.
