Hierarchical classification of all life on Earth. It is one of the most persistent conceptions about the nature of life and the universe in Western history. Originally presented by Roman philosopher Plotinus in the third century B.C.E., the theory states that all life indeed, everything in the known universe is connected in a single hierarchical structure, like a ladder or a chain. The great chain begins with the smallest single- celled amoeba and continues through increasingly complex animals, including sponges, worms, invertebrates, insects, fishes, amphibians, reptiles, birds, and mammals. Humankind represents the culmination of the great chain, the link between the material and the spiritual world.
Some historians believe that the great chain-of-being theory is a precursor to evolutionary theory. In its earliest incarnation, however, the chain of being was strictly hierarchical, static, and antievolutionary, assigning every creature a particular place in a world created and ordained by God. This chain of being represented the eternal order of things. It attracted conservative thinkers because it described a complex world that was in accord with the Bible. It also appealed to the human ego by placing humankind at the top of the hierarchy, giving humans pride of place at the top of the chain. Other thinkers expanded the chain to include biblical and spiritual creatures such as angels.