A balloon returned from a high-altitude flight this year covered in microscopic life forms which seemed not to be of this world - and reignited the debate over whether life on Earth actually began here, or somewhere else.
This year, other scientists have argued that life originated on Mars, due to a mineral found in Martian meteorites, thought to be crucial to the genesis of life. Another experiment showed that amino acids could have arrived in impacts with comets - which suggests life might be widespread in the solar system.
But a new book by American ecologist Dr Ellis Silver argues that humans may well not be from Earth - and may have arrived separately. Silver offers arguments, based on human physiology, that suggest we may not have evolved alongside other life on Earth - but arrived from elsewhere, brought here by aliens as recently as a few tens of thousands of years ago.
Silver, an environmentalist who is currently working with the effort to clean plastic debris from the Pacific, says his book aims to provoke debate - and is based on scientific work on the difference between humans and other animals.
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“The Earth approximately meets our needs as a species, but perhaps not as strongly as whoever brought us here initially thought,” Silver said in an interview with Yahoo news.
“Lizards can sunbathe for as long as they like - and many of them do. We can just about get away with it for a week or two. But day after day in the sun? Forget it. You might as well just lie down on the freeway and wait for a bus to hit you.” We are dazzled by the sun, which is also odd, says Silver - most animals are not.
Silver claims that some chronic illnesses that plague the human race - such as bad backs - could be a sign we evolved on a world with lower gravity. Silver points to other unique human traits - such as the fact that babies’ heads are so large that women have trouble giving birth - in earlier eras, this was often fatal for mother, child or both.
“No other truly native species on this planet has this problem,” he says. Silver also points out to the “extra” 223 genes in human beings, which are not found in any other species, and to the lack of a fossil “missing link”.
Silver chose not to publish in a scientific imprint, wanting to inspire open debate. Reviewers have compared Silver to other space-gazing theorists such as Erich von Däniken, while others have said, “it is possible to drive a coach and horses through several of his arguments.”
Silver also claims that the human race has defects that mark us out as being possibly “not of this world”.
“We are all chronically ill,” says Silver. “Indeed, if you can find a single person who is 100% fit and healthy and not suffering from some (perhaps hidden or unstated) condition or disorder (there's an extensive list in the book) I would be extremely surprised - I have not been able to find anyone.”
“I believe that many of our problems stem from the simple fact that our internal body clocks have evolved to expect a 25 hour day (this has been proven by sleep researchers), but the Earth's day is only 24 hours. This is not a modern condition - the same factors can be traced all the way back through mankind's history on Earth.”
Silver does not suggest one answer - but a possibility that early pre-humans such as homo erectus were crossbred with another species. He also suggests several possible origins, including Alpha Centauri.
“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 (raw) foods, ridiculously high rates of chronic disease, and more. Plus there's a prevailing feeling among many people that they don't belong here or that something "just isn't right".
“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, discussed in the book, 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.”
“Humans are not from Earth was published mainly to gauge reaction from readers and to provoke thought, particularly among those who might not have considered such a possibility before.”
Ellis hopes that readers will contact him with more evidence for a more extensive follow-up work.
The claim that bacteria are arriving from space has also caused controversy - and revived the idea of “panspermia”, where life from Earth might have “pollinated” other planets nearby..
"There is probably truth to the report that they find curious stuff in the atmosphere," Chris McKay, an astrobiologist at NASA told SPACE.com in an interview. "The jump to the conclusion that it is alien life is a big jump and would require quite extraordinary proof."
Professor Wainwright and his colleagues at the University of Sheffield aim to conduct further tests.
“In the absence of a mechanism by which large particles like these can be transported to the stratosphere, we can only conclude that the biological entities originated from space," Wainwright added.
"Our conclusion then is that life is continually arriving to Earth from space, life is not restricted to this planet and it almost certainly did not originate here."
Silver’s more radical idea is presented as polemic, intended to inspire argument - “Initial reaction has been positive, although one reviewer thought it was a parody, while another found the writing style heavily dictatorial,” he admits.
The debate over the origin of life looks set to intensify. Simulations on supercomputers at the Lawrence Livermore National Laboratory in the U.S. have found that amino acids, the building blocks of life, could have arrived on Earth via comets.
This would suggest that life might be found elsewhere in our solar system - or even beyond.
Nir Goldman suggests that the simple molecules found in comets (such as water, ammonia, and carbon dioxide) could have supplied the raw materials, and the impact with early Earth could have “ignited” a prebiotic reaction.
A series of experiments where projectiles were fired into a cometary ice mixtures formed amino acids - the building blocks of life.
"These results confirm our earlier predictions of impact synthesis of prebiotic material, where the impact itself can yield life-building compounds," Goldman said. "These results present a significant step forward in our understanding of the origin of the building blocks of life. This increases the chances of life originating and being widespread throughout our solar system," Goldman said.
Silver wants to pose the question of whether humans arrived separately, “Recent scientific reports suggest that life itself might not be from Earth but might have arrived here on meteors or comets. This primitive form of life then evolved over billions of years into what we find on Earth 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.”
“Little in the book can be proven - it can only be supposed or suspected. But there is more than enough indisputable evidence to make further study worthwhile.”