WASHINGTON (Reuters) - A single, gigantic asteroid slammed into Earth 65 million years ago, dooming the dinosaurs and many other species, scientists said on Thursday in a new study rebutting theories that multiple impacts did the deed. An examination of rock sediments drilled from five sites at the bottom of the Atlantic Ocean strongly supports the notion that one massive hunk of space rock caused the mass extinction, a research team led by University of Missouri-Columbia geology professor Ken MacLeod found.

"It's a completely straightforward, single-impact scenario," MacLeod, whose findings appear in the Geological Society of America Bulletin, said in an interview. "It was a haymaker that nobody saw coming. One shot, and that's all you need to explain it."

Scientists believe that an asteroid about 6 miles wide hurtled to Earth 65.5 million years ago at the end of the Cretaceous Period, plunging into what is now Mexico's Yucatan Peninsula to carve out the Chicxulub (pronounced CHIK-shu-loob) crater measuring about 110 miles across.

To put it mildly, it was a bad day to live on Earth.

The impact triggered a worldwide environmental catastrophe, many scientists believe, expelling vast quantities of rock and dust into the sky, unleashing giant tsunamis, sparking global wildfires and leaving Earth shrouded in darkness for years.

The dinosaurs, which had ruled for 160 million years, were wiped out. So were large marine reptiles like the mosasaurs and the plesiosaurs, the flying reptiles known as pterosaurs, the tentacled ammonites that populated the seas and many species of marine plankton. The birds suffered losses but survived.

The mammals made it through as well, allowing these warm-blooded, furry little creatures to eventually dominate the land and ultimately setting the stage for the rise of human beings.

EVIDENCE IN THE ROCK

Evidence of the single-asteroid calamity, in the form of debris from the impact scattered worldwide, is contained in rocks dating back to 65 million years ago.

Scientists 26 years ago found a band of iridium -- a metal rare on Earth but common in meteorites -- dating to the end of the Cretaceous Period that suggested a big space rock had smashed into the Earth and blasted its remains around the globe. The subsequent discovery of the Chicxulub crater, dating to the same time, was hailed by many as the smoking gun.

But a group of researchers led by Princeton University's Gerta Keller has advanced a competing theory that the impact that created the Chicxulub crater actually predated the end of the dinosaurs by 300,000 years and did not cause the mass extinction.

They propose that one or more additional big hunks of space rock later hit the Earth and finished the job, but the impact craters they would have left behind have not yet been found.

MacLeod's team examined sediment drilled far below the sea surface about 2,800 miles from the Yucatan impact site, a location they believed to be ideal. Any rock samples taken too close to the crater may be altered by events that occurred immediately after the impact, like waves, earthquakes and landslides. Samples taken too far away may contain too little debris evidence from the impact.

The samples they examined boasted a telltale layer of impact-related material, but there was none on top or below -- indicating, they argued, there were no other impacts.Keller said MacLeod's research does not settle the matter."Unfortunately, these claims are rather hyper-inflated and do not withstand close examination," Keller said by e-mail.

LONDON - A new videotape shows two of the Sept. 11 hijackers smiling for a camera and reportedly reading a will in footage taken more than 18 months before they carried out the worst terrorist attack on U.S. soil. Mohamed Atta and Ziad Jarrah look much different in the tape than they do in photographs made famous after the attacks in New York and Washington.

Both seem younger, are bearded, and the infamously bleak gaze of Atta, the ringleader of the attacks five years ago, is replaced by a somewhat softer expression.Osama bin Laden

The soundless video appears to be a departure from previous releases by al-Qaida, which is "normally, very professional in their media," said Paul Beaver, an independent defense and security expert.

It did not appear on Web sites commonly used by the group. The newspaper quoted an unidentified American source who said that lip readers had been unable to decipher what the men were saying.

The Sunday Times said it had obtained the video "through a previously tested channel" but gave no further details. It said sources from al-Qaida and the United States had confirmed the video's authenticity on condition of anonymity.

A U.S. intelligence official, who declined to be identified, citing government protocol, told The Associated Press, "We're aware of the tape and we're reviewing it." The official declined to answer further questions.

The newspaper said the hourlong video was made at an al-Qaida training camp in Afghanistan, is dated Jan. 18, 2000, and contains the only known footage of Atta and Jarrah together.

Ben Venzke, head of the Virginia-based IntelCenter, which monitors terrorism communications, said the video was probably raw footage that al-Qaida had intended to edit into a package similar to one released last month showing the last testament of two of the Sept. 11 hijackers, Wail al-Shehri and Hamza al-Ghamdi.

For more than 30 minutes, the video shows Atta, who flew one of the planes that brought down the World Trade Center, and Jarrah, who piloted United Airlines flight 93, which crashed into a Pennsylvania field, sitting in front of a white wall, alternately alone and together.

The Egyptian-born Atta is wearing a dark sweater and pats his hair into place after trying on a hat for the camera. At one point, the camera pans out to show a machine gun leaning against the wall next to him.

Atta appears to be the more reticent of the two hijackers. During a portion of the tape showing them together, the Lebanese-born Jarrah laughs and smiles broadly as Atta shakes his head slightly. The conversation then seems to turn serious, and the tape shows the two sitting on the floor, hunched over papers, which The Sunday Times reported was Jarrah's will.

Bin Laden said a few years ago that he was saving Atta's last testament to release for a special occasion, Venzke said.

"It is highly unlikely that al-Qaida wanted the material to be released in this manner, and it is not consistent with any previous release," he said.

Diaa Rashwan, an Egyptian expert on militant groups, said he found it strange that the cameraman focused not only on bin Laden but also on his audience. He said normally al-Qaida videos of bin Laden just focus on him.

"Was this a video by al-Qaida or by a security agency?" Rashwan asked. "I have never seen such a video."

Although the video has no sound, it could contain valuable information, Beaver said.

"It helps build up a profile, so you can ID people in the future," he said.

But Robert Ayers, an international security expert, said the tape was more curiosity than valuable resource.

"The fact that these guys changed their facial appearance? Any actor on any stage in the world knows how to change their appearance, so why are we so surprised these guys changed their appearance?" Ayers said.

Previous mug shots released of the two men show them clean-shaven and with closely cropped hair.

The video also includes images of a man who appears to be bin Laden speaking to an audience outdoors. A time stamp indicated that footage was shot on Jan. 8, 2000, and The Sunday Times said it appeared to have been made at Tarnak Farm, once the base for bin Laden's family in the Afghan desert near Kandahar's airport.

It shows about 75 men, many in turbans or caps, sitting on the ground as bin Laden arrives to address them. A few children are in the crowd. The man who appears to be bin Laden stands in front of an expanse of bare dirt dotted with a few trees and windowless, one-story mud-colored buildings, some of them partly in ruins.

He appears calm, with a long beard and a tan cloak over a white robe that covers his head. He speaks for more than 10 minutes, although the camera frequently cuts away from him and onto the audience.

The Sunday Times said those shown listening to bin Laden included Ramzi Binalshibh, who allegedly helped plan the Sept. 11 attacks and is now being held in the U.S. prison camp at Guantanamo Bay, Cuba.

Also reportedly present was Nasir Ahmad Nasir al Bahri, a security guard who The Sunday Times said has claimed he was authorized to shoot bin Laden in the head if the leader was in danger of being captured.