The Drake Equation: The fl Term fl: The fraction of these planets on which life arises. This is where, perhaps, we can be rather more optimistic. It has recently become apparent that life arose here on Earth, some 3.8 billion years ago, exceedingly quickly after conditions became suitable. It does seem that, given the right conditions, life will emerge by some, as yet unexplained, process. If the process were difficult, one would expect life to have arisen much later in the Earth's history. We know that the building blocks of life - amino acids and complex organic compounds - are widely distributed in space and are found in meteorites, comets and the gas and dust of the interstellar medium. This suggests that most planets will contain the building blocks of life and it thus seems reasonable to believe that given a suitable location life will, sooner or later, arise. If so, we can give this factor the value one. Life from other planets? Should life arise once in a solar system, then it might even be carried to other planets of that system in rocks blasted off the surface by the impact of an asteroid. Rocks from Mars which reached the Earth in this way are being investigated to see if they show any evidence of life once having existed within them. As Mars is half the size of the Earth it would have cooled more quickly and thus may have had conditions allowing the emergence of life far sooner than here on Earth. It could just be that life first originated on Mars and then was brought here in a meteorite!