The Drake Equation: The fi Term fi: The fraction of these planets on which "intelligent life" evolves. This is probably the most argued-about factor in Drake's equation! Optimists felt that evolution would naturally drive life towards intelligence, but this may not necessarily be true - a non-intelligent simple life form may be very well adapted to its environment and may not evolve significantly. It is also true that, although simple forms of life on Earth arose quickly, it took billions of years before complex life forms appeared and longer still before intelligent life arose. Thus even if evolution will eventually bring about intelligent life, it could well require a stable environment for a significant fraction of the lifetime of a suitable star. Planets, like our Earth, which have maintained a temperate climate for nearly 4 billion years may be a requisite for the evolution of intelligent life and may only occur rarely, given a happy occurrence of unlikely circumstances. We should not be influenced too much by the principle of mediocrity - that is, the common belief that there is nothing special about us and our planet Earth. It was a reaction against the placing of human beings and the Earth at the centre of the Universe when it was realised that ours was just one planet around a fairly typical star (actually rather better than average). If we could be here, why could not other civilisations exist elsewhere amongst the vast number of stars in the Galaxy? The fact that intelligent life has arisen once in the Galaxy does not, of necessity, mean that it must be common, and the fact that there are 100,000 million stars in the Galaxy does not mean that simply because that number is so large there must be other advanced civilisations elsewhere. Have we been lucky? It appears that a number of factors have helped to maintain a suitable and stable climate for the evolution of life here on earth: The Moon, which is relatively large compared to the Earth, has stabilised the Earth's rotation axis preventing it from swinging rapidly in its orientation and causing rapid climate changes across the Earth. Plate tectonics, the motions of the Earth's crust, have helped to mix the upper layers of the crust and recycle carbon dioxide so keeping up the mean temperature of the Earth's surface. The Sun is in a relatively sparse stellar neighbourhood on the outskirts of a spiral arm. As a result, the likelihood of close encounters with other stars is low. Such encounters would perturb the Oort cloud of comets and greatly increase the rate at which comets come into the inner solar system thus increasing the chance of an impact with the Earth. The massive gravitational field of the planet Jupiter has had the effect of shielding the inner planets from cometary impacts. As a result of (3) and (4), we have suffered fewer impacts than otherwise. Impacts cause severe changes in climate and may destroy an emerging intelligent species. However, it seems likely that some cometary impacts are a good thing as they encourage biodiversity and, for example, without the demise of the dinosaurs some 65 million years ago, we might not be here now! We do not really know how important each of these factors has been, but together they have enabled one branch of life here on Earth to evolve into an advanced technological civilisation. It may well be that the coincidence of these and other, as yet unknown, effects will occur only rarely. We do not know. It is thus virtually impossible to make an accurate estimate for this factor and it could be the one, along with an estimate of L, that really determines how likely we are to come into contact with another civilisation.