Aristotle introduced the first hierarchical classification system. He divided the life forms he saw into eidos and genos, establishing the basis for taxonomy. He classified 520 animal species, which were primarily from Greece.
From the time of Aristotle until the Enlightenment, biologists followed Aristotle's taxonomic system of classifying plants and animals. As they were working prior to the invention of the microscope, they were unaware of microorganisms, which made their task of classifying all known life forms easier.
While many different early scientists worked on the classification system, Carl Linnaeus receives credit for developing the system of taxonomy that continues in use in 2014. His system ties directly to that developed by Aristotle, but it recognizes three major divisions of life that Linnaeus called kingdoms. Each kingdom breaks down into smaller groupings, but Linnaeus bases his taxonomic system of species and provides the binomial system for naming animals. Through this system, humans have the two names Homo sapiens for their species name, and their companions are Canis familiaris.
Fifty years after Linnaeus developed the classification system, Darwin published his major work. Through Darwinian evolution, it is possible to see constantly changing species. Darwin uses natural selection to describe the changes within kingdoms that took place in developing life that fits into these classification systems.