About two million years ago, a giant star in Earth’s neighborhood exploded. If australopithecines and other early hominins were prone to skywatching, they surely would have noticed the sudden appearance of a star blazing brighter than the full moon—an eerie bluish beacon that may have even been visible during the day.

At about 300 light-years away, the explosion wasn’t close enough to harm life on Earth. But that doesn’t mean the planet escaped without a scratch: When that star exploded, it spattered our world and the moon with a form of unstable, radioactive iron.

Now, astronomers studying the iron’s decaying fingerprints in seafloor sediments have deduced that multiple supernovas probably exploded between 1.5 and 2.3 million years ago, and they think