Abstract
The aim of this commentary article is to discuss several problems in the distribution map for conscious organisms and suggest a different strategy to address their evolutionary development. I propose to complement the model of Unlimited Associative Learning (UAL) that Jablonka and Ginsburg present in their Target Article with the additional consideration of how body plans constrain the possibilities of evolution of the brain— in some cases blocking, in some others enabling its complexification and corporal integration. In my view, the specific body plan of vertebrates has played a key role in the complexification and diversification of consciousness because it has favored an increasing integration between brain and body.
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Notes
A review of the most recent literature shows how questionable these “lower cases” are and the wide acceptance that more research is needed when dealing with them.
In addition, the capacity for centralization/integration will not only depend on the number of neurons, but also on factors like their packing density, the inter-neural distance and axonal conduction velocity.
Here I have focused on neurological features (like the number of neurons in the brain, the presence of a specific brain structure or its functional equivalent, the degree of centralization, re-entrant processing, neural assemblies…). Yet, many authors construct their explanations from behavioral observations. I prefer to take the behavioral data with caution, unless supported by the latter. Ethograms have limited utility for determining the cognitive capacities of animals.
Compared with an aquatic environment, motility in a terrestrial environment requires, for a big size animal, exerting a significantly stronger force.
For the importance of the specific form of vertebrate’s BBB see Monahan-Earley (2013).
This is an interesting point that connects my own view about the emergence of C with the bio-semiotic research on the role played by information and signals in the study of living systems, since the essence of the nervous system is informational. But this is not the place to extend myself on this point.
“What I mean by “strange loop” is (…) an abstract loop in which, in the series of stages that constitute the cycling-around, there is a shift from one level of abstraction (or structure) to another, which feels like an upwards movement in a hierarchy, and yet somehow the successive “upward” shifts turn out to give rise to a closed cycle. That is, despite one’s sense of departing ever further from one’s origin, one winds up, to one’s shock, exactly where one had started out. In short, a strange loop is a paradoxical level-crossing feedback loop” (Hofstadter, 2007; pp. 101–102).
However, of the 500 million neurons in an octopus’ nervous system, 300 are in the arms. The tentacles of an octopus can function somewhat independently, and severed tentacles have been shown to be controllable by mechanical or electrical stimulation, suggesting that the basic motor program for voluntary movement is embedded within the neural circuitry of the tentacle itself (Hochner et al., 2006).
Octopuses have integration centers of 45 million neurons.
Jablonka and Ginsburg affirm that the capacity for C in cephalopods evolved 250 million after that of the vertebrates and arthropods (p. 21).
Just to be a bit more specific on some of these limitations. Fast motility (at the considerable size scale of these cephalopod bodies) implies a jet propulsion system that provides thrust, which in turn requires bioenergetic, physiological, and biochemical adaptations which maximize their metabolic rates (O’Dor & Webber, 2011). Predator-prey competition with rapid fishes generates strong energetic pressure on the basic molluscan physiology of coleoids cephalopods. These animals have evolved a complex closed circulatory system (i.e., enlargement of the mantle cavity was coupled with supplementation of the systemic heart through the addition of twin accessory branchial (gill) hearts, originating from the nautilus pericardial glands, allowing the increased oxygen demands required for greater mobility). However, as argued by several authors (O’Dor, 2002; Seibel, 2016) oxygen transport is lower in cephalopods compared to fish, due to their smaller hearts relative to body size, and because they use haemocyanin (Hcy), instead of the vertebrate’s haemoglobin. Yet, Hcy—which is found only in arthropods and in molluscs– is not cell-bound as is haemoglobin in the red blood cells of vertebrates, but remains free in the blood plasma. Hcy has only half of the oxygen carrying capacity, compared to haemoglobin. In addition, as Seibel (op. cit.) has pointed out, cephalopods also have little or no venous oxygen reserves. Thus, as Jaitly and colleagues have recently argued, “under conditions of high oxygen demand, cephalopods must make maximal use of blood-borne oxygen at every circulatory cycle, as facilitated by increased gill ventilation, heart rate and stroke volume. Jet propulsion is also highly inefficient relative to the caudal fin swimming of fish.” (Jaitly et al., 2022, p 14). So it seems quite obvious that these animals are pushing their limits to the extreme.
Somehow, in brief, the view presented here could be considered as a much more radical version of the embodied approach to C that their work also supports.
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Funding
The author acknowledges Kepa Ruiz-Mirazo for critical remarks and suggestions after his careful reading of previous versions of this commentary, and funding from the John Templeton Foundation (research project ‘Agency, Directionality & Function’ - Science of Purpose).
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Moreno, A. Some Reflections on the Evolution of Conscious Agents: The Relevance of body Plans. Biosemiotics 16, 35–43 (2023). https://doi.org/10.1007/s12304-023-09525-y
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DOI: https://doi.org/10.1007/s12304-023-09525-y