The closest supernovas to Earth may have blasted the planet with enough radiation to influence human evolution, researchers say.

Supernovas are the most powerful star explosions. These outbursts from enormous dying stars are visible all the way to the edge of the cosmos, and are bright enough to briefly outshine all of the stars in their host galaxies.

For more than 50 years, scientists have suggested that nearby stellar fireworks could have influenced life on Earth by disrupting global climate and even triggering mass extinctions. Previous research calculated that supernovas occurring within about 325 light-years of Earth, which are expected about once every 2 million to 4 million years, could deposit radioactive debris on this planet. (For perspective, the sun's closest stellar neighbor lies about 4.3 light-years away.) [Supernova Photos: Great Images of Star Explosions]

In 1999, researchers discovered significant levels of a mildly radioactive variety of iron known as iron-60 in deep-ocean rocks, indicating that supernovas have indeed sprayed Earth with radioactive material. Supernovas generate huge amounts of iron-60, whereas other natural ways of creating iron-60 produce only up to one-tenth as much.

Previous research suggested that the effects of high-energy particles from supernovas that explode within about 30 to 45 light-years of Earth would be catastrophic for life on the planet. However, prior work also estimated that explosions close enough to cause mass extinctions were very rare, "on the order of one every few billion years," study co-author Anton Wallner, a nuclear physicist at Australian National University in Canberra, told Space.com.

Now, researchers have pinpointed when and where the most recent supernovas closest to Earth might have occurred, and found that they happened recently enough to potentially influence human evolution.

Scientists investigated the origins of the "Local Bubble," the region of the Milky Way in which Earth's solar system is embedded. The Local Bubble—which measures about 600 by 600 by 1,200 light-years—is faintly lit by X-rays from hot plasma up to 1.8 million degrees Fahrenheit (1 million degrees Celsius), which likely came from a series of supernovas, said study lead author Dieter Breitschwerdt, an astrophysicist at the Berlin Institute of Technology. [Stunning Photos of Our Milky Way Galaxy (Gallery)]

The researchers focused on the origin of iron-60 in deep-sea rock that likely came from one or more supernovas 195 to 425 light-years from Earth about 2.2 million years ago.

"We can do a sort of galactic archaeology—like terrestrial archaeologists, we dig somewhere for traces of past events," Breitschwerdt told Space.com.

Using supercomputer models to calculate the likely masses of the dying stars and the complex trajectories this radioactive matter took, study team members were able to pinpoint the most probable times and sites of the explosions—two supernovas between about 290 and 325 light-years from the sun.

"Running several models took us about three to four years in total," Breitschwerdt said.

Breitschwerdt and his colleagues estimated that the closer of these two supernovas originated from a star with a mass about 9.2 times that of the sun and happened about 2.3 million years ago. The other supernova originated from a star with a mass about 8.8 times that of the sun and occurred about 1.5 million years ago. (In comparison, the oldest known member of the genus Homo—which includes modern humans, Homo sapiens—dates back to about 2.8 million years ago.)

These explosions were two of the 14 to 20 supernovas that supplied the hot plasma making up the Local Bubble, researchers said. The other supernovas likely showered iron-60 onto Earth as well, albeit to a smaller extent because they happened farther away and longer ago.

In a separate new study, Wallner and his colleagues focused on deep-sea rock from the Atlantic, Pacific and Indian oceans that had iron-60 levels about 40 times greater than average. Detecting this iron-60 was difficult—at best, "this extraterrestrial radioactive iron-60 is a million billion times less abundant in our samples than terrestrial stable iron," said Wallner, who led this second study. "We need extremely sensitive single-atom counting techniques to identify these iron-60 atoms," he said.

The researchers found that this radioactive debris was carried to Earth within interstellar dust grains in two relatively recent events—one 1.5 million to 3.2 million years ago, and the other 6.5 million to 8.7 million years ago. They suggested that these supernovas were less than 300 light-years away, close enough to be visible during the day and comparable to the brightness of the moon. This iron-60 may have come from supernovas that directly sprayed the solar system, but it's also possible that the solar system passed through interstellar clouds polluted with the remnants of multiple supernovas, researchers said.

Although Wallner noted that the supernovas he and his colleagues investigated probably would not have been close enough to Earth to cause mass extinctions, he said that radiation from these explosions might have influenced the planet's climate. For instance, he noted that the older event they discovered coincided with temperature changes about 8 million years ago during the late Miocene epoch, when global changes in Earth's fauna occurred. In addition, the more recent event happened at about the same time that Earth began getting colder, near the end of the Pliocene epoch, about 3 million years ago—a climatic shift that might have helped spur the rise of the human lineage.

Moreover, Breitschwerdt noted that radiation from these explosions also might have triggered mutations in life-forms on Earth. "It might be possible that an increased rate of mutations directly influenced evolution—for example, increase in brain size," Breitschwerdt said.

In the future, Wallner and his colleagues will try to pinpoint exactly when supernova iron-60 fell onto Earth, to get a better idea of whether it might have influenced life here, Wallner said.

The scientists detailed their findings in two separate papers in the April 7 issue of the journal Nature.

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