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." (...)
The concept of a shadow biosphere was first outlined by Cleland and her Colorado colleague Shelley Copley in a paper in 2006 in the International Journal of Astrobiology, and is now supported by many other scientists, including astrobiologists Chris McKay, who is based at Nasa's Ames Research Centre, California, and Paul Davies.
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. (...)
"Billions of years ago, life based on different types of carbon biochemistry could have arisen in several places on Earth," says 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 lifeforms based on different bases and proteins could still have survived – in the shadow biosphere."
A different prospect is highlighted by Sasselov, who points out that a complex organic chemical can come in two different shapes even though they have the same chemical formula. Each is a mirror-image of the other and are said to have a different chirality. "Amino acids are an example," says 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."
But how can scientists pinpoint this weird life? Microbes are usually detected in laboratories by feeding nutrients to suspected samples so they grow and expend. Then the resulting cultures can be analysed. A weird lifeform – such as one made only of proteins formed out of right-handed amino acids – will not respond to left-handed nutrients, however. It will fail to form cultures and register its existence.
by Robin McKie, The Guardian | Read more:
Photo: BWAC Images/Alamy
