[LINK] "Life on Earth… but not as we know it"

[rfmcdonald]

Writing in The Observer, Robin McKie introduces his audience at length to the idea of shadow biospheres, of ecosystems co-existing with the DNA-based ecosystems and life forms we're familiar with on our own Earth but unrecognized on account of profound differences. The idea was raised most recently in the popular media in 2010 with the purported discovery of arsenic-based life in California's Lake Mono--I've written about the fallout of what was at best a premature accident here on my blog (see 1, 2, 3). The idea remains fascinating for reasons other than their applicability to the search for extraterrestrial life, and potentially plausible.

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!"