Svante Arrhenius theorized that bacterial spores propelled through space by light pressure were the seeds of life on Earth.
British astronomers Fred Hoyle and Chandra Wickramasinghe rekindled interest in panspermia. They also proposed that comets, which are largely made of water-ice, carry bacterial life across galaxies and protect it from radiation damage along the way.
Not necessarily a view I subscribe to, but an interesting one nevertheless.

On reading an article by Wickramasinghe and Hoyle1 it seems that their comet theory extends, and intends to explain, the origins of life on earth. Not satisfied with the traditional 'warm pond' explanation (a straw man, really, since more pieces have been added to this theory, such as Hydrothermal Vents) they propose that comets 'seeded' the earth with life]. Additionally, they reject the possibility of Life surviving catastrophic events - such as meteor impacts! This seems a bit circular, but also strangely neat and tidy; Life is wiped out and reseeded in one stroke.

Unfortunately, they fall into the classic trap of using simple statistics to 'prove' their argument: basically a slightly updated form of Levinthal's Paradox. This does not detract from the power of an argument based on cosmological observation that there are interstellar bacteria - that is, bacteria (of normal size and shape) that live in or on interstellar dust. Indeed, there are bacteria that can resist radiation (Deinococcus radiodurans for example). Also, bacteria have been known to survive a short trip on satellites and other (earthly) space vehicles.

However, this does not explain how these 'interstellar bacteria' came into being. Although it might seem to solve the bootstrap problem of life it ceratinly doesn't - it merely displaces it.

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