Antonie van Leeuwenhoek
The first discoveries of viroids in the 1970s triggered the historically third major extension of the biosphere—to include smaller lifelike entities —after the discoveries, in 1675 by Antonie van Leeuwenhoek (of the "subvisible" microorganisms) and in 1892/1898 by Dmitri Iosifovich Ivanovsky and Martinus Beijerinck (of the "submicroscopic" viruses).