Supernovae that exploded in the cosmic neighborhood millions of years ago littered the oceans of the Earth with radioactive dust. This event may have affected the evolution of early human beings, according to a new study.

Deposits of iron-60 have been recorded on the seabed, as predicted 25 years ago. This material appears to have been produced in the massive explosions of a pair of supernovae that took place roughly 325 light years from the Earth. The first of these occurred somewhere between 8.7 and 6.5 million years ago, while the more recent event took place 3.2 to 1.7 million years before our own time.

"Our local research group is working on figuring out what the effects were likely to have been. We really don't know. The events weren't close enough to cause a big mass extinction or severe effects, but not so far away that we can ignore them either. We're trying to decide if we should expect to have seen any effects on the ground on the Earth," said Adrian Melott of the University of Kansas.

Supernovae are explosive events that take place around stars subjected to specific conditions. They come in two varieties. The first of these take place in binary star systems, as material falls from one member of the pair onto its partner. When this deposit reaches a critical stage, it explodes in a violent outburst. The second type of supernova takes place in massive stars as they die, and collapse toward their centers, triggering an explosive reaction.

If a supernova takes place within 30 light years of our planet, the event could result in a mass extinction on Earth. If they take place far from our home world, the effects are minimal. It remains to be seen what effect these explosions may have had on the development of our own species. These two events, which took place as our species was just starting to evolve, may have altered the evolution of our own species.

The sun and our local family of planets are housed within a bubble of hot, diffused plasma that formed long ago from between 14 and 20 supernovae.

The notion that iron-60 could provide evidence of a nearby supernova was first developed in the 1990s. Production of that isotope by supernovae was confirmed just five years after the theory was released.

Analysis of the evidence for the nearby supernovae was published in the journal Nature.