A star that exploded nearly three million years ago left traces of debris on Earth and might have affected the course of human evolution, a new study suggests.

When particles from the explosion bombarded Earth's atmosphere over a long stretch of time, climate change could have forced early humans to fan out in search of food, the reasoning goes.

The evidence is in the form of extra doses of iron-60, a radioactive isotope of iron that normally occurs on Earth in lesser quantities. Researchers found the supernova debris in layers of soil dated to 2.8 million years ago, building a case they opened five years ago with less concrete data.

Visible in daylight

The star that exploded was several times more massive than our Sun.

"For a very short time this explosion released as much light as a whole galaxy," explained study leader Gunther Korschinek of the Technical University of Munich, in Germany. Much of the debris -- newly formed elements -- was absorbed by interstellar dust and gas. But some washed over our solar system.

The same research group found abundancies of iron-60 five years ago, but the new findings are at a different site 3,000 miles away and in layers that are more accurately dated. The results are detailed in the Oct. 22 issue of the journal Physical Review Letters.

According to an article at the online site of the British journal Nature, the discovery "represents an experimental triumph and a milestone in this field," said astrophysicist Brian Fields of the University of Illinois at Urbana-Champaign. Fields said the result marks the birth "supernova archaeology."

Scientists have estimated that somewhere between one and three stars go supernova in our galaxy every 100 years. One probably occurs near our solar system every five million to 10 million years, researchers estimate.

"Our finding shows now for the first time traces from a supernova close to the Earth," Korschinek said in an email interview. "It would have been so bright, that it was easy to see during daytime."

But not too close.

We're still here

The supernova lit up somewhere between 30 light-years and 300 light-years from Earth, the study concludes. The rough estimate is due to the fact that the star's exact mass isn't known, and the extent of iron-60 is also not fully known. There is one pretty solid clue that helps provide a minimum distance: Our ancestors survived and even thrived.

"We know if the star would have exploded too close, we could not talk by e-mail as I do now," he told SPACE.com.

Astronomers cannot accurately lay out the local effects of a supernova. But one can speculate.

Ernst Dorfi, of the University of Vienna, calculates the supernova in Korschinek's study would have generated an increase in cosmic radiation hitting Earth, probably of several percent over a few hundred thousand years.

Other scientists believe extra cosmic rays would fuel increased cloudiness and a drop in temperatures, though the idea needs further research, Korschinek says.

"Interestingly, a temperature drop at that time can be seen in geological records," Korschinek points out, adding that there are other theoretical explanations for the change. Whatever the cause, evolutionary theorists think the climate change of that time forced early humans to adapt, and to leave drying Africa in search of wetter climates.

"So, perhaps -- but not proven -- this supernova was one reason for our existence."

This article is part of SPACE.com's weekly Mystery Monday series.