Stardust found deep beneath the Pacific Ocean has led German scientists to speculate that a supernova explosion 3 million years ago might possibly have helped bring about human evolution.
Gunther Korschinek and colleagues at the Technical University of Munich in Germany reported on Wednesday they found debris from an exploding supernova that could have changed the climate on Earth around the time that humanity's ancestors first began to walk.
Depending on how far away the supernova was, it might have caused an increase in cosmic rays for about 300,000 years that in turn could have heated up the Earth, they wrote in the latest issue of Physical Review Letters.
The timing of the star explosion coincides with a change in the climate in Africa, when drier conditions caused forests to retreat and the savannah to emerge. Anthropologists and other experts believe this change brought early hominids out of the trees, forcing them to walk upright.
The most famous pre-human, a skeleton nicknamed "Lucy," dates back just about 3 million years. Lucy and her Australopithecus afarensis kin would have walked upright.
Korschinek's team was the first, five years ago, to find real matter from a star on Earth, in Pacific sediments.
This time they looked for stardust at a site much deeper, in the middle of the Pacific Ocean near the equator and away from land roughly south of the Hawaiian islands.
There, 15,750 feet (4,800 meters) below the surface, they found a layer of iron-60, stable layers under the sea that are easy to date. This one can be dated to about 2.8 million years ago, they said.
Iron-60 is an isotope or chemical variant of iron that is rare on Earth and which scientists believe is unlikely to have come from anything other than a supernova.
It has a decay rate or half-life of about 1.5 million years, which can help pinpoint when the star exploded, sending out not only solid matter in the form of iron and other elements, but cosmic rays.
Korschinek and colleagues noted that other scientists have suggested a cosmic ray bombardment could affect the ozone layer, letting in more of the Sun's ultraviolet rays.
This in turn could make it hotter and drier in places.
"It has not yet been established that such an increase of the cosmic ray intensity could have had a significant influence on the Earth's climate," they wrote.
But they do note a coincidence.
"The African climate shifted towards more arid conditions about 2.8 million years ago," they wrote, adding, "some of the major events in early hominid evolution appear to be coeval with the African climate changes."