Chapter 1: Cosmic Forces at Play

An artistic depiction of a supernova. Credit: NASA/CXC/M.Weiss

Could the explosive death of stars have nudged early humans towards walking on two legs? This intriguing hypothesis suggests that approximately 163 light-years away, a series of stellar explosions may have bombarded Earth with heightened cosmic rays, peaking around 2.6 million years ago. Researchers propose that this event could have triggered "an avalanche of electrons in the lower atmosphere," leading to a dramatic increase in cloud-to-ground lightning strikes, which set off global wildfires, as discussed in a study published on May 28 in the online Journal of Geology.

The resulting fires likely transformed the forested regions of northeast Africa into savannas. As early human ancestors struggled to find trees to climb, they may have refined their ability to walk upright. "There was likely already a tendency for hominins to walk bipedally before this occurrence," explains Adrian Melott, the lead author of the study and a professor emeritus of physics and astronomy at the University of Kansas. "However, they were primarily adapted for arboreal living. Once the environment shifted to savanna, these early humans would need to traverse greater distances between trees, enhancing their upright walking skills. This adaptation allowed them to see above the grass, improving their ability to spot predators. It’s believed that this environmental shift contributed significantly to the evolution of bipedalism in our ancestors."

Other studies suggest that bipedalism emerged in human ancestors at least 4.2 million years ago. However, developing proficiency in walking upright would have required significant anatomical evolution. Chris Stringer from the Natural History Museum in London noted in a 2012 article in New Scientist that such changes would have resulted in a slow and unstable mode of movement during the transition.

The evidence for these ancient cosmic events, known as supernovae, was uncovered in 2016 when scientists discovered radioactive iron-60 in sediment and crust samples from oceans worldwide. This iron-60 was found in layers deposited between 3.2 million and 1.7 million years ago. Due to the intensity of these supernovae, researchers estimate that they occurred about 163 light-years away from Earth, relatively close in cosmic terms, considering the Milky Way Galaxy spans roughly 100,000 light-years.

This discovery raises important questions: "We were quite surprised to find debris distributed over a span of 1.5 million years," stated Anton Wallner, a researcher at the Australian National University and leader of the 2016 study. "It implies there was a series of supernovae occurring in succession, coinciding interestingly with the cooling of Earth as it transitioned from the Pliocene to the Pleistocene epoch."

Among our early ancestors, Homo habilis, emerged during the late Pliocene, a period marked by significant climatic shifts. Following the wildfires, the Great Ice Age ensued, compelling both flora and fauna to migrate extensively to adapt. As the Pleistocene epoch drew to a close and Earth began to warm, numerous large mammal species faced extinction.

Chapter 2: The Impact of Supernovae on Earth's Atmosphere

Yet, it is the broader implications of the supernovae that captivated Melott and his colleague, Brian Thomas from Washburn University. "We argue that this could have led to a 50-fold increase in ionization in the lower atmosphere," Melott elaborates. "Typically, cosmic rays do not penetrate that deeply into the atmosphere, but the more energetic rays from supernovae reach the surface, knocking electrons loose from atoms and molecules."

These freed electrons would have facilitated lightning strikes, according to the researchers. "The lower mile of the atmosphere is influenced in ways that are usually non-existent," Melott explains. "When high-energy cosmic rays collide with atmospheric atoms, they dislodge electrons, allowing current to flow more easily during the lightning formation process. This enhanced ionization would have significantly increased the frequency of lightning strikes."

Supporting this theory is the abundance of soot and charcoal found in geological layers dating back several million years. However, it's crucial to note that even experts in lightning research are still uncertain about the precise mechanisms behind its initiation and how it strikes the ground.

If Melott's hypothesis holds true—though it remains speculative—it could clarify why widespread wildfires occurred during this period. "The rise in fire activity likely contributed to the shift from wooded areas to savanna landscapes across many regions," he concludes. "This transformation is thought to be linked to human evolution, particularly in northeast Africa, specifically in the Great Rift Valley, where numerous hominin fossils have been discovered."