When a massive star explodes as a supernova, it does more than release extraordinary amounts of energy. Supernova explosions are responsible for the formation of some heavy elements, including iron, that are ejected into space during the explosion. On Earth, there are accumulations of two iron isotopes, Fe60, in seafloor sediments, which scientists trace to two to three million years ago and another about five to six million years ago.

Iron-producing explosions also exposed Earth to cosmic radiation.

In a new study presented Astrophysics Journal LettersScientists are investigating how much energy reaches Earth from these explosions and how this radiation may have affected life on Earth. The article is titled “Life in a bubble: How a supernova that exploded nearby left temporary traces in the spectrum of cosmic rays and left an indelible mark on life.” The lead author is Caitlin Nojiri of the University of California at Santa Cruz.

“Life on Earth is constantly evolving under the constant influence of ionizing radiation of both terrestrial and cosmic origin,” the authors write.

Earth’s radiation has been slowly decreasing for billions of years. But it’s not cosmic radiation. As our solar system moves through the galaxy, the level of cosmic radiation to which Earth is exposed changes.

“Nearby supernova (SN) activity can increase radiation levels at the Earth’s surface by several orders of magnitude, which is expected to have profound effects on the evolution of life,” they write.

The authors explain that the two-million-year accumulation was the direct result of a supernova explosion, while the older accumulation occurred when the Earth passed through the bubble. The bubble in the study’s name comes from a particular type of star called OB stars. OB stars are massive, hot and short-lived stars that often form in groups.

These stars emit strong outflow winds that form “bubbles” of hot gas in the interstellar medium. Our solar system is inside one of these bubbles, called the Local Bubble, which is about 1000 light-years in diameter and was created several million years ago.

Earth entered the local bubble approximately five to six million years ago, explaining the older deposition of Fe60. According to the authors, the younger cluster of Fe60, two to three million years ago, resulted directly from a supernova.

“It is very likely that the 60Fe peak about 2-3 million years ago was caused by a supernova in the Upper Centauri Lupus association (~140 units) or the Tucana Horologium association (~70 units) in Scorpius Centauri. “The ~5-6 Myr peak is probably explained by the Solar System entering the bubble,” the authors write.

Local Bubble is not a quiet place. It took several supernovae to create this. The authors write that 15 SN explosions over the last 15 million years were required to create the LB.

“From reconstructions of LB history, we know that at least 9 SNs have erupted in the last 6 million years,” they write.

The researchers collected all the data and calculated the amount of radiation from several SNe in the LB.

“It is unclear what the biological effects of such radiation doses will be,” they write, but they discuss some possibilities.

The radiation dose can be strong enough to create double-strand breaks in DNA. This is a serious damage that can lead to chromosomal changes and even cell death. But there are other implications for the development of life on Earth.

“Double-strand breaks in DNA can potentially lead to mutations and jumps in species diversity,” the researchers write. A 2024 paper found that “the rate of viral diversity in Africa’s Lake Tanganyika accelerated 2-3 million years ago.” Could this be due to SN radiation?

“It would be interesting to better understand whether this could be explained by the increase in cosmic radiation dose that we estimate occurred during this period,” the authors said.

SN radiation was not strong enough to trigger the extinction. But it could have been powerful enough to cause more mutations, leading to greater species diversity. Radiation is always part of the environment. It rises and falls as events unfold and the Earth moves through the galaxy. It must somehow be part of the equation that creates the diversity of life on our planet.

“There is therefore no doubt that cosmic radiation is an important environmental factor in assessing the viability and evolution of life on Earth, and given the evolution of species, there is an important question regarding the threshold of radiation as a beneficial or harmful trigger.” the authors conclude.

Unfortunately, we do not have a clear understanding of how radiation affects biology, what thresholds can be determined, and how these may change over time.

“A precise threshold can only be determined by a clear understanding of the biological effects of cosmic radiation (especially muons, which predominate at ground level), and this remains largely unexplored,” Nojiri and his co-authors write.

Research shows that the space environment has a powerful impact on life on Earth, whether we see it in our daily lives or not, and whether we even realize it or not. SN radiation may have helped shape evolution by influencing mutation rates at critical moments in Earth history.

Without supernova explosions, life on Earth might look very different. A lot of things had to go right for us to get here. Perhaps supernova explosions in the distant past played a role in the evolutionary chain that has reached us.