Was Earth’s evolution shaped by a dying star? A 2.5-million-year-old clue emerges

Also called cosmic rays, cosmic radiation comprises highly charged particles that travel through space almost at the speed of light. “It’s basically like if you put a drop of ink in water, it diffuses,” added Globus, who is also co-author of a study published in The Astrophysical Journal Letters investigating the potential effects of a nearby supernova on terrestrial life. 

As the Solar System travels through remnants of ancient supernovae, particles of heavy metals fall to Earth, where they become embedded in ocean-floor sediments. Thus, Globus and her colleagues turned their gazes from the cosmos to the bottom of our oceans to study the most recent supernova. 

Of the heavy metals spewed by dying stars, the astronomers were particularly interested in iron-60. That’s because iron-60 is radioactive, meaning that over time, its atoms decay into a nonradioactive state. Specifically, half of any quantity of iron-60 decays in 2.6 million years.

With that information, called its half-life, astronomers can calculate when that sample of iron-60 first formed—in other words, when the supernova occurred. Using this method, Globus and her colleagues estimated that the last nearby supernova occurred about 2.5 million years ago.

They also dated another sample of iron-60 to around six million years ago, coinciding with Earth’s entry into the “Local Bubble”—a vast region surrounded by a shell of heavy metals left behind by multiple supernovae. According to Globus, Earth remains near the center of this bubble today.

A Sherlock Holmes investigation

After dating the last nearby supernova, the team set out to pinpoint its location—a crucial factor in determining how much cosmic radiation may have reached Earth.

“It was really like a Sherlock Holmes investigation,” Globus said. By reconstructing what Earth’s corner of the universe would have looked like 2.5 million years ago, the team concluded that the supernova likely occurred in one of two groups of young stars called stellar associations: the Upper Centaurus Lupus or the Tucana-Horologium. 

Around 2.5 million years ago, Upper Centaurus Lupus lay about 457 light-years from Earth, while Tucana-Horologium was closer at 228 light-years. Using these distances, the team simulated how much cosmic radiation a supernova from each region could have sent toward Earth’s surface.

While our atmosphere does a formidable job of blocking harmful ultraviolet and gamma-ray radiation, when cosmic rays emitted by supernovas hit our atmosphere, they create showers of secondary particles.

Among these, muons are responsible for about 85 percent of the cosmic radiation that reaches ground level, according to the team.

Based on their simulations, the researchers estimated that if the supernova occurred in Tucana-Horologium, the average annual radiation dose on Earth would have been about 10 milligrays (a unit of measurement for absorbed radiation) during the first 10,000 years, falling to two milligrays after 100,000 years.

If the explosion originated from Upper Centaurus Lupus, the corresponding doses would have been lower—around two milligrays initially and 0.5 milligrays after 100,000 years.

“It is not clear what the biological effects of such radiation doses would be,” the researchers admitted in the paper. However, “the study of populations living in Kerala, India, where the background radiation level was observed to vary between 0.1 and 45.0 mGy yr⁻¹ [milligrays per year], showed that 5.0 mGy yr⁻¹ (mean dose) may be the threshold dose for double-strand break induction,” they added. “Double-strand breaks in DNA can potentially lead to [genetic] mutations and jumps in the diversification of species.” 

This implies that if the supernova 2.5 million years ago took place in the Tucana-Horologium association, the yearly radiation dose to reach Earth in those first 10,000 years might have been enough to overcome that threshold. 

Around this time, Globus and her colleagues came across a paper on viruses infecting fish in East Africa’s remote Lake Tanganyika, the world’s longest freshwater lake. The study reported that between two and three million years ago, viral diversity among Lake Tanganyika’s fish rose dramatically, peaking around 2.5 million years ago.

The timing led the astronomers to wonder whether the supernova and the viral surge might somehow be connected.

Globus, however, quickly admitted that coincidence “is not proof of anything” and that there is no other evidence besides the correlation of time. Nevertheless, she pointed out, “Wherever you are, you cannot escape from cosmic radiation unless you go deep in the ocean or underground.” 

Cosmic radiation shapes life

Henrik Svensmark, a physicist at the Danish National Space Institute, agrees that supernovae have a substantial—if indirect—impact on life on Earth.

Svensmark is best known for his hypothesis that cosmic rays promote cloud formation, with clouds playing a key role in reflecting sunlight and cooling the planet.

“I found that the fraction of organic material in sediments follows supernovas during the whole history of the Earth quite remarkably,” Svensmark wrote in an email to Interesting Engineering. “In the case of a cold climate, there are stronger winds and better distribution of nutrients in the oceans. One can have a larger biomass and, thereby, a larger fraction of organic material gets locked into sediments,” he added.

“The changes in climate on geological time scales are so large that even marine diversity appears to be affected.” Interestingly, Svensmark pointed out that Africa’s climate changed abruptly around 2.8 million years ago.

While this theory isn’t consistent with the UCSC researchers’ speculations, Svensmark “can’t exclude their explanation of increased mutation rates due to ionising radiation.” Ionizing radiation, such as gamma waves and muons, alters the composition of atoms.

However, other researchers were more critical of the proposed link between cosmic radiation and viral diversification.

“A virus with double-stranded breaks in its genome will not be functional when it tries to infect a cell,” Ken Stedman, a biologist at Portland State University specialized in astrovirology, explained to Interesting Engineering in an email. In other words, even if cosmic radiation had caused double-strand DNA breaks in the viruses, Stedman argues they wouldn’t have remained functional enough to infect cells. 

Furthermore, Edward Holmes, a virologist at the University of Sydney and co-author of the fish viruses study, told Interesting Engineering that his and his colleague’s research demonstrates that the jump in diversification of the fish viruses was driven by that of their fish hosts.

He also emphasized that an increase in diversification “is not the same as an increased rate of mutation,” Holmes wrote in an email. “It’s really just wild speculation and almost certainly not true.” 

Nevertheless, Globus and her colleagues maintain that it is “certain that cosmic radiation is a key environmental factor when assessing the viability and evolution of life on Earth,” they wrote in the study. The lingering question is whether, under the right conditions, cosmic radiation could ever play a favorable role in shaping life.

“The next step would be to get the biologists involved,” Globus concluded. 

It remains to be seen what future research will reveal about the intersection between astronomy and biology.

Perhaps the story of life on Earth is shaped not only by events on our planet but also by those unfolding among the stars.