In August 1881, the journal Science (a short-lived predecessor of the modern journal) published an article based on letters exchanged between two amateur geologists – British Charles R. Darwin and the German Otto Hahn – discussing the possibility of extraterrestrial life. Just some years earlier, Darwin had published On The Origin of Species, arguing that complex life forms evolved very slowly over time from simple ones.
However, Darwin faced a major problem with his theory. At the time, based on erroneous calculations of the cooling rate of earth by physicist Lord Kelvin, the Earth was believed to be just some million years old.
Accordingly, the planet seemed too young to explain the modern complexity and diversity of life. However, if already complex microorganisms existed in space (the existence of which would predate the formation of Earth), and only later they evolved in terrestrial animals, could solve this apparent contradiction.