Comet impact 'linked' to rise of mammals
A comet impact 55 million years ago may have helped mammals dominate the Earth.
It could have triggered a rapid phase of global warming linked to the expansion of mammal groups during the Eocene time period.
Writing in the journal Science, a team of American researchers outlines new evidence for the theory.
They found spherical fragments of glass thought to form when molten debris flung out by an impact solidifies in mid-air.
But the team's interpretation remains controversial with other experts.
Space impacts have had profound effects on Earth's ecosystems. For example, an asteroid which slammed into Mexico's Yucatan Peninsula 66 million years ago was responsible for wiping out the dinosaurs.
Dennis Kent, from Rutgers University, a co-author of the new study, thinks the glass found in sediment cores drilled along the New Jersey coast could have come from a 10km-wide comet slamming into the Atlantic Ocean.
This could be behind the mysterious release of CO2, and other greenhouse gases, which warmed the planet very rapidly 55.6 million years ago. During this event, global temperatures rose by about 6C in less than 1,000 years.
"It got warm in a hurry. This suggests where it came from," said Prof Kent.
The warm period, known as the Palaeocene-Eocene Thermal Maximum (PETM), is often compared to today's rapid human-induced climate change. It is recorded as an injection of an "isotopically light" form of the element carbon into the Earth's system.
This spike in temperature closely coincides with the dispersal of mammal groups to new parts of the world, and their diversification into three groups that are still with us today.
These are the Artiodactyla, the Perissodactyla and the Primates - the mammalian order that includes humans. Modern Artiodactyls include sheep, pigs, camels and giraffes, while today's Perissodactyls include horses, tapirs, rhinos and zebras.
The drivers behind this rapid phase of mammal evolution are not completely understood. But the planet became essentially ice-free during the PETM, with sea levels that were dramatically higher than now. Many small, single-celled ocean-bottom creatures became extinct.
But on land, the mammals adapted by moving their ranges towards the poles, which would have opened up new opportunities for them.
Mainstream theories suggest the global warming phase, which lasted about 200,000 years, was caused by sources closer to home - such as volcanism.
But the authors of the Science study identified an interesting mineral within the glassy spherules known as microtektites. This mineral, called lechatelierite, "forms at really high temperatures - about 1,700C", said Prof Kent.