Astrophysicists have traced radioactive elements on the ocean floor to cosmic explosions from which they may have come, and linked the event to evolutionary changes in viruses in a lake in Africa. The evidence comes from an iron isotope called iron-60, which is found in large amounts on the seafloor. Being radioactive, any such material that formed with the Earth would have broken down long ago, so its presence points to a more recent origin for the Earth. A nearby star that recently exploded as a supernova may have scattered it across Earth.
Researchers at the University of California, Santa Cruz (UCSC) first calculated the age of iron-60. Interestingly, they found two main ages for these atoms - a large peak around 2.5 million years ago and a smaller but still significant excess produced around 6.5 million years ago.
Suspecting that the spikes were cosmic in origin, the team traced the positions of Earth and other nearby objects over the past few million years. Our solar system currently sits in the center of a large, empty space known as the Local Bubble, which is thought to have been created by a series of supernova explosions 10 to 20 million years ago.
Earth appears to have entered this bubble about 6 million years ago. The UCSF team believes that traveling through outer walls, where radiation is concentrated, may be responsible for the earlier spike in iron-60.
The second, larger peak appears to have been produced by a supernova. By calculating the positions of nearby star clusters, the researchers identified two star clusters that may be home to the supernova progenitor. The Tucana-Dracula star cluster was then about 228 light-years from our solar system, while Wolf's Tail Upper was about 457 light-years from our solar system.
The simulations showed that wherever the explosion bathed the Earth in high-energy cosmic rays for up to 100,000 years, the peak in iron-60 could easily be explained. This additional radiation may be enough to cause double-strand breaks in an organism's DNA, thereby increasing the incidence of cancer, or causing mutations that lead to new evolutionary changes.
Such damage may be harmful by increasing the incidence of cancer, for example. In fact, previous research suggested that the supernova may have wiped out the megalodon through this mechanism. But in other cases, DNA mutations may act as "accelerators of evolutionary changes or mutations in cells," said Caitlyn Nojiri, first author of the new study.
The researchers investigated other events occurring at the same time that may be related to the radiation rain. Intriguingly, they found a paper describing how viruses in Lake Tanganyika in eastern Africa quickly began diverging into new species between 2 million and 3 million years ago.
"We can't say they're related, but they have similar time frames. There's been an increase in the diversity of viruses, which is interesting," Nojiri said.
The team says further research should explore how cosmic radiation may have historically affected evolution on Earth, which could also help in our search for life on other planets.
The research was published in The Astrophysical Journal Letters.