About 2 million years ago, a star roughly 325 light years away from our sun—pretty close, as cosmic things go—gave up the ghost. Its fuel finally spent, the star’s core collapsed under its own weight and it exploded in a supernova. The earliest human ancestors had not quite stepped down onto the savannas, but they likely would have noticed the star’s brilliant blue death throes, which would have outshone the full moon.
Radiation and pulverized star bits flowed across the cosmos, past nebulas and through interstellar dust clouds, and finally streamed into the solar system. For a few millennia, some of this material rained onto the Earth and moon. Dust from the perished star could have seeded clouds, and radiation may have stripped away protective ozone in the atmosphere, altering Earth’s climate. It sounds far-fetched, but scientists say it’s possible that these supernova-induced changes gave rise to the human lineage. We are all star stuff, indeed.
Supernovas from billions of years ago forged the elements that made the sun and solar system, and all of us. Our hearts pump blood full of iron born in stars. But well after the sun formed, local stars kept exploding. For a few decades, scientists have thought that these explosions could have triggered dramatic climate changes and mass extinctions, says Adrian Melott, an astrophysicist at the University of Kansas. Explosions close enough to wreak such havoc are pretty rare, but Melott thinks at least one of the major extinction events in the last 500 million years was probably caused by a supernova.
Definitively tying one of these older explosions to a mass extinction would be next to impossible, because the radioactive atoms that serve as supernova tracers would have long been lost to time. But that’s not the case with more recent explosions. Their relics persist, both in lunar soil and on the seafloor.
“You can use sea sediments as a telescope to learn about the supernova, and conduct supernova archaeology,” says Brian Fields, an astrophysicist at the University of Illinois who was one of the first people to propose doing this. “You’re digging into the earth to look at the cosmic past.”
A trio of new studies takes supernova archaeology into new territory, tracing different sets of stellar shrapnel to their sources.
When looking for dead star remains, scientists use a radioactive isotope called iron-60, which supernovas churn out in vast quantities (Earthly sources produce only one-tenth as much). Astrophysicists first located it in ocean-floor rocks in 1999, but now a team led by Anton Wallner at the Australian National University in Canberra found it in four separate locations, and at earlier times. The global distribution suggests that not only does the iron come from recent supernovae, but it comes from many of them.
Wallner’s team found two distinct peaks in iron-60, one of them between 1.5 and 3.2 million years ago and the other 6.5 to 8.7 million years ago. They used a particle accelerator to meticulously count single atoms of iron-60—“basically finding needles in haystacks,” says Fields.
Many of these atoms landed on the moon, too, according to a separate study published this week. Samples from two Apollo missions contain an order of magnitude more iron-60 than what cosmic rays would have deposited. It presumably came from the same supernovas as the ones buried on Earth.
Meanwhile, a third set of researchers used modeling to improve the stellar excavation. Astrophysicist Dieter Breitschwerdt at the Berlin Institute of Technology found evidence for two different star cataclysms within the younger iron deposits. The researchers used statistical analysis to figure out the stars' sizes, using numbers like the local neighborhood’s star population and existing stars’ brightnesses. They found the more distant one originated with a star about 8.8 times the mass of the sun, which blew up about 1.5 million years ago. The closer ex-star was about 9.2 times the mass of the sun and exploded 2.3 million years ago.
According to Breitschwerdt’s calculations—which took several years of work—the stardust would have taken about 100,000 years to shower Earth. Around the same interval, Earth experienced a sharp decline in global temperatures, and the onset of the Pleistocene ice ages. The cause of these climate changes is still under debate, but some anthropologists argue that the shift contributed to the evolution of human ancestors. The creep of glaciers was linked to a great drying throughout Africa, which caused forested ecosystems to become arid grasslands. Some argue this could have spurred human ancestors to walk upright rather than climb. There is evidence that early Homo species diets changed, incorporating more animal meat to ensure a diverse food supply. It’s even possible that hominid brains grew larger to enable them to figure out where to find food.
Could the supernova have been one of the things that made us human?
We aren’t sure. It’s not clear that there is a link between supernova activity and colder temperatures, let alone human evolution. Though grounded in good science, this idea remains a reach, says Peter de Menocal, who studies sea sediments and climate change and directs Columbia University’s Center for Climate and Life. While it's true that we don't know what caused the Pleistocene ice age, “Coincidence doesn’t mean causality," he says.
“It’s not that their explanation is flawed in any way; I just don’t think it impacted faunal evolution. If it did, we would expect to see a lot of discontinuity in the fossil record,” he adds. “Trying to invoke it as an answer to these Pleistocene puzzles that we have—I just don’t see it.”
Still, Melott and others plan to incorporate the new data to figure out how it could work, including how supernova remnants could interfere with Earth’s atmosphere. Other than loss of the ozone layer, X-rays, gamma rays, ultraviolet light, cosmic rays, and even radioactivity could have played some role, Melott says: “All of those are things you need to look at.”
The good news is that we won’t find direct proof anytime soon, because no stars in our neighborhood will go supernova. The closest one to go off next will be Betelgeuse, one of the night sky’s brightest, and a key fixture in the constellation Orion. It is about as big as the stars that Breitschwerdt studied, but twice as distant, and its violent ending will amount to little more than a spectacular light show here on Earth.
Fields says supernova archaeologists would love to associate a supernova with a mass extinction; it’s an “unholy grail” of the research, he says. But the flip side—supernovas as architects of human evolution—sounds even more provocative. Fields argues it’s at least worth a conversation.
“It takes this speculation out of the realm of science fiction into something that really deserves further thought,” he says. “We Earthlings are also citizens of the cosmos. Sometimes the cosmos intervenes. Not all the time, but it happens. It’s a reminder that we live in a larger universe, and it can affect us.”