For scientists studying the history of human evolution, determining when certain changes happened and what may have prompted those changes are very important questions to answer. It’s only natural that we wonder where we came from, but there’s only so much that fossils can teach us, and sometimes we have to make big assumptions about what happened in our distant past.

One of the most puzzling mysteries has long revolved around the period of time in which human ancestors first began to walk upright. Archaeologists believe that as some of the environments in which humans first took root began to change from forest to grasslands, bipedalism became a big advantage. Now, a new study suggests that the explosion of a nearby star may have contributed to those environmental changes in a big way.

A new paper published in the Journal of Geology focuses on evidence of increased soot and carbon deposits found to date back some 2.6 million years ago which, the authors argue, is evidence that a neighboring star bombarded Earth with cosmic rays as it died.

The energy from the dying star in the form of cosmic rays may have dramatically charged the atmosphere, causing an increase in lightning strikes which resulted in a spike in wildfires. The fires — which are thought to be the reason for the leftover soot from millions of years ago — could have played a major role in converting large portions of Africa from forest to open grasslands.

As for why that may have pushed human ancestors up onto two feet, the researchers have a pretty strong hunch.

“It is thought there was already some tendency for hominins to walk on two legs, even before this event,” lead author Adrian Melott said 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 become better at walking upright. They could see over the tops of grass and watch for predators. It’s thought this conversion to savanna contributed to bipedalism as it became more and more dominant in human ancestors.”