There are various scientists which have contributed to biological classification and based on their observations we have the current system of classification:
Aristotle (384-322 BC): Also known as the father of classification, he classified animals on the basis of two points: presence or absence of blood, wether they live on land, air or water.
Carolus Linneaus (1707-1778): Also known as the father of taxonomy, he classified living things into two kingdoms- Plantae and Animalia.
Ernst Haeckel (1834-1919): He proposed the three kingdom classification- Plantae, Animalia and Protista.
Robert Whitakker (1920-1980): He gave the five kingdom classification- Monera, Protista, Fungi, Plantae and Animalia.
Carl Woose (1928-2012): He gave the classification where he classified all the organisms into three categories- Eukarya domain, Bacteria domain and Archaea domain.