The biological spectrum consists of all living organisms divided into three domains, coexisting across various levels of biological organization. Microbiologist Carl Woese organized all known organisms into a phylogenetic tree of life based on RNA and common ancestor comparisons. The three domains are bacteria, archaea and eukaryota, which are further subdivided into kingdoms. All three domains exist on various levels of biological organization, from a cellular level to its biosphere.
The single cell is life's fundamental unit of structure and function. Some organisms are single-cell, in which one cell performs all the functions of life. Other organisms are multicellular and have a division of labor among specialized cells. Multicellular organisms have a biological complexity that adheres to a hierarchy of life. Groups of cells working together to perform a specialized function are tissues. Two or more tissue types working together to perform a function are an organ. Organs cooperating together for a larger function are an organ system.
A multicellular organism is an individual form of life, composed of multiple organ systems. Both multicellular and single-cell organisms populate communities, and this array of life inhabits a particular ecosystem. An ecosystem consists of all living things in a particular area, along with all nonliving components of the environment with which life interacts. All of life on Earth, and the places in which this life exists, is the biosphere.