Posted on 06/19/2003 4:06:07 PM PDT by blam
Mammals' lucky space impact
By Paul Rincon
BBC science
A comet collision with Earth around 55 million years ago may have kick-started a crucial early phase of mammal evolution.
Did a comet strike deliver carbon to heat up the Earth
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.
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.
Methane feedback
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.
Iron-rich particles (black dot) could be evidence of impact debris
The sudden warming made these northern climes habitable, allowing mammals to disperse across the land corridors into new continents.
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.
Drilling project
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.
Advantage mammals: Propalaeotherium, a horse-like early perissodactyl
But Professor Kent acknowledges that methane release from the sea floor probably prolonged the warm spell.
"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.
Different outcome
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.
Then how do explain the starving idiots that live in a desert? They and their ancesters were too stupid to move their tent 10 feet every few hundred years?
Some would remain, but humans like animals radiate toward suitable environments, the places we lived 1,000 years ago may not be as suitable as they once were.
The deserts of SE California were covered with lakes and had a temperate climate 4,000 - 5,000 years ago. This changed and the most natives moved on, those remaining had to adopt different strategies.
OBTW: I don't think that this migration is a cognitive decision.
Ooh, I like your optimism!
Much prior to that the Sahara desert was a populated grass land.
I think we need to give early man more credit. (Maybe if they were as stupid and lazy as some of us are now, we wouldn't exist.) I believe that early man migrated far more than what we think. In a mobile society, of coarse you would move if conditions became adverse. (And heck, no government to demand they fix it.)
1,000 years = about 50 generations is plenty of time for groups to make a move to "greener pastures" as would be the want of hunter/gatherer societies.
Mankind, bless 'em, has always wondered what lay over that next mountian. The frightened, weak, slow hairless ape forced to use his wits to survive in a world ruled by tooth and claw managed to come out of Africa and walk the moon in something like 5,000 generations.
Assuming the amount of water on earth was relatively constant, wouldn't the absence of ice caps mean more water for the oceans and thus fewer land bridges?
Gee, you folk call this science? "could have" and "thought" in one sentence make this whole article just evo propaganda for the unintelligent masses.
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