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.

The presence of this mineral is hard to explain without a highly energetic event such as an impact. Volcanic magma, for example, has much lower temperatures.

A second line of evidence comes from the discovery of grains of shocked quartz in one of the spherules. These "shocked grains" occur when the mineral quartz is knocked out of shape under the kind of intense pressure produced by a cosmic collision. But pressures in a volcano are not sufficient to produce these grains.

But Prof Christian Koeberl, an impact specialist at the University of Vienna who was not involved with the study, noted that the identification of shocked quartz had been made using a technique known as Raman spectrometry.
He told the BBC News website: "This is not a standard method to identify shocked quartz, so maybe it is, maybe not."

Dennis Kent first floated the comet idea in a 2003 study published in the journal Earth and Planetary Science Letters.

He based his argument on magnetised clay particles found in New Jersey that he said could have been altered by space impact. However, many colleagues were unconvinced by his hypothesis.

The discovery of the glassy spherules, reported in Science, provides additional evidence favouring the comet theory, according to Prof Kent.

"The glassy spherules don`t demand a comet, rather than an asteroid, but the idea is attractive because of the increase in isotopically light carbon which acts as a greenhouse gas," he told the BBC News website.

"About a fifth of comets by mass is carbon... there are also indications from spectral analyses of comets in the Solar System that this carbon tends to be on the isotopically light side."

The dinosaur-killing impact 66 million years ago - almost certainly caused by an asteroid - left a pronounced layer of the element iridium in rocks far from the crater site.

Iridium is rare on Earth but common in asteroids. However, the impact of a comet - with less rock and more ice than an asteroid - could explain why scientists have not found a significant iridium layer at the Palaeocene-Eocene boundary.

In addition, "comets come from the outer reaches of the Solar System, so their impact velocity is something like three times faster than an asteroid`s", said Prof Kent.

This high speed impact may act to dilute the iridium present in the space object. And an impact into the ocean may explain why no crater has yet been discovered.

Prof Koeberl told the BBC News website that the spherule evidence "indicates that an impact event may have occurred at that time - but only if the spherules are really unambiguously of [Palaeocene-Eocene] age.

"The information given so far is not conclusive on this point - it is not impossible that the spherules are derived from another stratigraphic layer (I hope contamination can be excluded, which is not uncommon either - spherules have been found in many strange locations). So age dating would be helpful."

He added: "What I am also noting is that the spherules are very small and very rare... this indicates either a far-away source (but the limited size range is still unusual) or a rather small impact event.

"Both options do not indicate a major influence on the Earth`s climate. Even much larger events... have had no severe or discernible influence on the climate. In this respect I think any suggestion that an impact event caused the PETM is not supported by the data."

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