Never mind aliens in outer space. Some scientists believe we may be sharing the planet with ‘weird’ lifeforms that are so different from our own they’re invisible to us

Across the world’s great deserts, a mysterious sheen has been found on boulders and rock faces. These layers of manganese, arsenic and silica are known as desert varnish and they are found in the Atacama desert in Chile, the Mojave desert in California, and in many other arid places. They can make the desert glitter with surprising colour and, by scraping off pieces of varnish, native people have created intriguing symbols and images on rock walls and surfaces.

How desert varnish forms has yet to be resolved, despite intense research by geologists. Most theories suggest it is produced by chemical reactions that act over thousands of years or by ecological processes yet to be determined.

Professor Carol Cleland, of Colorado University, has a very different suggestion. She believes desert varnish could be the manifestation of an alternative, invisible biological world. Cleland, a philosopher based at the university’s astrobiology centre, calls this ethereal dimension the shadow biosphere. “The idea is straightforward,” she says. “On Earth we may be co-inhabiting with microbial lifeforms that have a completely different biochemistry from the one shared by life as we currently know it.”

It is a striking idea: We share our planet with another domain of life that exists “like the realm of fairies and elves just beyond the hedgerow”, as David Toomey puts it in his newly published Weird Life: The Search for Life that is Very, Very Different from Our Own. But an alternative biosphere to our own would be more than a mere scientific curiosity: it is of crucial importance, for its existence would greatly boost expectations of finding life elsewhere in the cosmos. As Paul Davies, of Arizona State University, has put it: “If life started more than once on Earth, we could be virtually certain that the universe is teeming with it.”

However, by the same token, if it turns out we have failed to realise that we have been sharing a planet with these shadowy lifeforms for eons, despite all the scientific advances of the 19th and 20th centuries, then we may need to think again about the way we hunt for life on other worlds. Robot spacecraft – such as the Mars rover Curiosity – are certainly sophisticated. But what chance do they have of detecting alien entities if the massed laboratories of modern science have not yet spotted them on our own planet? This point is stressed by the US biologist Craig Venter. As he has remarked: “We’re looking for life on Mars and we don’t even know what’s on Earth!”

Animal figures cut into desert varnish by Native Americans in Utah. Photograph: BWAC Images/Alamy

Cleland – working with her Colorado colleague Shelley Copley – outlined her vision of the shadow biosphere in a paper in 2006 in the International Journal of Astrobiology. Other astrobiologists have also proposed ideas along these lines. They include Chris McKay, who is based at Nasa’s Ames Research Centre, California, and Paul Davies, who put forward his vision of this alternative living zone in a paper in Astrobiology in 2005.

These researchers believe life may exist in more than one form on Earth: standard life – like ours – and “weird life”, as they term the conjectured inhabitants of the shadow biosphere. “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.”

Science’s failure to date to spot this weird life may seem puzzling. The natural history of our planet has been scrupulously studied and analysed by scientists, so how could a whole new type of life, albeit a microbial one, have been missed? Cleland has an answer. The methods we use to detect micro-organisms today are based entirely on our own biochemistry and are therefore incapable of spotting shadow microbes, she argues. A sample of weird microbial life would simply not trigger responses to biochemists’ probes and would end up being thrown out with the rubbish.

That is why unexplained phenomena like desert varnish are important, she says, because they might provide us with clues about the shadow biosphere. We may have failed to detect the source of desert varnish for the simple reason that it is the handiwork of weird microbes which generate energy by oxidising minerals, leaving deposits behind them.