We are the only primates walking exclusively on two legs. Doing so leaves our hands free to make and use tools.
This, the experts claim, was a crucial development in the evolution of early humans. But there is little consensus as to how the radical change from walking on all fours to standing upright came about. In a Journal of Geology paper just published, Kansas-based physicists Adrian Melot and Brian Thomas put forward a novel theory.
Supernovae, they claim, forced our ancestors to become bipeds.
When a large star reaches the end of life, the nuclear fuel sustaining it runs out. The star expands as gravity weakens and, finally, collapses in on itself, resulting in a gigantic explosion known as a supernova.
An enormous burst of energy is released into space. When this radiation strikes the Earth’s atmosphere it causes ionisation, knocking electrons out of atoms. The build-up of free electrons leads to lightning strikes. These result in thunderstorms causing forest fires.
Iron layers in the seabed contain evidence of radiation impacts. Most of the world’s iron is stable, but a radioactive isotope, known as iron-60, is produced by radiation. Its atomic nucleus has four additional neutrons, compared to ‘normal’ iron-56. The isotope is unstable and decays slowly.
The rate of decay is known, so the time at which the radiation struck can be calculated. From analysis of the deposits in seabed layers, scientists determine when blasts of radiation occurred.
Deposits of charcoal and soot, found throughout the world, show that there were massive fires during the last 8m years. These may have been triggered, the authors suggest, by supernova radiation.
“Multiple lines of evidence point to one or more moderately nearby supernovae, with the strongest signal at about 2.6m years ago,” they write.
Our ancestors, living in the decimated forests there, had to adapt to a more open habitat, or die. Lucy, a pre-human who lived 3.5m years ago, walked on two legs. Early humans such as Homo habilis, or ‘handy man’, appeared 1.5m years later.
The ‘hominins’, although perfectly adapted to living in trees, already had a tendency to walk on two legs. When the forests were depleted by fire, trees became isolated on the savannah. To get from one tree to the next, hairy apes had to descend and walk across open ground.
Forest-dwelling creatures are vulnerable when doing so. To keep an eye out for predators, hominins learned to walk upright so that they could see over the ground vegetation.
Could something like this happen again? Betelgeuse is an enormous star in the constellation of Orion, prominent in the Irish night sky in winter.
It is nearing the end of its life and will produce a supernova when it dies. In theory, this star could even have exploded already; being around 650 light years from us, it would be six and a half centuries for us to discover that its hour had come.
The sight of such a dramatic event would be spectacular but Betelgeuse is so far away that its death-throes won’t cause worldwide fires. All electrical systems on Earth, however, will be destroyed.
Adrian Melott and Brian Thomas. From ‘Cosmic Explosions to Terrestrial Fires?’ The Journal of Geology, 2019.