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.