Droplets of glass dug up in New Jersey and from the Atlantic seabed indicate a comet or some other extraterrestrial object may have smacked Earth 56 million years ago, roughly 10 million years after the asteroid impact that doomed the dinosaurs.
Scientists said on Thursday the collision may have triggered a particularly warm, ice-free period on Earth when important mammalian groups, including the primate lineage that led to humans, appeared for the first time.
The findings, published in the journal Science, marked the latest evidence of the profound influence that past impacts by celestial bodies have had on life on Earth.
The tiny spherical bits of dark glass, called microtektites, represent strong evidence of a collision with a comet or asteroid, the researchers said. They form when a space rock hits Earth's surface and vaporizes the spot where it lands, ejecting into the air bits of molten rock that solidify into glass.
The microtektites were excavated from a geological layer marking the start of the Eocene Epoch about 56 million years ago from three sites in southern New Jersey (Millville, Wilson Lake and Medford) and an underwater site east of Florida.
That coincided with the beginning of a warming event, called the Paleocene-Eocene Thermal Maximum, associated with an accumulation of atmospheric carbon dioxide. It lasted more than 100,000 years and drove up global temperatures about 9-14 degrees Fahrenheit (5-8 degrees Celsius).
The impact of an asteroid about six miles wide (10 km) off Mexico's Yucatan Peninsula 10 million years earlier killed off many marine and terrestrial creatures including the dinosaurs, and enabled mammals to gain supremacy.
No such mass extinction was associated with the event 56 million years ago, although many single-celled ocean-bottom creatures disappeared. During the warming period, primates and two mammal groups — one that includes deer, antelope, sheep and goats and another that includes horses and rhinos — first appear in the fossil record.
The researchers said they have not found the location of an impact crater linked to the collision. They said geological evidence suggested the object was a comet.
"We can't really say where it was, or how big, at this point," said geochemist Morgan Schaller of Rensselaer Polytechnic Institute, who led the study.
While the findings are not proof that the impact caused the warming period, they are "a rather dramatic finding in support of an impact trigger" for the climate changes, said planetary scientist Dennis Kent of Columbia University's Lamont-Doherty Earth Observatory and Rutgers University.