A U.S. ecologist has claimed that humans are not from Earth but were put on the planet by aliens tens of thousands of years ago.

Dr Ellis Silver points to a number of physiological features to make his case for why humans did not evolve alongside other life on Earth, in his new book.

They range from humans suffering from bad backs - which he suggests is because we evolved in a world with lower gravity – to getting too easily sunburned and having difficulty giving birth.

Dr Ellis says that while the planet meets humans’ needs for the most part, it does not perhaps serve the species’ interests as well as the aliens who dropped us off imagined.

In his book, HUMANS ARE NOT FROM EARTH: A SCIENTIFIC EVALUATION OF THE EVIDENCE, the ecologist writes the human race has defects that mark it of being ‘not of this world’.

‘Mankind is supposedly the most highly developed species on the planet, yet is surprisingly unsuited and ill-equipped for Earth's environment: harmed by sunlight, a strong dislike for naturally occurring foods, ridiculously high rates of chronic disease, and more,’ he told Yahoo.

Dr Ellis says that humans might suffer from bad backs because they evolved on a world with lower gravity.

He also says that it is strange that babies’ heads are so large and make it difficult for women to give birth, which can result in fatalities of the mother and infant.

No other native species on this planet has this problem, he says.

He also believes humans are not designed to be as exposed to the sun as they are on Earth, as they cannot sunbathe for more than a week or two – unlike a lizard – and cannot be exposed to the sun every day without problems.

Dr Ellis also claims humans are always ill and this might be because our body clocks have evolved to expects a 25 hour day, as proven by sleep researchers.

‘This is not a modern condition; the same factors can be traced all the way back through mankind's history on Earth,’ he says.

He suggests that Neanderthals such as homo erectus were crossbred with another species, perhaps from Alpha Centauri, which is the closest star system to our solar system, some 4.37 light years away from the sun.

Dr Ellis said many people feel that they don’t belong and feel at home on Earth.

‘This suggests (to me at least) that mankind may have evolved on a different planet, and we may have been brought here as a highly developed species.’

‘One reason for this … is that the Earth might be a prison planet, since we seem to be a naturally violent species and we're here until we learn to behave ourselves,’ he said.

Dr Ellis said the book is intended to create debate, instead of being a scientific study and hopes it will lead to people getting in touch with him with further suggestions of 'evidence'.

While other scientists have said some bacteria arrived on Earth from space, Chris McKay, an astrobiologist at NASA, said that to jump to the conclusion that it is alien life is ‘a big jump’.

Professor Wainwright from the University of Sheffield plans to investigate further, and believes that life is constantly arriving from space that did not originate on Earth.

Dr Ellis says that while his idea is an extreme evolution of that idea, it is intended to be thought-provoking and he claims to have had a largely positive response to it.

He is interested in whether humans came to Earth separately, perhaps by arriving on meteors and comets, before evolving into the species we know today.

‘My thesis proposes that mankind did not evolve from that particular strain of life, but evolved elsewhere and was transported to Earth (as fully evolved Homo sapiens) between 60,000 and 200,000 years ago,’ he says.