A comet collision with Earth around 55 million years ago may have kick-started a crucial early phase of mammal evolution.
By Paul Rincon
The impact could have triggered the greenhouse warming thought to have encouraged primitive mammals to disperse across the world and diversify into three important groups still with us today.
Did a comet strike deliver carbon to heat up the Earth
These groups were the Artiodactyla, the Perissodactyla and the Primates - the mammalian order that includes humans. Modern Artiodactyls include sheep, pigs, camels and giraffes. Today's Perissodactyls include horses, tapirs, rhinos and zebras.
This evolutionary branching event coincides with a clear boundary in the Earth's geological record dividing the Palaeocene and Eocene epochs.
North American scientists have put forward their comet hypothesis after studying sediments drilled on the East Coast of the US.
It is known from the composition of rocks and marine sediments laid down at the Palaeocene-Eocene boundary that global temperatures at the time rose by around 6 degrees Celsius in less than 1,000 years - an event known as the thermal maximum.
This is thought to have warmed the cold, northern latitudes where most of the major early Eocene land corridors were located.
The sudden warming made these northern climes habitable, allowing mammals to disperse across the land corridors into new continents.
Iron-rich particles (black dot) could be evidence of impact debris
As mammals dispersed, they diversified - perhaps to exploit different food sources.
The Palaeocene-Eocene boundary also coincides with a massive injection of the form, or isotope, of carbon known as 12C into the Earth's carbon cycle. Scientists believe this boosted carbon dioxide (CO2) levels in the atmosphere, contributing to the greenhouse warming responsible for the 100,000-150,000-year-long thermal maximum.
One of the injection candidates is a sudden release from the sea floor of ice-trapped methane gas; general warming of the climate and oceans could have triggered the melting of these so-called clathrates, sending a surge of gas into the atmosphere that pushed temperatures even higher.
But this version of the story is now challenged by new data.
The information would appear to support the idea that it was a comet impact which released directly into the atmosphere the massive quantities of carbon necessary to raise global temperatures so abruptly.
"The thermal maximum seems to be a transient event, unrelated to whatever's controlling long-term climatic trends," said Professor Dennis Kent, a geologist at Rutgers University, Piscataway, US, and a co-author of the new study.
But Professor Kent acknowledges that methane release from the sea floor probably prolonged the warm spell.
Advantage mammals: Propalaeotherium, a horse-like early perissodactyl
"We're suggesting that there was another source of 12C carbon to kick things off," he explained.
Professor Kent and his team say the impact may have been caused by an object measuring about 10 kilometres across - about the size of Halley's Comet.
The researchers looked at layers coinciding with the Palaeocene-Eocene boundary in three sediment cores drilled from beneath the Atlantic coastal plain of New Jersey, US.
They found tiny iron-rich particles similar to those found in 65-million-year-old sites associated with the comet or asteroid collision that supposedly killed off the dinosaurs.
The 65-million-year-old particles are thought to have condensed out of the vapour-rich plume of debris blown out by the impact, so the authors of the latest study suggest that iron-rich grains in their samples formed the same way.
Professor Kent speculates the object responsible for the Palaeocene-Eocene impact could have been a big snowball containing little rock.
This could account for a relative absence in the Atlantic cores of iridium, an element found abundantly in meteorites and in 65-million-year-old clays.
Dr William Clyde, a geologist at the University of New Hampshire, US, expressed scepticism about the theory, but said it was likely to prompt further investigation into the mechanisms behind the thermal maximum.
The new study could add weight to the theory that life on Earth was shaped by impacts from outer space. If one of these impacts killed off the dinosaurs, it is perhaps ironic that another may have helped mammals flourish and diversify.
The research is published in the journal Earth and Planetary Science Letters.