When supernovae explode in our region of the galaxy the most immediate effect is a blast of light and cosmic rays, which can affect our upper atmosphere. Later, elements formed in the explosion and thrown out by its force can make their way to Earth. The most significant of these for research purposes is iron-60, spikes of which in ancient sediments have alerted us to ancient astronomical events.

An upsurge in iron-60 3.4-1.7 million years ago indicates something off-planet happened around that time, although competing explanations exist for why the peak lasted so long. As part of an effort to model the event responsible, one team has also linked it to viral diversification, suggesting a supernova from the Tucana-Horolgium association might have raised radiation levels sufficiently to affect DNA on Earth – specifically life in Lake Tanganyika in Central Africa – for about 10,000 years, even with all the shielding our atmosphere and magnetosphere provide, causing a notable rise in mutations. 

University of California Santa Cruz undergraduate student Caitlyn Nojiri is studying astronomy, not virology, but may have linked the two in a project attempting to identify the source of the latest iron-60 spike. “The iron-60 is a way to trace back when the supernovae were occurring,” Nojiri said in a statement. “From two to three million years ago, we think that a supernova happened nearby.”

Nojiri and two UCSC academics propose the iron-60 came from a supernova from one of two collections of stars: the Upper Centaurus Lupus association and the Tucana-Horologium association. Both associations are stars whose ages and movements indicate they formed together in star clusters 15 and 40 million years ago respectively, and have been slowly drifting apart since. 

Given the 2-3 million-year-old timing for the spike and what we know about the movements of these stars, the source would have been around 460 light-years away if from the Upper Centaurus Lupus association, and half that from the Tucana-Horolgium association. Most supernovae, particularly core-collapse supernovae, are thought to be from associations like these because only the largest, shortest-lived stars undergo these explosions, and such stars don’t have time to get far from the cluster where they were born.

Being more distant, one in the Upper Centaurus Lupus association would have exposed the Earth to about four times less radiation, putting it below what one study found to be the threshold to induce double-strand breaks in DNA. An explosion at Tucana-Horolgium, on the other hand, wouldn’t cause a mass extinction but might raise mutation rates enough to increase the rate at which new species emerge.

Knowing the distance to the supernova in question is important because it allows us to model the radiation produced in the context of bumps in the current high-energy cosmic radiation spectrum. “Nearby supernova (SN) activity has the potential to raise the radiation levels at the surface of the Earth by several orders of magnitude, which is expected to have a profound impact on the evolution of life,” the authors write. 

They note a previous study that revealed a sharp upsurge in viruses in Lake Tanganyika in Africa’s rift valley, which holds one-fifth of all the fresh water in surface lakes worldwide. “We can’t say that they are connected, but they have a similar timeframe,” Nojiri said. 

“It’s really cool to find ways in which these super distant things could impact our lives or the planet’s habitability,” Nojiri said.

Virus diversification around the same time needs to be demonstrated at other locations to confirm an astronomical cause. Nevertheless, the work is considered so impressive Nojiri became the first UCSC undergraduate to be invited to give a Center for Cosmology and AstroParticle Physics seminar. Senior author Professor Enrico Ramirez-Ruiz previously encouraged Nojiri, a former community college student, to apply for UC LEADS, a program to overcome the disadvantage students from underrepresented groups experience in science. 

“People from different walks of life bring different perspectives to science and can solve problems in very different ways,” Ramirez-Ruiz said. “This is an example of the beauty of having different perspectives in physics and the importance of having those voices.”

The study is open access in the Astrophysical Journal Letters.