Astrophysicists have done a bit of crime scene investigation on what’s almost a reverse murder mystery. They’ve traced radioactive elements on the seafloor back to the cosmic explosions they might have come from – and potentially linked the event to evolutionary changes in viruses in a lake in Africa.
The smoking gun is an isotope of iron called iron-60, which has been found in large amounts on the seafloor. Being radioactive, any of this stuff that formed with Earth would have broken down long ago, so its presence indicates a more recent origin. Nearby stars exploding as supernovae relatively recently could scatter it over Earth.
Researchers at UC Santa Cruz (UCSC) started by calculating the age of the iron-60. Intriguingly, they found two main ages for the atoms – a large spike dating back about 2.5 million years, and a smaller but still significant excess that was seeded about 6.5 million years ago.
Suspecting cosmic origins for these spikes, the team traced back where Earth and other objects in our neighborhood were over the past few million years. Currently, our solar system sits in the middle of a big empty patch of space called the Local Bubble, which is thought to have been blown by a series of supernova explosions 10 to 20 million years ago.
Earth seems to have entered this bubble about 6 million years ago. Passing through the outer wall, where radiation would have been concentrated, could have accounted for the older spike in iron-60, the UCSC team suggests.
The second and larger spike seems to have been delivered by a supernova. By calculating the positions of nearby stellar clusters through time, the researchers identified two main suspects that could have housed the supernova progenitor. The Tucana-Horologium cluster was about 228 light-years from our solar system at the time, while Upper Centaurus Lupus was about 457 light-years away.
Wherever it was, simulations suggest that the explosion would have bathed Earth in higher energy cosmic rays for up to 100,000 years, easily accounting for the iron-60 spike. That extra radiation could have been strong enough to cause double-strand breaks in the DNA of organisms, which can increase the rates of cancer, or mutations that might lead to new evolutionary changes.
This kind of damage can be detrimental by, for instance, increasing the rates of cancer. In fact, previous research has suggested that this same supernova could have wiped out the Megalodon through this mechanism. But in other cases, DNA mutations could act as “an accelerant for evolutionary changes or mutations in cells,” according to Caitlyn Nojiri, lead author of the new study.
The researchers investigated other events that occurred at the same time, which might be associated with the radiation shower. Intriguingly, they found a paper that described how viruses in Lake Tanganyika in eastern Africa rapidly began diversifying into new species between two and three million years ago.
“We can't say that they are connected, but they have a similar timeframe,” said Nojiri. “We thought it was interesting that there was an increased diversification in the viruses.”
The team says that further research should investigate how cosmic radiation might have influenced evolution on Earth throughout history, which could help us in the hunt for life on other planets too.
The research was published in The Astrophysical Journal Letters.
Source: UC Santa Cruz