Taxonomy and common classification alike have tended to the identification of organisms as perceived entities, as singular things defined by clear boundaries. Until the development of the microscope a presumed individual organism might be highly structured, and in fact individual, or it might actually be an association of microscopic and more or less independent cells. The science has become much more sophisticated and discriminating with the greater resolution of the microscope and the development of techniques for analyzing genetic relationships. But except in obvious cases where an organism is single-celled and free-living, or found to be a simple colony of homologous cells, it is still the composite entity at the level of perception that is usually identified and described, not the individual cells by which it is constituted.
The increasing focus on cellular kinship has provided a more reliable basis for the classification of organisms than perceived similarities (homologies or synapomorphies), but with regard to cells and their levels of organization, taxonomy remains almost entirely indiscriminate. According to both taxonomy and common perception, a tree is an individual plant, not an organization of interdependent cells, if only because it has the appearance of singular thinghood. A jellyfish is considered an individual animal because it appears to be a distinct and motile organism that feeds by ingestion.
While the primary emphasis on phylogeny remains especially advantageous for specialized purposes, its indiscriminate combination with perceptual identification tends to obscure significant differences in the way multi-cellular organisms are structured, a consideration fundamental to the more general purpose of taxonomy, the heuristic classification of organisms. If taxonomy is, after all, about classification, it is worthwhile considering when an organism can properly be classified as an individual entity or an aggregate of individual entities.
2. The Issue of Identity
There are two somewhat incongruous aspects of the philosophy of science that have likely contributed to inconsistencies in the classification of organisms as either individuals or composites. The characteristic empiricism of science has influenced the identification of organisms by their evident singularity, but the virtual indifference to the discrimination between apparent singularity and microscopic plurality is contrary to the more typical reductionism of science, whereby a multi-celled organism would be routinely described in terms of its cells.
In standard taxonomic practice, individual protists are distinguished from colonies, and colonies from multicellular organisms. But whether an organism is an individual or a colony is treated as one characteristic among many, not significant enough to qualify as a distinguishing feature of a class. Metazoans (multicellular organisms) are commonly distinguished from colonies of protista, somewhat arbitrarily, on the basis of their having more than one functional type of non-reproducing cell; but except in evolutionary studies, where individuality has been identified as a unit of selection (Buss, 1987), the question of what criteria by which multicellular bodies can properly be considered individual organisms, as more than a matter of convenience or convention, has not been definitively raised. In a fairly common philosophy of science perspective, Hull (1988) describes individuals variously as “spatiotemporal particulars,” “cohesive wholes,” “regularities,” and even goes so far as to allow individuality to evolutionary lineages, the planet Mars, and the Empire State Building. Ironically, even morphological studies that seek to carefully define and classify organs, or modules (Winther, 2001), quite permissively allow individuality to include everything from protists to societies of hymenopterans (colonies of ants and wasps).
The indifference in taxonomy and biology in general to the level of organization by which an individual entity is defined is a tendency shared by other branches of science. It may be attributed to an underlying presumption that an aggregation of sufficient size and complexity is sufficient to generate a higher-level identity. A most striking example of this can be found in the field of artificial intelligence, where there is an influential belief that if an information system is sufficiently large it is possible that an individual intelligence will spontaneously arise, an entity that exists in no particular element, but which in some way transcends its constituents. In biological sciences the analogous, usually implicit belief is that a sufficiently large and complex aggregation of cells is capable of generating a singularity that is no longer just an association, but an actual individual that operates as a whole, dependent on, and yet somehow transcendent of its components. This belief is usually, if not always, more than just an expedient manner of treatment. A tree is literally considered a tree, not an organization of specialized and interdependent tree cells, in common parlance and in taxonomy, and for no apparent reasons other than perceived singularity and a presumption of spontaneous individuality.
3. Cellular Cooperation and Coordination
Being multicellular rather than colonial is the character that generally defines metazoan animals. Colonies, as aggregations of cooperating individuals, are commonly, if somewhat arbitrarily distinguished from true metazoans by the number of types of specialized cells. Most, if not all the cell-types of colonies must feed themselves, as they possess no effective means of transporting and sharing nutrients. In more rigorous definitions, multicellular organisms are distinguished from colonies for having developed cell junctions, mechanisms for distributing nutrients, and complex organic molecules to mediate intercellular communications (Nielsen 1995, 19).
Sponges are notable as borderline organisms between colonies and more advanced organizations of cells. Being virtually plant-like in appearance, and lacking overt behavior, they weren’t classified as animals until the 18th century, when their generation of internal water currents as a means of gathering nutrients was discovered. Although they are often acknowledged to be aggregations of somewhat independent cells which (as shown by Ruthmann and Terwelp, 1979) have been found capable of rearranging themselves if disassociated, the porifera (sponges) are in their aggregations routinely treated as singular organisms, based, evidently, on their coherence as singular things. The question here is whether the definition of the aggregation of sponge cells as a sponge can be justified as any more than convention, and whether that convenience has led to a broad neglect, if not misunderstanding, of the wondrous capabilities of cellular organizations, and the profound significance of supra-cellular individuality.
Consider the ctenophores, the comb jellies for example, a much more complex organization than the porifera, being comprised of a number of highly specialized and interdependent cell-types. Ctenophores are characterized by having two networks of nerve cells operating with, at most, a limited interaction to coordinate feeding and locomotion. They have a distinct apical organ at the anterior pole that is suggestive of a brain, being a distinct mass positioned where a head would be in a typical animal, but it is evidently limited to providing bodily orientation relative to gravity, and to having its determinations communicated via nerve cells to other cells responsible for maintaining orientation. Here the question of individuality is more interesting than it is with a sponge: Where is the comb jelly? Is it in any way appropriate to regard the system of cells that seems to present itself as a discrete, singular organism as more than an organization of comb jelly cells? Is there anywhere to be found in its physiology something more involved than a discrete signal from one cell to another, a signal which evokes anything more than a discrete reflex? The ctenophores are capable of a high degree of sensory discrimination and remarkably complex behavior patterns, suggestive of a unified awareness and intentionality, but there is no evident anatomical basis for believing that such an individuality actually exists. Clearly more so than a colony, a ctenophore might best be described as a singular system of highly organized and interdependent cells.
Whereas ctenophores may be said to have approached the limits of cooperative systemic organization, echinoderms – starfish, or sea stars, for example – have achieved a highly effective consolidation of decentralized hierarchies of coordination. They are characterized by two largely independent nerve rings which coordinate overall motor and sensory functions, and five aggregations of ganglial consolidation which serve to actuate their characteristic arms – but they lack anything like an integrative center, or brain. In echinoderms as in ctenophores, cellular interaction is entirely discrete and reflexive. In both of these most complex and intricate coordinated associations of cells there is no physiological basis for more than specific reflexes between individual cells in response to specific types of stimuli.
4. The Limits of Coordination
Multicellular bodies are, in general, able to react to local stimulation by at least some sort of general reflexive avoidance, as in the communication of a contraction between cells. Even some plants transmit wilting reflexes between cells without the benefit of nerves, much less nervous systems. Sensory and nerve cells, by themselves, can be regarded as no more than specialists in the transmission of signals among a system of cells. Intercellular communication is an interaction between individual cells as such, and in the absence of a centralized nervous system, individual sensory cells, nerve cells, or nerve networks only contribute a more precise reception and more efficient communication of information.
Going beyond simple coordination, numerous examples can be found of the concentration of nerve centers, as with echinoderms, where there is evidence of a consolidation of both sensory and motor functions. But even here there is no justification for a description of anything more than a complex, reflexive, and consolidated coordination. Sensory pathways may be switched off to inhibit or dampen reflexes, enabling hierarchies of reflexes, as for example when an escape reflex takes precedence over feeding (Wiersma 1964). Consolidation allows for a more sophisticated coordination of behavior, but it remains, on analysis, a coordination among individual cells.
Again, the question is: If a multi-cellular body is a qualitatively different level of association of cells than a colony, is it meaningful to refer to such a body as an individual organism, and if so, what are the criteria? How does the identification of a tree compare against the standard of organic unity? Multicellular plants may be specialized to the point of being highly coordinated, interdependent, and exclusive, but strictly speaking, the most intricate flowering plant is by all indications no more than a collection of highly organized cells. A tree is a vast system of specialized and interdependent cells, but there is no evidence of a systemic integration that would give it an individuality such as exists within the cells themselves.
I propose the concept of the cellular system to distinguish an organization of cells that goes beyond the organizational grade of the colony, beyond cooperation to coordination, but which offers no indication of a true integration and individuality such as is characteristic of the higher animals (discussed below). The concept of the cellular system can serve to resolve the present arbitrariness whereby, among organizations in which the many individuals are more easily discernible than the organization as a whole, the many are routinely identified as many organisms; and where in an organization in which the individuals are normally indiscernible, it is the organization as a whole that is commonly identified as a single organism. Regardless of the perceptibility of the entity and its constituents, a cellular system can be defined as an exclusive, coordinated association of individual cells that may be expressly treated as an individual entity only by informal convention.
5. The Centralized Nervous System
I have only suggested until now that supra-cellular individuality might be a legitimate concept. It is a meaningful issue, by scientific standards: For coordination to be transformed into integration, and thereby into a true individuality, we would expect to find a structural basis, and we would expect to find evidence of behaviors that cannot be reduced (except by impetuous analysis) to discrete communications between individual cells. Accordingly, some multicellular phenomena can be regarded as patently irreducible: There is no single cell in our bodies that perceives a mountain vista, and no one cell that has a thought. By comparison, in the most highly centralized and coordinated body so far considered, the ctenophore, the cells of the apical organ draw from particular sensations (their own individual cilia) to coordinate cooperative reflexes (synchronized cilial motions or cellular contractions across the body). And in contrast to such discrete, coordinated activity between cells, a central nervous system – the brain and its ancillary networks of sensory organs and nerves – provides for the integration of cellular communication and the subordination of the cells of the body to global considerations, facilitating and controlling integrated perceptions and patterns of behavior, rather than chains of reflexes.
The brain controls reactions and levels of arousal based on integral situations, and it provides the basis of adaptive behavior by storing and subsequently utilizing memory as a supplemental source of information. As the integrative organ of sensory and motor nerves, as the unifying organ of the body, the brain is capable of exercising overriding control of reflexes based on global conditions – not just inhibiting or moderating the intensity of activity, but actually initiating and conducting behavior, subordinating reflexes to integral considerations, responding more or less resourcefully to the uniqueness of present circumstances. The brain thus integrates, not just coordinates the activity of the organism. One might say it involves the integration of experience and the formulation of response. And it is this capability that may justifiably be said to constitute an actual supra-cellular individuality.
The brain and its central nervous system enable a relationship of an animal with its environment on an entirely new level, no longer as an association of organisms, but as an organism of organisms, to initiate individual activity in its constituent cells and organs – not specific activity in response to specific conditions, but individual behavior in response to global conditions. As such, supra-cellular individuality involves a most significant transformation of cellular interaction, an innovation that can be easily overlooked in dissection and analysis, but which cannot perhaps be overestimated for its importance in the understanding of life-forms.
My hypothesis is that the central nervous system – the brain and its sensory and nervous extensions – forms the structural basis of actual supra-cellular individuality, constituting a distinct level of biological existence. By this definition an animal can be said to be formed as an individual when there is an integration of a sensory system with a motor system in a unified level of experience and determination above (transcendent of) cellular interaction. Unlike systems of cells, where interactions are between individual cells or between membranes and molecules, true animals have a supra-cellular presence, a capacity for interaction with larger, more complex structures, as determined by an integral perceptibility and responsiveness. An animal, defined as a supra-cellular organism, thus relates to its environment on a higher level that its constituents, of which it is in some degree distinct, and it manipulates them as subsystems of givens in its projection as an individual entity.
It is worth noting that this hypothesis is at least no more presumptive than the established alternatives which either presume, however implicitly, that individuality simply happens, or that individuality is some sort of subjective, epiphenomenal delusion. In view of the evident development from cells and colonies to systems of cells, and from systems of cells to the consolidation of systems into wholes, it appears that individuation is something that nature does, or rather that nature is able to achieve, provided there is a developed structural foundation for integration. The consideration of the difference between individual organisms and their associations profiles both the remarkable coordinated capabilities of cellular systems and the special significance of actual individuality. If individuality is natural, if evolution can be described as a process of developing from one level of individuality to a system of increasingly interdependent and mutually supportive individuals to another level of organization, then individuality is a characteristic worthy of appropriate classification and deliberate investigation.
In this hypothesis, the level of individuality – not the manner of feeding, not relative motility, not even phylogenetic kinship – is the most significant character of living organisms. It qualifies as a difference of grade as significant as that between procaryote (bacteria and archaea) and eucaryote, and indeed, as a difference of taxonomic level.
7. A Proposal for a Revised Taxonomy
It has been widely acknowledged in the field of taxonomy that there is a practical value to admitting grades, or levels of development, to the consideration of classifications. If individuality distinguishes organisms from systems of organisms, it seems reasonable that individual cells and their systems should be considered together as one level of organisms, and that animals are best treated as a distinct level of organisms in the same manner.
The strict phylogenetic classification of cells is unquestionably a useful procedure for specialists, regardless of whether animal individuals are distinguished from non-animal or pre-animal multicellular organisms. But for more general purposes of classification, a figurative vertical dimension of taxonomic classification, where levels of individuality are recognized, is an important discrimination.
Taxonomy on the basis of the present hypothesis would be divided first of all into levels, of procaryotes (which might most consistently be regarded as molecular systems), then of eucaryotic cells and their systems, and then of animals and their associations.
A revised taxonomy of Kingdom Animalia in terms of individuality would, admittedly, not be so orderly as one of the current trees based more or less consistently on phylogeny. Paraphyletic (partial) and polyphyletic (compound) groupings would be necessary, as supra-cellular individuality has evidently developed along at least two paths, among both the protostomia (e.g., octopus) and the deuterostomia (e.g., mammals). But although it would be more complex than the current taxonomy, this proposed deviation from a more strict systematics would not actually be an unconventional procedure. The Eucaryotes already form a polyphyletic group in taxonomy if, as it is maintained, they ascend from the association of bacteria and archaea (Woese 1990). And the conventional paraphyletic discrimination of mammals and birds from Reptlia is based on little more than custom and convenience – and perhaps a little mammalian vanity.
In seeking to accommodate individuality to systematic taxonomy it seems clear that animals, defined as individual supra-cellular organisms, would not be a monophyletic group (of single descent), unless the present phylogeny is seriously in error and the cephalic bilaterals (including both octopus and mammals) are more closely related than all those that are non-cephalic. Although all animals thus defined are bilaterals, not all protostomia and not all deuterostomia (as they are currently classified) are animals. It seems reasonable therefore to rename the Kingdom Animalia as the Metazoa, redefining Animalia as a polyphyletic sub-kingdom of Metazoa, perhaps renamed as the Euanimalia. The excluded members of the present taxon of Animalia, those without actual brains, could accordingly constitute the paraphyletic sister sub-kingdom Proanimalia.
The division of metazoans into the sub-kingdoms Proanimalia and Animalia may be unsatisfactory from a strictly phylogenetic point of view, requiring as it would the introduction of paraphyletic and polyphyletic groupings. But from a recognition of the differences among cells, cellular systems, and individuals, the revision is conventional, and indispensable to a better understanding of individuality as a natural phenomenon, and to an orientation toward concentrated research into the specific characteristics of cellular systems and individualities.
Current taxonomy has been described as a largely indiscriminate classification of cells, cellular systems, and supra-cellular individuals, any of which may be customarily treated as individual organisms. It has been hypothesized that in true animals, nature has constituted itself into wholes, into individualities, by means of the development of integrative structures. Individuality has been presented as a most significant feature of organisms, for which accommodation should be made in a revised taxonomic system.
The cells of jellyfish and the cells of humans may be more closely related than those of oak trees. But if oak trees and jellyfish are to be recognized as non-individuals, as vast cellular systems, and if the distinctiveness of animal individuality is to be fully appreciated, taxonomy may contribute to an enlightening and exciting reordering of the way we view ourselves and the life-forms around us.
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Hull, David L. (1988), Science as a Process, University of Chicago Press.
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Woese, C., O. Kandler, and M.L. Wheelis (1990), “Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya”, Proceedings of the National Academy of Sciences, 87: 4576-4579.