Animals cannot be considered individuals by anatomical or physiological criteria because a diversity of symbionts are both present and functional in completing metabolic pathways and serving other physiological functions.
These discoveries have profoundly challenged the generally accepted view of “individuals.” Symbiosis is becoming a core principle of contemporary biology, and it is replacing an essentialist conception of “individuality” with a conception congruent with the larger systems approach now pushing the life sciences in diverse directions. These findings lead us into directions that transcend the self/nonself, subject/object dichotomies that have characterized Western thought.
Animals are composites of many species living, developing, and evolving together. The discovery of symbiosis throughout the animal kingdom is fundamentally transforming the classical conception of an insular individuality into one in which interactive relationships among species blurs the boundaries of the organism and obscures the notion of essential identity.
What would biological science be if symbiosis were seen as the rule, not the exception? What scientific questions would become paramount and how might this change our view of life if intimate cooperation between species were a fundamental feature of evolution?
The entity we call a cow is an organism whose complex ecosystem of gut symbionts—a diverse community of cellulose-digesting bacteria, ciliated protists, and anaerobic fungi—informs its specialized anatomy, defines its plant-digesting physiology, regulates its behaviors, and ultimately determines its evolution.
What constitutes the individual organism? How can a worker termite be considered an individual when it is the hive that is the reproductive unit of the species, and the worker cannot even digest cellulose without its gut symbiont, Mixotricha paradoxa, which is itself a genetic composite of at least five other species? Neither humans, nor any other organism, can be regarded as individuals by anatomical criteria.
The coevolution of mammals and their gut bacteria has in effect resulted in the “outsourcing” of developmental signals from animal cells to microbial symbionts. Thus, the symbionts are integrated into the normal networks of animal development, interacting with the eukaryotic cells of their “host”. Development then becomes a matter of interspecies communication. We are not individuals from the viewpoint of developmental biology.
On classical physiological grounds, animals are not individuals.
We are not individuals by genetic criteria.
Organisms are anatomically, physiologically, developmentally, genetically, and immunologically multigenomic and multispecies complexes. Can it be that organisms are selected as multigenomic associations? Is the fittest in life’s struggle the multispecies group, and not an individual of a single species in that group?
We are not genetic or anatomical individuals; and if there is no “individual organism,” what remains of classic notions of “individual selection”?
If the immune system serves as the critical gendarmerie keeping the animal and microbial cells together, then to obey the immune system is to become a citizen of the holobiont. To escape immune control is to become a pathogen or a cancer.
Immunity does not merely guard the body against other hostile organisms in the environment; it also mediates the body’s participation in a community of “others” that contribute to its welfare.
There is no circumscribed, autonomous entity that is a priori designated “the self.” What counts as “self” is dynamic and context-dependent.
Animals can no longer be considered individuals in any sense of classical biology: anatomical, developmental, physiological, immunological, genetic, or evolutionary. Our bodies must be understood as holobionts whose anatomical, physiological, immunological, and developmental functions evolved in shared relationships of different species. Thus, the holobiont, with its integrated community of species, becomes a unit of natural selection whose evolutionary mechanisms suggest complexity hitherto largely unexplored. As Lewis Thomas commented when considering self and symbiosis: “This is, when you think about it, really amazing. The whole dear notion of one’s own Self—marvelous, old free-willed, free-enterprising, autonomous, independent, isolated island of a Self—is a myth.”
For animals, as well as plants, there have never been individuals. This new paradigm for biology asks new questions and seeks new relationships among the different living entities on Earth. We are all lichens.