The search for extraterrestrial life has been an ongoing quest for decades, but finding alien life relies on our ability to recognize it.

Is there any guarantee that other life forms would manifest in a form that we would immediately be able to identify? Are we wrong to assume that all life must be derived from the same chemical elements, or require the same environmental factors?

These are questions that are being posed by scientists at Colorado University, after the discovery of a mysterious bloom of chemicals on boulders and rocks in dry areas such as the Mojave desert in California and the Atacama desert in Chile. The sheen, a colorful mixture of manganese, arsenic and silica, has become known as "desert varnish." Despite extensive research, it is not known how the varnish forms, though the most common theory suggests that it is produced by a series of chemical reactions that take thousands of years to develop, but one scientist has a radical new theory that shines a very different light onto the subject.

Professor Carol Cleland, a philosopher based at Colorado university’s astrobiology centre, believes that the desert varnish is very significant as it could hold the key to unlocks the door into the shadowy worlds that lie just beyond our comprehension. She is postulating that the sheen could actually form part of an invisible biological world that exists in an ethereal dimension known as the shadow biosphere.

"The idea is straightforward," said Cleland. "On Earth we may be co-inhabiting with microbial life forms that have a completely different biochemistry from the one shared by life as we currently know it."

The concept, detailed by Cleland and her co-author Shelley Copley in a 2006 paper published in the International Journal of Astrobiology, is intriguing and opens up a whole new dimension to the way we approach the search for alien life. According to the paper, alien life may already be existing in many forms on our own planet, both in forms that follow physiological patterns that we understand, and those we have yet to recognise.

The authors of the paper classify the unknown organisms thought to inhabit the shadow biosphere as "weird life."

"All the micro-organisms we have detected on Earth to date have had a biology like our own: proteins made up of a maximum of 20 amino acids and a DNA genetic code made out of only four chemical bases: adenine, cytosine, guanine and thymine," says Cleland. "Yet there are up to 100 amino acids in nature and at least a dozen bases. These could easily have combined in the remote past to create lifeforms with a very different biochemistry to our own. More to the point, some may still exist in corners of the planet."

So, why has this "weird life" remained undetected by science to date? How can we search for life on other planets when we have yet to fully determine who resides on our own?

As US biologist Craig Venter commented: "We’re looking for life on Mars and we don’t even know what’s on Earth!"

Cleland explained that previous methods used to detect micro-organisms have been based solely on our own biochemistry, therefore a shadow microbe would not be registered by conventional biological probes and would most probably be disregarded. Despite the recognition of this fact, scientists are still viewing the unknown residents of the shadow biosphere in biological terms on the assumption that they will be carbon-based entities, though the possibility more complex, silicon-based life forms has been accepted.

"Billions of years ago, life based on different types of carbon biochemistry could have arisen in several places on Earth," said Cleland. "These varieties would have been based on different combinations of bases and amino acids. Eventually, one – based on DNA and on proteins made from 20 amino acids – formed multicellular entities and became the dominant form of life on Earth. That is why we find that life as we know it, from insects to humans and from plants to birds, has DNA as its genetic code. However, other life forms based on different bases and proteins could still have survived – in the shadow biosphere."

A recent book written by David Toomey and entitled Weird Life: The Search for Life that is Very, Very Different from Our Own, suggests that we may co-exist with a mystical shadow world which lives "like the realm of fairies and elves just beyond the hedgerow." It encompasses a whole array of weird life forms that appear to live outside the accepted physical boundaries within which life is thought to be possible, including micro-organisms that live out their entire life cycles in clouds.

Toomey backs Cleland on the subject of desert varnish. "No laboratory microbiologist has been able to coax bacteria or algae to make desert varnish," he states in his book. "It is also possible that the stuff is the end result of some very weird chemistry but no one has been able to reproduce that either."

Despite its many advocates, however, the weird life concept has its critics.

"I think it is very unlikely that after 300 years of microbiology we would not have detected such organisms despite the fact that they are supposed to have a different biochemistry from the kind we know about today," says Professor Charles Cockell, of the UK Centre for Astrobiology at Edinburgh University.

To quell the critics, Dimitar Sasselov, professor of astronomy at Harvard University and director of the Harvard Origins of Life Initiative, has attempted to put forward some proof that the shadow biosphere really exists:

"If you want a clue you can count up the amount of carbon that is emitted by living things – cows, sheep, grass, plants, forests and all the planet’s bacteria. When you do, you find there is a discrepancy of around 5% when you compare the amount given off from Earth’s standard biosphere and the amount you find in the atmosphere," he explained.

Sasselov believes that the discrepancy could be due to emissions from the shadow biosphere. If the calculations are correct, then there is more carbon dioxide in the atmosphere than would be generated by known life on Earth.

"There is plenty of room for a shadow biosphere. That is clear. Certainly, it is not true, as some allege, that we have strong evidence to show that it does not exist. In fact, the opposite is true: we do not have good enough evidence to dismiss it," he said.

Scientists are speculating regarding the type of life that might be found in the shadow side. Sasselov suggests that complex organic chemicals can form in two different shapes even though they have the same chemical formula, with each being chiral molecules, or mirror-images of the other.

"Amino acids are an example," said Sasselov. "Each comes in a right-handed version and a left-handed version. Our bodies – in common with all other lifeforms – only use left-handed versions to create proteins. Right-handed amino acids are simply ignored by our bodies. However, there may be some organisms, somewhere on the planet, that use only right-handed amino acids. They could make up the weird life of the shadow biosphere."

Detecting unknown forms of life with conventional equipment is difficult, as organisms are normally found by feeding samples until the cultures grow large enough for analysis, but right-handed weird proteins will not respond to left-handed nutrients.

Sasselov is pursuing a solution with colleagues through Harvard Origins of Life Initiative.

"They are building an artificial cell – or bionic system – made only of right-handed components including right-handed DNA and right-handed ribosomes. "If there are right-handed lifeforms out there, many of them will be viruses – which will attempt to hijack the DNA of our bionic cells," explained Sasselov. "When they do that they will leave evidence of their existence. Essentially we are building honey traps to catch any right-handed viruses that might live in the shadow biosphere and so reveal their existence."

So the shadow biosphere and its unique residents may soon be revealed, though extreme life forms that could support the weird life theory have already been found on this planet. Existing in the most inhospitable locations, such as hot vents on the sea bed, in highly-saline lakes, in ice sheets in Antarctica and in arid deserts, these hardy organisms are known as "extremophiles." Defying all previously-held beliefs about the conditions necessary to support life, they survive in physically or geochemically extreme conditions that would be detrimental to most other life forms on Earth.

Cleland is not the only astrobiologist to have embraced the "weird life" concept, as similar ideas have been proposed by Chris McKay, from Nasa’s Ames Research Centre, California, and also Paul Davies, who outlined his vision of an alternative living zone in an Astrobiology paper in 2005.

Davies, of Arizona State University, commented: "If life started more than once on Earth, we could be virtually certain that the universe is teeming with it."

We may need to expand our minds even further in the search for E.T., however, as some scientists hypothesize that we may even have to look outside the realms of biological entities. They theorize that biological intelligence may have merely paved the way for incredibly advanced forms of artificial intelligence, and that the majority of the intelligent beings in the universe could well be machines. These artificial entities would require large amounts of power to survive, so potential habitats are thought to be black holes, or in the neighborhood of large stars.

We may never be able to detect that these life forms are artificial; to quote Whitley Strieber’s enigmatic informant, whose teachings are outlined in his book The Key, "If I was an intelligent machine, I would deceive you."

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