A lichen, for example, is a symbiotic association of algae and fungus, allowing both organisms to colonize habitats in which they could not survive without their symbiotic partners.
Symbiotic relationships pervade all levels of biological systems.
Most animals, including humans, harbour symbiotic microorganisms which aid the digestion of food, while many plants provide shelter to symbiotic bacteria and fungi which enhance their ability to extract nutrients from the environment. Multicellular organisms such as plants and animals, and many single-celled organisms, such as protozoa, have cells which are described as eukaryotic and are distinguishable from prokaryotic cells such as bacteria because they possess organelles . These are bodies, surrounded by membrane, which are found within the eukaryotic cell and which are generally specialized in function. The observation that these organelles can divide independently of the cell, and that they appear to possess their own nucleic acid, has led to the suggestion that eukaryotic cells may have originated in a symbiotic relationship of several prokaryotic cells. This idea remains an unproven hypothesis, but there exists substantial evidence that many living organisms are based upon a primeval symbiosis.