As such, it will be a blow to proponents of intelligent design, who claim that the many gaps in the fossil record show evidence of some higher power.

Richard Dawkins, the evolutionary biologist, said: "Our emergence on to the land is one of the more significant rites of passage in our evolutionary history, and Tiktaalik is an important link in the story."

Tiktaalik - the name means "a large, shallow-water fish" in the Inuit language Inuktikuk - shows that the evolution of animals from living in water to living on land happened gradually, with fish first living in shallow water.

The animal lived in the Devonian era lasting from 417m to 354m years ago, and had a skull, neck, and ribs similar to early limbed animals (known as tetrapods), as well as a more primitive jaw, fins, and scales akin to fish.

The scientists who discovered it say the animal was a predator with sharp teeth, a crocodile-like head, and a body that grew up to 2.75 metres (9ft) long.

"It's very important for a number of reasons, one of which is simply the fact that it's so well-preserved and complete," said Jennifer Clack, a paleontologist at Cambridge University and author of an accompanying article in Nature.

Scientists have previously been able to trace the transition of fish into limbed animals only crudely over the millions of years they anticipate the process took place. They suspected that an animal which bridged the gap between fish and land-based tetrapods must have existed - but, until now, there had been scant evidence of one.

"Tiktaalik blurs the boundary between fish and land-living animal both in terms of its anatomy and its way of life," said Neil Shubin, a biologist at the University of Chicago, and a leader of the expedition which found Tiktaalik.

The near-pristine fossil was found on Ellesmere Island, Canada, which is 600 miles from the north pole in the Arctic Circle.

Scientists from the Academy of Natural Sciences in Philadelphia, the University of Chicago, and Harvard University led several expeditions into the inhospitable icy desert to search for the fossils.

The find is the first complete evidence of an animal that was on the verge of the transition from water to land. "The find is a dream come true," said Ted Daeschler of the Academy of Natural Sciences.

"We knew that the rocks on Ellesmere Island offered a glimpse into the right time period and were formed in the right kinds of environments to provide the potential for finding fossils documenting this important evolutionary transition."

When Tiktaalik lived, the Canadian Arctic region was part of a land mass which straddled the equator. Like the Amazon basin today, it had a subtropical climate and the animal lived in small streams. The skeleton indicates that it could support its body under the force of gravity.

Farish Jenkins, an evolutionary biologist at Harvard University said: "This represents a critical early phase in the evolution of all limbed animals, including humans - albeit a very ancient step." Tiktaalik also gives biologists a new understanding of how fins turned into limbs. Its fin contains bones that compare to the upper arm, forearm and primitive parts of the hand of land-living animals.

"Most of the major joints of the fin are functional in this fish," Professor Shubin said.

"The shoulder, elbow and even parts of the wrist are already there and working in ways similar to the earliest land-living animals."

Dr Clack said that, judging from the fossil, the first evolutionary transition from sea to land probably involved learning how to breathe air. "Tiktaalik has lost a series of bones that, in fishes, covers the gill region and helps to operate the gill-breathing mechanism," she said. "The air-breathing mechanism it had would have been elaborated and having lost the series of bones that lies between the head and the shoulder girdle means it's got a neck, it can raise its head more easily in order to gulp the air.

"The flexible robust limbs appear to be connected with pushing the head out of the water to breathe the air."

H Richard Lane, director of sedimentary geology and palaeobiology at the US National Science Foundation, said: "These exciting discoveries are providing fossil Rosetta stones for a deeper understanding of this evolutionary milestone - fish to land-roaming tetrapods."

A cast of the fossil goes on display at the Science Museum in South Kensington central London today.

That means dentistry is at least 4,000 years older than first thought and far older than modern anesthesia.

Macchiarelli came to his conclusion after finding nearly perfect holes that had been drilled into the back teeth of nine skulls in a Pakistani graveyard. He carbon-dated the skulls and found the patients lived between 5500 BC and 7000 BC.

What surprised Macchiarelli was the sophistication of the dental work. The ancient dentists managed to drill holes into the large molars at the back of their patients' mouths, a tricky job even with modern equipment.

Some holes were 3.5 millimetres deep.

"The holes were so perfect, so nice," said David Frayer, an anthropology professor at the University of Kansas who co-authored the study. "I showed the pictures to my dentist and he thought they were amazing holes."

He believes the ancient dentists learned their trade drilling ornamental beads, a technique that was in vogue at the time.

Researchers found drill bits that were fashioned out of tiny pieces of flint and used tiny bows to spin them. Macchiarelli tried the technique with a home-made bow and drilled through a human tooth in less than a minute.

"Definitely it had to be painful," Macchiarelli said.

Researchers were impressed by how advanced the society was at such an early period. One patient had a tooth that was drilled twice. Another patient had three teeth drilled, while other teeth showed signs of cavities around the drill holes.

Macchiarelli found no sign of fillings, but he told Nature that a soft, asphalt material could have been used.

"When they first saw us digging there, the local people just couldn't believe there was a pyramid," said archaeologist Jesus Sanchez. "It was only when the slopes and shapes of the pyramid, the floors with altars were found, that the finally believed us."

Ceramic fragments and the presence of other ceremonial structures on the hill suggested the possibility there was a pyramid or temple somewhere nearby, but the theory wasn't proved until a member of Sanchez's team, Miriam Advincula, started a project to map the site in 2004. Exploratory trenches dug in 2005 and 2006 confirmed the find.

"Both the pre-Hispanic structure and the Holy Week rituals are part of our cultural legacy, so we have to look for a way to protect both cultural values," said Sanchez, who, along with archaeologist Miriam Advincula, has been exploring the site since 2004.

The people of Iztapalapa - now a low-income neighborhood plagued by squatter settlements - began re-enacting the Passion of Christ in 1833, to give thanks for divine protection during a cholera epidemic.

During the ritual, which draws as many as a million spectators every year, a wooden cross is raised just a few yards from the buried remains of the Teotihuacan temple, and a man chosen to portray Christ is tied to the cross.

Archeologists said they will fill in the excavation pits that revealed the pyramid to prevent the structure from being damaged by Good Friday spectators.

Measuring nearly 500 feet on each side, the 60-foot-tall pyramid was carved out on a natural hillside around 500 A.D., the scientists said. It was abandoned about 300 years later when the Teotihuacan culture collapsed.

Mexico abounds with cases in which Spanish conquerors literally built their Catholic faith atop the remains of older religions.

But the case of Iztapalapa hillside, known as the Hill of the Star, appears to be mere geographical coincidence, Sanchez said.

Pre-Hispanic cultures chose the hills that dot the otherwise flat, mountain-ringed Mexico Valley for their ceremonial sites, and postcolonial communities did the same.

What's more, the collision was big enough to send up to 16 million billion tons (16 quadrillion tons) of Mercury's rocky material tens of millions of miles to Earth, new computer simulations suggest.

Some scientists believe that Mercury was much larger as it was forming than it is today. It had a lighter, rockier outer layer, similar to that of Earth's, which was blasted away in the great crash, they say.

This would explain why Mercury is so different from its neighbors Venus and Earth. Mercury is very heavy for its size, due to an unusually large amount of iron, scientists believe.

All the planets are thought to have formed in much the same way, so how Mercury ended up being so different is not completely understood.

A team of scientists in Bern, Switzerland, decided to tackle the mystery. They ran a pair of extensive computer simulations to test the collision theory.

"You always try to prove an idea wrong. This work shows it could have happened in this way,'' said astronomer Jonti Horner, who will present his results tomorrow at a Royal Astronomical Society meeting in Leicester, England.

First, the scientists simulated the catastrophic collision of the young Mercury with a giant asteroid traveling at 16 miles (25 kilometers) a second.

The collision would have been so violent and energy-packed that it would have caused Mercury's outer layer to melt. Shock waves would have flung the material from the planet.

The rapidly escaping debris would have eventually cooled into particles 1 centimeter (0.4 inch) in diameter or smaller, Horner said.

At the end of this hours-long process, Mercury would have been left at 35 percent of its original size.

"You've lost more [of Mercury] than what is left,'' Horner said.

A second simulation followed Mercury's jettisoned particles through space. The bits, Horner says, would have traveled for millions of years.

"Mercury particles would've ended up on everything in the solar system," he said.

Some particles were swept up by Jupiter's gravity and flung from our solar system. Some landed on Earth.

Most would have gone to Venus. "It's the nearest stop,'' Horner said.

Exactly where they ended up was heavily influenced by where Mercury may have been at the time of the collision and exactly where the planet was hit, the simulations found.

The simulations determined that the particles wouldn't have fallen back to Mercury.

Because of the distance the particles would have been flung during the collision, it would have taken four million years for 50 percent of the particles to fall back to Mercury. By then, they would have already been carried away by solar radiation.

John Chambers, a research astronomer with the Carnegie Institution in Washington, D.C., agreed that Mercury bits may have landed on Earth.

"If Mercury was hit, especially by something big, pieces could have escaped and hit other planets,'' he said.

The scientists acknowledge that the Sea of Galilee, in what is now northern Israel, has never frozen in modern times. But they say geological core samples suggest that average temperatures were lower in Jesus's day, and that there were at least two protracted cold spells in the region 1,500 to 2,500 years ago.

In addition to chilly weather, their explanation depends on a rare physical property of the Sea of Galilee, known to modern-day Israelis as Lake Kinneret. It is fed by salty springs along its western shore that produce plumes of dense water, thermally isolating areas that could freeze even if the entire lake did not, they assert.

"I don't know whether the story is based on someone seeing Jesus walk on ice," said Doron Nof, an Israel-born professor of oceanography at Florida State University. "All I know is that during that time, a freeze could have happened -- and it could have looked like someone was walking on water, particularly if it rained after the ice formed."

This is not the first time that Nof, 61, has attempted to debunk a biblical miracle. In 1992, he and Nathan Paldor, an atmospheric scientist at the Hebrew University in Jerusalem, wrote a scientific article proposing that strong winds across the narrow, shallow Gulf of Suez could have lowered the Red Sea by 10 feet, allowing the Israelites to cross to safety and then swallowing up an Egyptian army within a few minutes when the wind stopped, just as the book of Exodus says.

Nof, who described himself as a nonreligious Jew, said he hopes that critics will realize that he is an "equal opportunity miracle buster" who has taken on both Moses and Jesus.

"This isn't going to convince a believer not to believe, and nobody's trying to do that. At least, I'm not trying to do that," he said. "I personally believe that all these biblical stories are based on some truth."

To develop their theory of what they call "springs ice" on the Sea of Galilee, Nof and Paldor teamed up with Ian McKeague, a statistician at Columbia University. Their 23-page paper appears in this month's edition of the Journal of Paleolimnology, a peer-reviewed publication on the history of lakes.

In recent years, possible scientific explanations have also been proposed for several other Bible stories, including Noah's flood (seawater surging from the Mediterranean into the Black Sea as glaciers melted at the end of the last Ice Age) and Joshua's destruction of the walls of Jericho (an earthquake).

The reaction yesterday among biblical scholars to Nof's theory ranged from bemused detachment to real irritation.

"When I look at those verses, I don't need a scientific explanation. I'm a religious man, and I believe that God can do whatever he wants to do, that Jesus could do whatever he wanted to do," said Stanley M. Burgess, professor of Christian history at Regent University, an evangelical Christian school founded by Pat Robertson in Virginia Beach.

But Burgess added that he has "no problem at all" with scientists propounding alternative explanations for miracles. Rather than undercutting religious belief, such explanations may strengthen the faith of people who accept the gospel message but have difficulty accepting "signs and wonders" at face value, he said.

"If you need miracles to prop up your faith, then maybe your faith is weak to begin with," he said.

Wendy Cotter, professor of scripture at Loyola University Chicago, a Roman Catholic school, wrote her doctoral dissertation 15 years ago on biblical accounts of Jesus's stilling the wind and walking on the sea. When she heard about Nof's theory, she said, her first thought was: "Anything's possible -- but that's not what the writer means."

To the Romans, she said, the sea was "the ultimate force of nature, which was why the Caesars always claimed control over it." Jews in the time of Jesus also feared the sea and, moreover, were familiar with the Book of Job, in which God is described as the one who can "walk on the sea," she said.

In attributing to Jesus the power to walk across the waves, "Christians were using the imagery that had previously been used by both the Romans and the Jews to show that a person has been given authority by God," Cotter said. "Water, or ice, is not the point."