Alien life, if it exists at all, is likely to be just like us, a leading scientist has claimed. He also believes aliens would also share our human weaknesses for greed, violence and the exploitation of others.

The evolutionary paleobiologist's beliefs mean that science fiction films such as Star Wars and Star Trek could be more accurate than they ever imagined in depicting alien life.

Prof Conway Morris believes that extraterrestrial life is most likely to occur on a planet similar to our own, with organisms made from the same biochemicals. The process of evolution will even shape alien life in a similar way, he added.

"It is difficult to imagine evolution in alien planets operating in any manner other than Darwinian," he said.

"In the end the number of options is remarkably restrictive. I don't think an alien will be a blob. If aliens are out there they should have evolved just like us. They should have eyes and be walking on two legs.

"In short if there is any life out there then it is likely to be very similar to us."

Extra-terrestrials might not only resemble us but have our foibles, such as greed, violence and a tendency to exploit others' resources, claims Professor Conway Morris.

They could come in peace but also be searching for somewhere to live, and to help themselves to water, minerals and fuel he is due to tell a conference at the Royal Society, in London.

However he also thinks that because much of the Universe is older than us they would have evolved further down the line and we should have heard from them by now.

He believes it is increasingly looking like they may not be out there at all.

"It is about time they turned up," he said. "It is very, very quiet out there. Suspiciously quiet. Where on Earth are they? I personally don't think that there is anything out there."

His lecture is part of a two-day conference at which experts will discuss how we might detect life on distant planets and what that could mean for society.

Professor Paul Davies, a British physicist at Arizona State University, will reiterate his belief that life exists right under our noses - here on Earth.

He believes that the life forms, most likely microscopic, could exist in remote and hostile environments where it was previously thought no creature could survive.

They are so different to life as we know it that scientists would have overlooked them in the past.

At the heart of his theory is that life on Earth may have come and gone many times during the planet's existence.

These creatures would be the remnants of the previous inhabitants.

Not all are convinced by the "shadow life" concept. Professor Colin Pillinger, who led the Beagle 2 Mars landing mission, said: "I prefer to deal in scientific fact - this is wildly science fiction. You'd be off your trolley to go searching for arsenic-based life."

Professor Pillinger, who is also due to speak at the Royal Society, argues that Mars remains the best bet for finding alien organisms.

The conference will also address the social implications of the search for alien life.

Professor Albert Harrison, from the University of California, will discuss how human beings might respond to the discovery of extraterrestrial intelligence.

"It is easy to imagine scenarios resulting in widespread psychological disintegration and social chaos," he said.

"But historical prototypes, reactions to false alarms and survey results suggest that the predominant response to the discovery of a microwave transmission from light years away is likely to be equanimity, perhaps even delight."