Signs of life

Suppose we travel to a planet in a distant part of the universe. There we encounter an object that might be deemed "living". Let us call it Z. How can we decide whether Z is living? What might have suggested to us that it is an organism?

First, how big or small might Z be? No terrestrial organism has linear dimensions less than about one micrometre. This lower limit is set by the minimal equipment needed to maintain a living state (chapter 2). Organisms with the same chemistry as ours presumably cannot be much smaller, no matter where in the universe they live. Setting an upper size limit is more difficult. Mechanical restrictions give us some guidance - for example, animals with chitinous exoskeletons, such as insects, cannot grow beyond a certain size - but this is not sufficient. Apart from dramatic organisms such as Wellingtonia or Gigantosaurus, the Earth boasts colossal fungi, with hyphae spreading over almost a square kilometre of soil. The maximum possible sizes of organisms on other planets are beyond conjecture. However, alien organisms such as Z would presumably have cellular structures. We discussed earlier why the range of terrestrial cell sizes is so narrow (chapter 3 and following); we can extrapolate this argument, at least provisionally, to other worlds. No matter how big Z might be, we could (in principle) investigate whether it comprised cells with complex internal organisation. In any case, the simplest organisms (corresponding to prokaryotes on Earth) are highly likely to predominate on any life-bearing planet, so Z is statistically likely to be microscopic and to comprise a single cell.

This reasoning is tenuous and might not apply to life with a different chemistry. For instance, what about the possibility of organisms based on silicon or perhaps phosphorus instead of carbon? Compounds of these elements produce interesting polymers at high temperatures and pressures. Such possibilities allow no way of predicting minimum organism size. We cannot even be sure whether such hypothetical organisms would have cellular structures. Nevertheless, Gold has suggested that there might be silicon-based life in subterranean parts of the Earth, the "deep hot biosphere". If he were right, could we recognise them as living? For example, life as we know it cannot exist without liquid water, but would water be a prerequisite for silicon-based life?

Can we apply our criteria of "livingness" to objects such as Z on the hypothetical alien world? In principle we can, but there might be practical difficulties. For instance, organised structural complexity is an inevitable feature of life. However, in order to identify "structural complexity", we have to know what sort of structure and what sort of complexity to look for in Z. This entails judgement. How complex is complex? What objective criteria of "organisation" can we apply? Another aspect of "livingness" is metabolism - the exchange of energy and materials between organism and environment, on which the integrity of structure depends. If Z exchanges energy and material with its surroundings we might be expected to recognise the fact; nevertheless, the inputs and outputs might happen too quickly or too slowly for us to interpret them correctly. Such exchanges might be mistaken for inanimate processes. Would a mineral that (say) absorbed ultraviolet radiation and emitted heat, maintaining a highly elaborate crystal structure in the process, be regarded as an organism that ate ultraviolet and excreted heat, or as a lump of rock with a complicated chemistry? The other hallmark of internal state - internal transport - would never be observed in practice unless we had already decided that Z was an organism. Control of gene expression would be definable only if we knew what constituted "genes" in Z and what we meant by "expression". Responses to stimuli might be noticeable - but once again, only if the time-scale was neither too slow nor too fast. And what if the stimulus was elusive, such as a narrow wave-band in the far infrared - would we ever think of looking there?

Perhaps this is too pessimistic a view. If there were any chance that Z was alive, then surely we would examine it exhaustively against each of our criteria, making proper allowance for all the aforementioned difficulties. Irrespective of complications, a proper examination should suffice to reveal any reciprocal dependences among the aspects of internal state (structure, metabolism and transport) and among internal state, gene expression and responses to stimuli. According to our characterisation, the essence of "livingness" lies in these reciprocal dependences.

Finally, let us remember that no organism is an island; that all organisms belong to ecosystems. Z would not exist in isolation if it were alive. It would be one of many objects on the planet that might be deemed "alive", some similar Z itself, others perhaps different in outward appearance. All these objects would, if we studied them for sufficiently long and in the right ways, show a measure of mutual dependence. Overall, it seems that we could rely on our criteria of "livingness" to make a reasonable decision about the status of Z.

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