thousand million chemists in a few thousand million laboratories for a few million years and, so it is argued, life will be duplicated in a test tube.

Here, however, the appeal to large numbers lacks credibility because no effective model or theoretical framework is provided to demonstrate the difference between insurmountable problems and practical problems. Critics have pointed out that there are no satisfactory models of the origin of life. It is no longer believed that life occurred out of a jumble of chemicals in a soup plus energy, and it is known that the characteristics of life are found in the logical structure and organization of the complex  molecules. But to date we have no idea as to how this information kicks into the chemical components. How did the genetic code arise, and what is the mechanism for translating this code into protein synthesis? There are huge explanatory gaps here. Scientists have not explained how self-replicating molecules came into being. There is no certainty regarding the origin of nucleic acids, or of the process from nucleic acids to protein; no certainty regarding the origin of the cell; there are no adequate explanations of how large complex molecules, like DNA, evolved from smaller molecules. There may be ‘missing link’ molecules, precursors of DNA, which could have taken over the function of self-replication. Many candidates have been proposed (Shapere, 1991: 113) but none so far have achieved any form of consensus support. Concluding an inquiry into the origins of life on Earth, Jakosky says:

It is unlikely that we will ever fully understand the origin of  life on Earth. The transition from nonliving, prebiological chemistry to the existence of entities fully capable of reproducing or passing on genetic information, of changing through a ‘survival-of-the-fittest’ Darwinian evolution, and of metabolizing energy and using it to perform these functions, almost certainly was not a sharp transition. Rather, it must have involved a long series of steps, with only a few having been demonstrated experimentally.

(1998: 109)

There are, however, three certainties: [1] life did get started; [2] it could not get started here now; and [3] we have no satisfactory models as to how it got started. The third explanation – that life came to Earth from external sources – has several versions. One view is that diffusion of life occurred through colonization, or that living forms developed out of materials left behind by ET explorers several billions of years ago. The latter view – sometimes referred to as the ‘garbage theory of life’ – is unflattering, as it suggests that the evolution of complex forms of life originated in waste products similar to those left by explorers in the Antarctic or the Himalayas. There is no evidence to either support or refute the garbage theory.

Another version of the theory that life originated external to Earth is the panspermia or ‘seeding’ hypothesis. The term ‘panspermia’, which means ‘seeds

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everywhere’ was coined by the nineteenth-century Swedish physicist, Svante August Arrhenius, who held that life was seeded by micro-organisms from space. The English biologist, John Haldane, advanced a similar thesis in the early twentieth century. Philosophically the panspermia hypothesis might appear to be a weak explanation, as it only moves the question back: where did life originate from in the first place? The most obvious answer is ‘somewhere where the conditions required for the emergence of life are more favourable than those on Earth’. Given the difficulty of demonstrating how life emerged on Earth this might not be such a ridiculous suggestion.

One version of the panspermia hypothesis maintains that suitable spores, chemicals, and bacteria brought life from passing comets. Fred Hoyle (Hoyle and Crick, 1981) argued that comets have been carriers of biomolecules, virus and complex cell-type bodies. The main scientific objection to Hoyle was that organic matter would not survive molecular decomposition in the radiation environment of space. However, it could be replied that organic molecules could survive this, as well as the hazards of entry into the Earth’s atmosphere, if they were embedded within the ice of a large comet. Although no fossils had been found in cometary or meteoritic matter, organic molecules and amino acids had been discovered in meteorites, giving rise to speculations that life arose as a consequence of meteor bombardment. Thus Fred Hoyle and Chandra Wickramasinghe (1977: 402) argued that life on Earth originated in microbe invasion from outer space. Life on Earth, they held, began as the result of a cometary impact, and that continuous bombardment contributes to changes in living forms. In support of their hypothesis they appealed to the great epidemics throughout history. Evidence of ET life, they concluded, should be sought among the comets.

This view proved to be too outlandish for the scientific community at the time and as a result the panspermia hypothesis was discredited. A useful speculative approach could be based on an appeal to the random distribution and collision of organic molecules. Hoyle and Wickramasinghe located the origins of life in the early stages of stellar formation and argued that ‘the interstellar cloud of gas and dust in which our Solar System formed continues to add biomolecules long after the early high temperature phase of the solar nebula and of the planetary material has been completed’ (1978: 13). They maintained that the  interstellar cloud contains the building blocks of life and consequently life on Earth was derived from these building blocks which had been prefabricated in various parts of the galaxy. Such a theory is suggestive that life could be evolving in many locations throughout the universe and Hoyle and Wickramasinghe claimed that it is virtually certain that ‘similar experiments in biological assembly occurred on innumerable occasions in many other places in the Universe’ (ibid.: 20).

Hoyle also argued that fundamental building blocks for life, such as the amino acids, were unlikely to form naturally in the primordial ‘soup’ on Earth. In 1994 astronomers found evidence of the amino acid, glycine, in an interstellar cloud called Sagittarius B2, thus supporting Hoyle and Wickramasinghe’s theory that

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