| 1753 | Carl Linnaeus | Binomial nomenclature; two-kingdom system (Plantae + Animalia) | Origin of systematic classification |
| 1866 | Ernst Haeckel | Proposed three kingdoms — added Protista as third kingdom | First recognition of unicellular eukaryotes as separate |
| 1892 | D.J. Ivanowsky | Demonstrated that tobacco mosaic disease is caused by an agent smaller than bacteria (filterable) | Discovery of the first virus (TMV) |
| 1898 | Martinus Beijerinck | Coined the term "virus" (Latin for poison); described TMV as a "contagium vivum fluidum" | Named the virus concept |
| 1937 | Herbert Copeland | Proposed four-kingdom system — added Monera for prokaryotes | Separated prokaryotes from eukaryotes |
| 1959 | Robert Whittaker | Published evidence supporting five-kingdom concept | Precursor to the 1969 formal proposal |
| 1969 | R.H. Whittaker | Formally proposed Five Kingdom Classification in Science journal | THE classification system for NEET |
| 1971 | T.O. Diener | Discovered viroids — naked RNA infectious agents causing plant diseases | "Viroids" — NEET examiner's favourite |
| 1977 | Carl Woese & George Fox | Proposed Archaebacteria as a distinct domain based on 16S rRNA sequencing | Basis for distinguishing Archaea from Eubacteria |
| 1982 | Stanley Prusiner | Proposed the prion hypothesis — infectious proteins with no nucleic acid | Nobel Prize 1997; prions in NEET context |
| 1990 | Carl Woese | Proposed Three Domain System (Bacteria, Archaea, Eukarya) | Beyond NEET syllabus but useful context |