The idea that life can migrate naturally through space (Panspermia) is one of long standing. But many scientists simply refuse to even consider it. It is instructive therefore to ask why this is. Instead, many prefer to either believe that life originated solely here on Earth, or that the spontaneous origin of life is relatively easy so that it may start anywhere. Here the philosophical basis for believing (or not) in Panspermia is discussed vs. the counter arguments about the ease (or otherwise) of spontaneous generation of life. Traditionally there was only one relevant observation, namely life exists on Earth. Now however there are a variety of experimental results and observations relevant to the possibility that life may survive a transfer through space, but of course there is no example of it happening. It may simply be that some scientists are more comfortable with the one concrete observation (life on Earth - even though its origin is not understood), rather than with considering theoretical possibilities such as Panspermia. Nevertheless, given our lack of understanding of the origin of life it can reasonably be argued that this is a preference rather than anything more fundamental.

Key Words: Origin, life, Panspermia, abiogenesis, probability.

1. INTRODUCTION.

The idea that life can naturally migrate through space (Panspermia), annoys many people. The idea itself is an old one. In several European nations during the 19th century, leading scientists suggested that life may not have originated on the Earth. The logic they used had several key strands. It had long been "known" that life wasn’t spontaneously generated (e.g. the experiments of Spallanzani and Pasteur). Evolution was increasingly accepted, but the age of the Earth was still subject to debate, and some believed it was not old enough to have permitted the degree of evolution required to go from first generation of life to the profusion of life today. Others simply accepted the Copernican view that the Earth was neither unique, nor central to the Universe: Therefore to presuppose that life started here was just wrong.

By the early 20th century these ideas found an advocate in Svante Arrhenius, who discussed the idea that life could migrate through space and proposed that seeds came to the Earth from space (Arrhenius, 1908, or see Arrhenius, 2009). Throughout the 20th century the debate concerning Panspermia waxed and waned. The problems presented by the deleterious effects of radiation in space on living organisms were more fully understood. This was bad for Panspermia. But Martian meteorites were identified on Earth (Bogard and Johnson, 1983) and modelling showed that materials could move from planet to planet (e.g. Gladman et al., 1996 and Gladman, 1997).

Melosh (1988) made the conceptual leap and argued that this was a means for what he termed "rocky" or "litho-" Panspermia, i.e. life may move from planet to planet. To support this, Mileikowsky et al. (2000a;b) considered the various stresses any microorganisms might experience during such a transfer from Mars to Earth and concluded that this was not necessarily a sterilising event.

Various groups have also now undertaken extensive experimental work with relevance to Panspermia. Horneck et al. (2001a) (and similar work since) have shown that microbes can survive (short) expose in space, in real experiments in space! Survival under other stresses such as impact shock in the GPa range (Horneck et al., 2001b, Burchell et al., 2004, Stöffler et al., 2007) and in high speed impacts (> 1 km s^-1) (Burchell et al., 2001, 2004) has also been demonstrated in the laboratory. That microbes in targets can be cultured from ejecta after high speed impact events has also been demonstrated (Burchell et al., 2003 and Fajardo-Cavazos et al., 2009).

Not all experiments are positive. Whilst rocks with microbial content have survived launch and recovery on the exterior of sounding rockets (Fajardo-Cavazos et al., 2005), experiments with sandstones placed in the heat shield of capsules returned from orbit suggest more significant heat processing which may be more hazardous to life (Parnell et al., 2008).

Nevertheless, slowly, step by step, results and measurements are being obtained which show that passage of life through space by natural means is not necessarily implausible. It is therefore no surprise that the idea of Panspermia keeps cropping up (see Burchell, 2004; Davies, 1988; Parsons, 1996).

By contrast, what can we say about the origin of life from non-life (abiogenesis)? This is what is required for a local origin of life. We have never observed it. Further we do not even understand what life is, as is illustrated by the difficulty of producing a definition of life (see Cleland and Chyba, 2002 for a discussion of this philosophical point). Those who favour abiogenesis argue that it obviously happened and is not as difficult as some make out (e.g. see Menor-Salván, 2009). Or they point out that even if life originated someplace else and came to the Earth, abiogenesis must have occurred somewhere (e.g. Sidharth, 2009), in which case why not here? By considering the molecular structure of life, some deductions can be made, for example the jump from chemistry to life may not have occurred all at once, but via an intermediate "RNA-world" (Orgel, 2004). But then how did the RNA-world evolve? And which steps led to life as we know it? There is much we do not understand.

The arguments in favour of abiogeneis are not yet conclusive and certainly not at the level that would permit an estimate of its degree of difficulty. Hoyle and his colleague Wickramasinghe argue that the degree of complexity involved in even the most basic living organism is so great, that the chance of random combinations producing it in random circumstances is vanishingly small (Hoyle and Wickramasinghe, 1999a). Therefore having it occur once is hard enough. This led them to consider Panspermia as a more likely source of the origin of life on Earth (Hoyle and Wickramasinghe, 1999b, also see papers in Hoyle and Wickramasinghe, 1999c).

However, few are convinced by the arguments suggested by Hoyle and Wickramasinghe. Attaching a mathematical significance to an event (abiogensis) about which we know so little is difficult. Some argue that chemistry constrains the form that complex molecules can take (e.g. Menor-Salván, 2009), so the resulting complexity, if it occurs, is already programmed to follow certain paths. Indeed, as noted by many (see Sidharth, 2009 for a recent discussion) chemistry is universal and thus complex organic molecules are widespread in space. However, this still ignores the assembly of such molecules into life forms (see discussion above), and it is this step that involves an enormous change in the degree of complexity.

2. LOGIC LEADING TO CHOOSING PANSPERMIA OR LOCAL ABIOGENESIS AS THE ORIGIN OF LIFE ON EARTH.

It would seem sensible to try to understand why some view Panspermia in a positive, or at least permissive, light whilst many others simply reject it. What seems to be occurring is an almost sub-conscious choice. Few argue from statistical considerations such as Hoyle and Wickramasinghe have done, and indeed some maintain that you cannot apply such an analysis to abiogenesis until we understand it. Instead many form their opinion based on an intuitive grasp of the issues. Sometimes they pick holes in individual arguments for and against the issues, but the motivation for doing so often seems determined by their prior belief. The author has never seen anyone in this debate change their mind after a discussion.

We can consider two factors which are key to the issue: The ease of Panspermia occurring and the ease of abiogenesis. Each of these can be considered on a scale ranging from "so easy it happens all the time" to "impossible". The two scales can then be plotted orthogonally to each other to form a scatter plot – this is shown in Fig. 1. Depending what value you assign to each of the two factors, you will inhabit a different region of Fig. 1.

Figure 1. Scatter plot of degree of difficulty for Panspermia vs. degree of difficulty for abiogenesis.

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