They are often portrayed on screen as little green men with elongated limbs and saucer-like eyes.

From E.T to the X-Files, aliens from outer space have captured our imagination for decades.

Yet a new book from a leading evolutionary biologist argues that if they exist and we ever encountered them, they would look very similar to us.

Professor Simon Conway Morris said extra-terrestrials that resemble human beings should have evolved on at least some of the many Earth-like planets that have been discovered by astronomers.

In his new book published on 2 July, The Runes of Evolution, the University of Cambridge academic builds on the principle of convergent evolution – that different species will independently evolve similar features, with the comparison of the camera eye of an octopus and a human eye a favourite example – and argues it will not just took place on Earth.

The titular alien character from 2011's 'Paul' (Universal Pictures)

“An area of biology which is becoming popular, perhaps too popular, that the possibility evolution is becoming much more predictable than people thought,” he told The Independent. “The book is really trying to persuade the world that evolutionary convergence is completely ubiquitous. Wherever you look you see it.

“The theme is to try and drive the reader, gently of course, into the possibility that the things which we regard as most important, ie cognitive sophistication, large brains, intelligence, tool making, are also convergent. Therefore, in principle, other Earth-like planets should very much end up with the same sort of arrangement.”

Professor Conway Morris, a Fellow at St John’s College, said it follows that plant and animal life on other planets able to support life would also look similar to Earth’s.

He said: “Certainly it’s not the case that every Earth-like planet will have life let alone humanoids. But if you want a sophisticated plant it will look awfully like a flower. If you want a fly there’s only a few ways you can do that. If you want to swim, like a shark, there’s only a few ways you can do that. If you want to invent warm-bloodedness, like birds and mammals, there’s only a few ways to do that.

“The whole business of extra-terrestrial life, Kepler [the most Earth-like planet ever discovered], the search for extra solar planets, and of course the various missions to Mars. This is all going along at such a pace at the moment we can be reasonably sure [any current ideas] might be open to revision in a few years.”

His theory leads on to Enrico Fermi’s famous paradox – why, if aliens do exist, have they not made contact. He admits his book goes “off-piste for a bit of fun” in the last chapter when dealing with this problem.

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Professor Conway Morris said: “Fermi’s paradox seems to be coming rather sharply into focus. If I’m on the right track then the likelihood of intelligence is evolving and actively engaging in some sort of transgalatic expeditions doesn’t seem to be completely beyond the realm of possibility.

“What Fermi didn’t know when he asked that famous question was that the number of Earth-like planets is absolutely gigantic now. More problematic is that many of these solar systems far, far pre-date our solar system. They would have, in principle, a major head start of hundreds of millions, if not billions, of years.

“The problem is exceedingly acute: we shouldn’t be alone but, famous last words, all the evidence suggests we are. Maybe [aliens] are hiding, the Arthur C Clarke idea, or as Stephen Baxter mischievously suggested we live in a virtual world. I don’t honestly know. My suspicion is we have only begun to scratch at the surface of reality, for want of a better word.”

Professor Conway Morris said one of the main themes of his new book was his desire to “re-energise people and get their sense of curiosity running again”. Convergence is demonstrable at every point of evolutionary history, he argues, from early cells, through to the emergence of tissues, nervous systems, limbs and the ability to make tools.

He said: “There is a growing interest in convergent evolution but I’m always willing to push it as far as I can. I’ll always say ‘show me anything which has only evolved once’ and I’ll try and jump up and say ‘no I can give you another example’.