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"OH WATERS, TEEM WITH MEDICINE TO KEEP MY BODY SAFE FROM HARM, SO THAT I MAY LONG SEE THE SUN." - Rig Veda
We could be poised on another such philosophical precipice, about to discover a second important world hiding amid our own: alien life on our own planet. Today, scientists seek extraterrestrial microbes in geysers of chilled water shooting from Enceladus and in the ocean sloshing beneath the ice crust of Europa. They search for clues that beings once skittered around the formerly wet rocks of Mars. Telescopes peer into the atmospheres of distant exoplanets, hunting for signs of life. But perhaps these efforts are too far afield. If multiple lines of life bubbled up on Earth and evolved separately from our ancient ancestors, we could discover alien biology without leaving this planet.
The modern-day descendants of these ‘aliens’ might still be here, squirming around with van Leeuwenhoek’s microbes. Scientists call these hypothetical hangers-on the ‘shadow biosphere’. If a shadow biosphere were ever found, it would provide evidence that life isn’t a once-in-a-universe statistical accident. If biology can happen twice on one planet, it must have happened countless times on countless other planets. But most of our scientific methods are ill-equipped to discover a shadow biosphere. And that’s a problem, says Carol Cleland, the originator of the term and its biggest proponent.
The idea came to Cleland, a philosopher at the University of Colorado at Boulder, when she spent a sabbatical year at the Centro de Astrobiología in Spain. She was studying the scientists, who were studying microorganisms.
‘If you have a sample of soil,’ she asked them, ‘how will you recognise what’s in it?’ The scientists rattled off the usual answers: slide it under a microscope, put it in a Petri dish, make millions of DNA copies, catalogue the genes. But that party line disturbed Cleland. ‘You couldn’t detect anything that wasn’t almost identical to familiar Earth life,’ she said. Their methods assumed that all microbes have genetic material that works like ours. Isn’t it possible, Cleland wondered, that life arose more than once here? If so, organisms from a second (or third) genesis would never turn up in our tests, because our tests are only meant to turn up familiar life. ‘But these organisms, if they exist, would leave traces in the environment,’ Cleland says.
In 2007 in the journal Studies in History and Philosophy of Biological and Biomedical Sciences, Cleland wrote about just such a trace: desert varnish. It’s a strange sheen, like a hardened waterfall, that covers desert rocks all over the planet. The streaks run down rocks from the desert of El Azizia in Libya to Antartica’s Dry Valley. Desert varnish – into which people have scraped petroglyphs for thousands of years – appears layer by layer, growing only the width of a human hair each millennium. The varnish is replete with arsenic, iron and manganese, although the rocks it coats are not. No known geochemical or biological process can account for its ingredients. And yet there it is. Since that discovery, Cleland has urged scientists not to discount – but to seek out – such anomalies as the varnish, things that don’t quite seem to fit. Because maybe they don’t fit.
If you make amino acids and sugars in the lab, you get two mirror-image types, called ‘left-handed’ and ‘right-handed’. But inside living things, you’ll find only left-handed amino acids and right-handed sugars. Shadow life could be ambidextrous, or a mirror image of regular life: its molecules could be flipped as if Photoshopped. The astronomer Dimitar Sasselov and the geneticist George Church at the Harvard Origins of Life Initiative want to build such mirror-world organisms. One day, they hope, their manufactured microbes will fall ill. ‘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,’ Sasselov told The Guardian in 2013. And if viruses attack, we will know viruses exist. And if they exist, some of their prey must exist outside Harvard’s labs, having formed long before Harvard and long before labs.
Another mysterious finding, like the desert varnish, involves excess gas. If you catalogue all the carbon that living things burp out, and then you count the carbon you find in the atmosphere, the two don’t match: there’s 5 per cent more hanging over our heads than there ‘should’ be. Maybe that extra comes from shadow organisms whose excretions we aren’t counting. Maybe not. Maybe we have already found shadow organisms – some of the shy- and stubborn-seeming microbes that won’t reproduce in our labs, which scientists build as breeding grounds for dogma-obeying life. Maybe they really are just reticent. But we should wonder harder.
The discovery of life as we don’t know it would hint that biology is a universal law, like physics and chemistry. If you drop a trombone off a terrestrial building, it will fall to the ground – every time. If molecular clouds have just the right density, dynamics and ingredients, they will collapse into new stars – every time. If they collapse into new stars, the leftover gas and dust will always collapse into planets. Maybe when those planets have the right ingredients baked at the right temperature, they always make life. A shadow biosphere – evidence that biology emerged more than once on Earth – suggests that biology emerges as a normal consequence of Goldilocks or just‑right conditions, rather than being a mysterious lottery-ticket phenomenon.