The basic idea of panspermia is not that hard to imagine. Panspermia proposes that life can be distributed through the galaxy by interstellar dust, comets, debris from planetary impacts, among other methods. But the extension of that basic idea can lead to the idea that life on earth, or some life on earth, came from space: mushrooms or bacteriophages or human beings. Most folks don’t think that we cover our pizzas with tasty alien plants. But is the notion of panspermia plausible? Here’s more from Centauri Dreams:
Panspermia is an ancient concept. Indeed, the word first emerges in the work of Anaxagoras (born ca. 500–480 BC) and makes its way through Lucian of Samosata (born around 125 AD), through Kepler’s Somnium, to re-emerge in 19th Century microbiology. Accidental propagation of life’s building blocks was considered by Swedish chemist Svante Arrhenius in the early 20th Century. Fred Hoyle and Nalin Chandra Wickramasinghe developed the idea still further in the 1970s and 80s.
So how do we approach a subject that has remained controversial, likely because it does not appear necessary in explaining how life emerged on our own Earth? As the paper notes, modern work falls into three distinct categories, the first involving whether or not microorganisms can survive ejection from a planetary surface and re-entry onto another. Remarkably, hypervelocity impacts are not show-stoppers for the idea, suggesting that a small fraction of spores could survive impact and transit.