Panspermia: Myth or Reality?

Panspermia, the hypothesis that life reached Earth from outer space, was developed by Svente Arrhenius between 1903 and 1906. This is the "strong" version of panspermia, and it sidesteps the question of how did life originate. There were several precursors who had suggested that life existed elsewhere in the Universe (Anaxagoras, Giordano Bruno, Fontanelle). There were also precursors such as Berzelius who, in 1834, suggested meteorites as carriers of life. But a scientific approach to these questions had to await the demise of the long-held concept of spontaneous generation and the development of Darwin's theory of evolution. As understanding of astronomical and astrophysical observations progressed, the concept of life elsewhere than on Earth had fluctuating fortunes, as did estimations of the probability of survival of space transit and atmospheric entry. Thus a more chemical/biochemical approach to the origin of life on Earth developed, one variant of which, involving the infall of prebiotic chemicals, constitutes a "weak" panspermia hypothesis. The scientific and, in part, philosophical background to these developments will be discussed and will include remarks on directed panspermia.