Andrew Watson: Forget about aliens: we're all alone in the universe

From a lecture given by the Professor of Environmental Sciences at the University of East Anglia to the Geological Society, London

Around other stars there are, presumably, many potential Earths – rocky planets at the right distance from their sun, with a supply of liquid water at the surface. How many of them actually harbour life, and what are the chances of complex, sentient species analogous to ourselves developing on other planets?

Around other stars there are, presumably, many potential Earths – rocky planets at the right distance from their sun, with a supply of liquid water at the surface. How many of them actually harbour life, and what are the chances of complex, sentient species analogous to ourselves developing on other planets?

We can make some general deductions about the probability of life arising on other planets. For example, we know that on Earth, bacterial life was established very early indeed – shortly after the planet became habitable or perhaps simultaneously with it. This suggests either that bacteria evolve relatively easily from inorganic precursors, or that they evolved elsewhere and were "seeded" on to the planet near the time of its formation. In either case, bacterial biospheres might be expected to be relatively common, at least in our neighbourhood of the universe.

However, structurally complex organisms took a long time to arise – about 3 billion years after the first bacteria are found. This is a good fraction of the total period that life will have on the Earth – since present theory suggests that life will last only about 1 billion years more on Earth before the increasing output of the sun causes a "runaway greenhouse". This arrival of complex life very late in the day is consistent with the fact that complex life is very rare and that most planets never reach that stage of evolution. Sentient life is rarer still, but on the very rare occasions when sentient life does arise, it will almost always find itself "awakening" towards the end of the tenure of life on that particular planet.

Like that of all similar stars, the sun's output has not been constant but has increased substantially over time. If nothing had compensated for this change, life on Earth could not have survived for billions of years and complex life would not have arisen, because the temperature would have risen too high. This has been prevented only because the natural greenhouse effect of the Earth's atmosphere has adjusted to compensate for the increasingly luminous sun, and this seems partly due to a "Gaia tendency" – life, and non-living processes, have tended to regulate the planetary environment. This tendency may be quite strong, but it is not fail-safe: there is clear evidence that it sometimes breaks down. It seems, therefore, that a considerable amount of good luck is also necessary to explain why life on Earth has survived so long.

My conclusions are that, given present knowledge, we can make a number of "defensible speculations". The first is that bacterial life is relatively common. A corollary is that Mars, which once had water at its surface, may well once have possessed a primitive biosphere – but just because the planet could sustain bacteria does not mean it could sustain aliens. We owe our existence to the "Gaian" tendency of the Earth system to regulate the planetary environment and thus extend the biosphere's lifetime. However, it may be just luck, and another example of observer self-selection, that this tendency has worked so well for so long.

Out beyond our own special planet, complex life is rare, and sentient life (aliens) rarer still. That a large number of planets probably exist does not make it reasonable to assume that sentient life is inevitable on at least some planets if the chances of it arising are infinitesimally low. Our evolution at a late stage of our planet's history is consistent with beings like us being so rare that we are very unlikely to contact any others. Whether we like it or not, therefore, we are probably, in effect, alone in the universe, and this planet the only place we will ever know where the universe has come into self-awareness.

I think we dream of contacting aliens because we are scared of being alone. We seek companionship, and it is comforting to believe that civilisations older and wiser than ours will one day show us knowledge and wisdom. Growing up is hard, but perhaps it's time we grew up and accepted that we really are alone.