How to define what we recognise as 'life' must be one of the most intriguing questions of science - alien or otherwise. It is tantalising to ponder if celestial bodies might harbour biochemical phenomena recognisable as alien life - proto-organic spores in comets, primitive microbes in the Martian soil, or good old-fashioned spacecraft-eating monsters. But the question of how to recognise a life-form also applies closer to home - even to Earth, the planet, itself.
According to James Lovelock's Gaia hypothesis (or theory), the biological, chemical and physical components of planet Earth are part of a single system, which we could identify as a single life-form or super-organism. For example, planet Earth can be seen to be self-regulating, in the sense that it tends to maintain its temperature in the face of external influences. As John Gribbin notes in his book Deep Simplicity, life on Earth has not just boiled away as the sun has got hotter, but the planet's ecosystem has adapted to maintain what we would regard as a habitable temperature. In this sense, planet Earth could be viewed as being analogous to a 'warm-blooded' organism.
Perhaps planet Earth really is a living 'organism', at least if we allow an expanded, generalised concept of 'life' that could accommodate any kind of alien life-form that might conceivably inhabit universe. That is, if we imagine coming across some alien entity somewhere in outer space, we would attempt some sort of objective test to see if it was in any sense alive. For example, does it show signs of self-regulating chemical activity, exhibit local entropy reduction, or eat spacecraft with monstrous relish?
Our own planet, viewed from afar by some alien intelligence, could also qualify using the same credentials of life. The existence of the Gaia 'organism' could be apparent to an alien scientist that had its own 'test for life' applied to distinguish living from non-living entities.
The Gaia hypothesis has sometimes been (mis)interpreted as having mystical, even quasi-religious overtones, as if the self-regulating, apparently organism-like status makes the planet a kind of Mother Earth or even Earth-Goddess. However, this should not obscure the serious scientific issues that the concept of Gaia addresses.
The idea that planet Earth is self-regulating has sometimes been interpreted as if the planet were somehow self-consciously looking after itself. In this case, Gaia might grow alarmed at humanity's harmful tampering with the eco-system, and might consider redirecting - or punishing - its wayward offspring for the good of the planet as a whole.
But rather than Earth's children, perhaps we are more like Gaia's bacteria, who may be beneficial, harmful or neutral with respect to our host. In this case, Gaia might be no more aware of us than we are of our own corporeal bacteria: not really knowing or caring about us, unless we cause trouble. (Conversely, if our host really were somehow conscious, then we should not expect to be any more aware of this than a bacterium inside us would be aware of our own human consciousness.)
The idea of Gaia as Mother Earth seems rather like trying to give an inanimate object human qualities - an anthropomorphic projection on an astronomical object, like wishfully interpreting the image of a 'man' in the moon (or deities in the constellations).
But the original suggestion that planet Earth could be a life-form - of its own kind - could actually challenge the anthropocentric assumption that sees creatures 'like us' as the primary models for defining life, and sees creatures that are less like us (algae, viruses, and so on) as less worthy of the status of life. It could be that this anthropocentric conceit hinders us from recognising life in the more universal sense. So recognition of Gaia as a living thing could potentially be more outward looking and scientifically revealing than any sentimental notion that the lump of rock we call home is somehow consciously caring for us.
So, the idea of our Earthly Gaia might open our eyes to the potential diversity of life-forms elsewhere in the universe. We could imagine alien beings not only as microbes or monsters, but in the form of 'Gaian aliens' - planetoid beings with cool self-breathing atmospheres and funky biotic body-fauna. Instead of just looking for living beings on other planets, we could look for other planetary beings. Perhaps an intelligent alien planet is already gazing at Gaia from afar, wondering whether she is worth a closer look.
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