Neural Darwinism

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Professor Edelman giving a lecture, September 30, 2010.

Neural Darwinism is a biological, and more specifically Darwinian and selectionist, approach to understanding global brain function, originally proposed by American biologist, researcher and Nobel-Prize recipient[1] Gerald Maurice Edelman (July 1, 1929 – May 17, 2014). Edelman's 1987 book Neural Darwinism[2] introduced the public to the Theory of Neuronal Group Selection (TNGS) – which is the core theory underlying Edelman's explanation of global brain function.

Owing to the book title, TNGS is most commonly referred to as the Theory of Neural Darwinism, although TNGS has roots going back to Edelman and Mountcastle's 1978 book, The Mindful Brain – Cortical Organization and the Group-selective Theory of Higher Brain Function – where Edelman's colleague, the American neurophysiologist and anatomist Vernon B. Mountcastle (July 15, 1918 – January 11, 2015), describes the columnar structure of the cortical groups within the neocortex,[3] while Edelman develops his argument for selective processes operating among degenerate primary repertoires of neuronal groups.[4] The development of Neural Darwinism was deeply influenced by Edelman's work in the fields of immunology, embryology, and neuroscience – as well as, his resolute commitment to Charles Darwin and the ingenuity of selection as the unifying foundation of the biological sciences.

Introduction To Neural Darwinism[edit]

Charles Darwin (February 12, 1809 – April 19, 1882) – Darwin drew inspiration not just from nature, but from those around him. As an aristocratic naturalist, Darwin was in the company of breeders of a multitude of organisms. Plants, flowers, livestock, dogs, birds all have a long history of artificial selection by man to suit his purposes. By the time of Darwin there were an astonishing number of breeds and varieties that had been produced by horticulture and animal husbandry. Darwin, stimulated and inspired by his journeys aboard the Beagle and seeking to explain the relational diversity that he observed, drew inspiration from the work of breeders, co-opted their methodology, and combined it with Malthusian population dynamics. He replaced the selective mechanism of man in artificial selection with the conditions of nature in natural selection.
Charles Darwin's first diagram of an evolutionary diversification taken from his Notebook B – On the Transmutation of Species (1837). Neural Darwinism would be published 150 years later – a fitting tribute to the enduring power of Darwin's ideas. Although many will claim that this is the first evolutionary tree diagram, it is more of a bush, if anything, and Darwin's initial instincts are prescient.

Neural Darwinism is really the neural part of the natural philosophical and explanatory framework Edelman employs for much of his work – Somatic selective systems. Neural Darwinism is the backdrop for a comprehensive set of biological hypotheses and theories Edelman, and his team, devised that seek to reconcile vertebrate and mammalian neural morphology, the facts of developmental and evolutionary biology, and the theory of natural selection[5] into a detailed model of real-time neural and cognitive function that is biological in its orientation – and, built from the bottom-up, utilizing the variation that shows up in nature, in contrast to computational and algorithmic approachs that view variation as noise in a system of logic circuits with point-to-point connectivity.

The book, Neural Darwinism – The Theory of Neuronal Group Selection (1987), is the first in a trilogy of books that Edelman wrote to delineate the scope and breadth of his ideas on how a biological theory of consciousness and animal body plan evolution could be developed in a bottom-up fashion. In accordance with principles of population biology and Darwin's theory of natural selection – as opposed to the top-down algorithmic and computational approaches that dominated a nascent cognitive psychology at the time.

The other two volumes are Topobiology – An Introduction to Molecular Embryology [6] (1988) with its morpho-regulatory hypothesis of animal body plan development and evolutionary diversification via differential expression of cell surface molecules during development; and The Remembered Present – A Biological Theory of Consciousness[7] (1989) – a novel biological approach to understanding the role and function of "consciousness" and its relation to cognition and behavioral physiology.

Edelman would write four more books for the general lay public, explaining his ideas surrounding how the brain works and consciousness arises from the physical organization of the brain and body – Bright Air, Brilliant Fire – On the Matter of the Mind[8] (1992), A Universe of Consciousness – How Matter Becomes Imagination[9] (2000) with Giulio Tononi, Wider Than The Sky – The Phenomenal Gift of Consciousness[10] (2004), and Second Nature – Brain Science and Human Knowledge[11] (2006).

Neural Darwinism is a tour-de-force of biological thought and philosophy as well as fundamental science; Edelman being well-versed in the history of science, natural philosophy & medicine, as well as, robotics, cybernetics, computing & artificial intelligence. The multidisciplinary aspects of Neural Darwinism require a great deal of patience to absorb because of the scope and breadth of what it is trying to encompass. In the course of laying out the case for Neural Darwinism, or more properly TNGS, Edelman delineates a set of innovative concepts for rethinking the problem of nervous system organization and function – all-the-while, demanding a rigorously scientific criteria for building the foundation of a properly Darwinian, and therefore biological, explanation of neural function, perception, cognition, and global brain function that is capable of adequately supporting primary and higher-order consciousness.

Population thinking – somatic selective systems[edit]

The immune system has an ancient history within animals. All animals have an innate immune system, but only vertebrates have an "active" antibody-based immune system. The innate (left branch) immune system is ancient and anchored around the phagocytic white blood cells discovered by the pioneering biologist, embryologist, zoologist, immunologist, gerantologist Élie Metchnikoff (May 15, 1845–July 15, 1916).[12] The antibody-based system (right branch) arose at the origin of vertebrates and is associated with the genome duplication events[13] that provided the duplicate copies of NCAM which eventually resulted in the emergence of genetically recombinant antibodies.[14][15] Paul Ehrlich (March 14, 1854–August 20, 1915)) was the discoverer of antibodies – and, along with Elie Metchnikoff, is considered to be one of the founders of Immunology. In 1908, they would share the first Nobel Prize in Physiology or Medicine. 64 years later, Edelman and Porter would share this very same prize.
Illustration of disulfide bridges (red) linking the light (L, green) and heavy (H, purple) chains of Immunoglobulin G (IgG) antibody. The variable (V) regions are located at the antigen-binding end; and, the constant (C) domains form the primary frame of the IgG molecule. Another disulfide bridge holds the two symmetrical units made up of a light chain (VL+CL) and a heavy chain (VH+CH1+CH2+CH3) together to form the completed antibody.[a]
Clonal selection theory (CST): hematopoietic stem cells (1) differentiate and undergo genetic rearrangement to produce a population of cells possessing a wide range of pre-existing diversity with respect to antibody expression (2). Lymphocytes expressing antibodies that would lead to autoimmunity are filtered from the population (3), while the rest of the population represents a degenerate pool of diversity (4) where antigen-selected variants (5) can be differentially amplified in response (6). Once the antigen has been cleared, the responding population will decrease, but not by as much as it was amplified, leaving behind a boosted capacity to respond to future incursions by the antigen – a form of enhanced recognition and memory within the system.

Edelman was inspired by the successes of fellow Nobel Laureate[16] Frank MacFarlane Burnet and his Clonal selection theory (CST) of acquired antigen immunity by differential amplification of pre-existing variation within the finite pool of lymphocytes in the immune system. The population of variant lymphocytes within the body mirrored the variant populations of organisms in the ecology. Pre-existing diversity is the engine of adaption in the evolution of populations.

"It is clear from both evolutionary and immunological theory that in facing an unknown future, the fundamental requirement for successful adaption is preexisting diversity"[17] – Gerald M. Edelman (1978)

Edelman recognizes the explanatory range of Burnet's utilization of Darwinian principles in describing the operations of the immune system - and, generalizes the process to all cell populations of the organism. He also comes to view the problem as one of recognition and memory from a biological perspective, where the distinction and preservation of self vs. non-self is vital to organismal integrity.

Neural Darwinism, as TNGS, is a theory of neuronal group selection that retools the fundamental concepts of Darwin and Burnet's theoretical approach. Neural Darwinism describes the development and evolution of the mammalian brain and its functioning by extending the Darwinian paradigm into the body and nervous system.

Antibodies and NCAM – The emerging understanding of somatic selective systems[edit]

Edelman was a medical researcher, physical chemist, immunologist, and aspiring neuroscientist when he was awarded the 1972 Nobel Prize in Physiology or Medicine (shared with Rodney Porter of Great Britain). Edelman's part of the prize was for his work revealing the chemical structure of the vertebrate antibody by cleaving the covalent disulfide bridges that join the component chain fragments together, revealing a pair of two-domain light chains and four-domain heavy chains. Subsequent analysis revealed the terminal domains of both chains to be variable domains responsible for antigen recognition.[18]

The work of Porter and Edelman revealed the molecular and genetic foundations underpinning how antibody diversity was generated within the immune system. Their work supported earlier ideas about pre-existing diversity in the immune system put forward by the pioneering Danish immunologist Niels K. Jerne (December 23, 1911 – October 7, 1994); as well as supporting the work of Frank MacFarlane Burnet describing how lymphocytes capable of binding to specific foreign antigens are differentially amplified by clonal multiplication of the selected preexisting variants following antigen discovery.

Edelman would draw inspiration from the mechano-chemical aspects of antigen/antibody/lymphocyte interaction in relation to recognition of self-nonself; the degenerate population of lymphocytes in their physiological context; and the bio-theoretical foundations of this work in Darwinian terms.

By 1974, Edelman felt that immunology was firmly established on solid theoretical grounds descriptively, was ready for quantitative experimentation, and could be an ideal model for exploring evolutionary selection processes within an observable time period.[19]

"Two major developments have profoundly altered immunological research in the last decade: the theory of clonal selection and the chemical analysis of antibody structure... As a result of these developments, it has become clear that the central problem of immunology is to understand the mechanisms of selective molecular recognition in a quantitative fashion. Aside from evolution itself, there are few such well-analyzed examples of selective systems in biology or in other fields for that matter. For this reason, the immune system provides a unique opportunity to analyze the problem of selection under defined and experimentally measurable conditions that have so far been hard to achieve in other Eukaryotic systems. It is fortunate that the characteristics of the molecules and cells mediating selection in the immune response are known or can be known, and above all, that the time scale of the selective events is well within that required for direct observation and experimentation."[20] – Gerald M. Edelman (1974)

His studies of immune system interactions developed in him an awareness of the importance of the cell surface and the membrane-embedded molecular mechanisms of interactions with other cells and substrates. Edelman would go on to develop his ideas of topobiology around these mechanisms – and, their genetic and epigenetic regulation under the environmental conditions.

During a foray into molecular embryology and neuroscience, in 1975, Edelman and his team went on to isolate the first neural cell-adhesion molecule (N-CAM), one of the many molecules that hold the animal nervous system together. N-CAM turned out to be an important molecule in guiding the development and differentiation of neuronal groups in the nervous system and brain during embryogenesis. To the amazement of Edelman, genetic sequencing revealed that N-CAM was the ancestor of the vertebrate antibody[14] produced in the aftermath of a set of whole genome duplication events at the origin of vertebrates[13] that gave rise to the entire super-family of immunoglobulin genes.

Edelman reasoned that the N-CAM molecule which is used for self-self recognition and adherence between neurons in the nervous system gave rise to their evolutionary descendants, the antibodies, who evolved self-nonself recognition via antigen-adherence at the origins of the vertebrate antibody-based immune system. If clonal selection was the way the immune system worked, perhaps it was ancestral and more general – and, operating in the embryo and nervous system.

Variation in biological systems – Degeneracy, Complexity, Robustness, and Evolvability[edit]

The degeneracy of the genetic code buffers biological systems from the effects of random mutation. The ingenuous 1964 Nirenberg and Leder experiment would identify the mRNA codons, a triplet sequence of ribonucleotides, that coded for each amino acid; thus elucidating the universal genetic code within the DNA when the transcription process was taken into account. Changes in the third position of the codon, the wobble position, often result in the same amino acid, and oftentimes the choice comes down to purine or pyrimidine only when a choice must be made. Similar, but variant, codon sequences tend to yield similar classes of amino acid – polar to polar, non-polar to non-polar, acidic to acidic, and basic to basic residues.
The four major classes of biological amino acids – polar (hydrophilic), nonpolar (hydrophobic), acidic, and basic side chain residues. The amino acid backbone is amino group linked to an alpha carbon, on which resides the side chain residue and a hydrogen atom, that is connected to a terminal carboxylate group. Aside from the disulfide bridge, there are quite a number of degenerate combinations of sidechain residues that make up the tertiary structure (H-bonding, Hydrophobic, and Ionic bridges) in the determination of protein structure.
Relationships between degeneracy, complexity, robustness, and evolvability – 1) Degeneracy is the source of Robustness. 2) Degeneracy is positively correlated with Complexity. 3) Degeneracy increases Evolvability. 4) Evolvability is a prerequisite for Complexity. 5) Complexity increases to improve Robustness. 6) Evolvability emerges from Robustness.

Degeneracy, and its relationship to variation, is a key concept in Neural Darwinism. The more we deviate from an ideal form, the more we are tempted to describe the deviations as imperfections. Edelman, on the other hand, explicitly acknowledges the structural and dynamic variability of the nervous system. He likes to contrast the differences between redundancy in an engineered system and degeneracy in a biological system. He proceeds to demonstrate the how the "noise" of the computational and algorithmic approach is actually beneficial to a somatic selective system by providing a wide, and degenerate, array of potential recognition elements.[21]

"Degeneracy, the ability of elements that are structurally different to perform the same function or yield the same output, is a well known characteristic of the genetic code and immune systems. Here, we point out that degeneracy is a ubiquitous biological property and argue that it is a feature of complexity at genetic, cellular, system, and population levels. Furthermore, it is both necessary for, and an inevitable outcome of, natural selection."[22] – Gerald M. Edelman & Joseph A. Gally (2001)

Edelman's argument is that in an engineered system,

  • a known problem is confronted,
  • a logical solution is devised
  • an artifice is constructed to implement the resolution to the problem.

To insure the robustness of the solution, critical components are replicated as exact copies. Redundancy provides a fail-safe backup in the event of catastrophic failure of an essential component but it is the same response to the same problem once the substitution has been made.

If the problem is predictable and known ahead of time, redundancy works optimally. But biological systems face an open and unpredictable arena of spacetime events of which they have no foreknowledge of. It is here where redundancy fails – when the designed answer is to the wrong problem...

Variation fuels degeneracy – and degeneracy provides somatic selective systems with more than one way to solve a problem; as well as, the ability to solve more than one problem the same way. This property of degeneracy has the effect of making the system more adaptively robust in the face of unforeseen contingencies, such as when one particular solution fails unexpectedly – there are still other unaffected pathways that can be engaged to result in the comparable final outcome. Early on, Edelman spends considerable time contrasting degeneracy vs. redundancy, bottom-up vs. top-down processes, and selectionist vs. instructionist explanations of biological phenomena.

Rejection of computational models, codes, and point-to-point wiring[edit]

Edelman was well aware of the earlier debate in immunology between the instructionists, who believed the lymphocytes of the immune system learned or was instructed about the antigen and then devised a response; and the selectionists, who believed that the lymphocytes already contained the response to the antigen within the existing population that was differentially amplified within the population upon contact with the antigen. And, he was well aware that the selectionist had the evidence on their side.

Edelman's theoretical approach in Neural Darwinism was conceived of in opposition to top-down algorithmic, computational, and instructionist approaches to explaining neural function. Edelman seeks to turn the problems of that paradigm to advantage instead; thereby highlighting the difference between bottom-up processes like we see in biology vis a vis top-down processes like we see in engineering algorithms. He sees neurons as living organisms working in cooperative and competitive ways within their local ecology and rejects models that see the brain in terms of computer chips or logic gates in a algorithmically organized machine.

Edelman's commitment to the Darwinian underpinnings of biology, his emerging understanding of the evolutionary relationships between the two molecules he had worked with, and his background in immunology lead him to become increasingly critical and dissatisfied with attempts to describe the operation of the nervous system and brain in computational or algorithmic terms.

Edelman explicitly rejects computational approaches to explaining biology as non-biological. Edelman acknowledges that there is a conservation of phylogenetic organization and structure within the vertebrate nervous system, but also points out that locally natural diversity, variation and degeneracy abound. This variation within the nervous system is disruptive for theories based upon strict point-to-point connectivity, computation, or logical circuits based upon codes. Attempts to understand this noise present difficulties for top-down algorithmic approaches - and, deny the fundamental facts of the biological nature of the problem.

"One conclusion we can draw (...) is that, while there are close similarities in certain regions, there are no absolutely specific point-to-point connections in the brain. The microscopic variability of the brain at the finest ramifications of its neurons is enormous, making each brain unique. These observations provide a fundamental challenge to models of the brain based on instruction or computation."[23] – Gerald M. Edelman (1998)

Edelman perceived that the problematic and annoying noise of the computational circuit-logic paradigm could be reinterpreted from a population biology perspective – where that variation in the signal or architecture was actually the engine of ingenuity and robustness from a selectionist perspective.

Population Thinking – From the bottom up[edit]

Darwin's theory of Natural Selection put population dynamics at the center of biology and distinguished it from the other sciences in its epistomology. Edelman points out that population thinking freed biologists from the formalism of Platonic Essentialism, or typology that other sciences relied upon. For Edelman, "population thinking states that evolution produces classes of living forms from the bottom up by gradual selective processes over eons of time."[24] He points out that biology is somewhat unique in its particular "mode of thought",

Edelman lists the following primary features of a somatic selective system:

  • A degenerate population of response elements,
  • One or more mechanisms of adaptive selection of elements from within the degenerate population,
  • Differential amplification of adaptive response elements within the population in the next generation.

Selective systems can take many forms, not only at the inter-organismal level but also within organismal structures that are constructed of "sub"-organisms, i.e. cells and cell populations within a multicellular organism, or organelles within an cells... enzymes and macromolecular structures within the biochemistry of the cell, et cetera.

These systems operate in terms of response amplification or suppression under historically contingent circumstances, where there is a population of potential response networks from which a particular response network is selected and strengthened upon realization of threshold conditions, thereby differentially amplifying that particular system potential in response to similar future events.

Population Biology – Individuals, groups, and the somatic evolution of a distributed population[edit]

The immune system is an ideal model for the study of population dynamics. Within the immune system one can observe individual lymphocytes that possess a specific form of a variant antibody within the organism. We can observe two types of groups that emerge over time:

  • Groups of multiple individual lymphocytes that arise from clonal amplification, all of which possess the same specific form of antibody.
  • Groups of multiple individual lymphocytes, each with is own specific form of the variant antibody, that all respond to the same antigen in a degenerate fashion.

We can observe the somatic evolution of the entire lymphocyte population over time by observing how the distribution of specific antibody possessing, and/or antigen-recognizing, lymphocytes change in response to ecological exposure.

Similarly, we could treat the nervous system. Among other things, Neural Darwinism posits that the evolution and behavior of cell populations in the body of an organism, operating over the lifetime of the organism, are determined by the rules of natural selection operating in the local cellular and physiological context. Just as the development and evolution of cooperative and competitive interactions between populations of organisms in the ecology are governed by the rules of natural selection – for cell populations in the body, the body is their ecology and their interactions within that context is still governed by the principles of population biology and natural selection.

Very few, if any, neurons in the vertebrate nervous system act solo. Most individual neurons undergo somatic selection during embryogenesis and ultimately find themselves a member of a neuronal group, array, or ensemble, whether that be in the form of ganglia, nuclei, or laminae. Like organisms in the environment, neurons are born into a population of other cells and find themselves members of a community or group.

Edelman describes a neuronal group as "a collection of similar or variant types, ranging in number from hundreds to thousands, that are closely connected in their intrinsic circuitry and whose mutual dynamic interaction may be further enhanced by increases in synaptic efficacy."[25] Groups are defined by the relative strengths and interactivity of group members amongst themselves via intrinsic connections; and, a concerted output along extrinsic connections to other neuronal groups.

Neuronal groups are organized regionally into topobiological histological maps based upon the source of their input connections, whether it be a sensory sheet, motor ensemble, or another neuronal group. Not only is there a great deal of plasticity within a neuronal group on the basis of input stimulation and the learning of sensorimotor discriminations, but there is also a great deal of plasticity associated with the connections between the neuronal groups globally facilitating multimodal association, memory, and learning. The global population of neuronal groups comprises a distributed system of interconnected groups capable coordinating and modulating multi-modal sensorimotor integration for the organism as a whole – and, evolving over time on the basis of the organisms experience in the ecology in relation to the hedonic needs of the organism.

Somatic Selective Memory systems[edit]

At the end of Bright Air, Brilliant Fire, Edelman lists the evolutionary emergence of four distinct types of biological memory that are based upon somatic selection that have arisen within vertebrates:[26]

  • Hereditary – DNA provides a stable molecular substrate for replication
  • Immunological – differential amplification of adaptive lymphocytes within a population
  • Reflexive – phylogenetically constrained and deeply experiential canalized neural transduction pathways
  • Recategorical – neuronal group selection within reentrant topobiological maps.

Recognition links cognition to memory and serves as the basis for evolutionary learning in a somatic system.

"Each memory reflects a system property within a somatic selection system. And each property serves a different function based upon the evolution of the appropriate neuroanatomical structure. These higher-order systems are selective and are based on the responses to environmental novelty of populations of neuronal groups arranged in maps. They are recognition systems."[27] – Gerald M. Edelman (1992)

Completing Darwin's program – The problems of evolutionary and developmental morphology[edit]

In Topobiology, Edelman reflects upon Darwin's search for the connections between morphology and embryology in his theory of Natural Selection. He identifies four unresolved problems in the development and evolution of morphology that Darwin thought important:[28]

  • Explaining the finite number of body plans manifested since the Precambrian.
  • Explaining large-scale morphological changes over relatively short periods of geological time.
  • Understanding body size and the basis of allometry.
  • How adaptive fitness can explain selection that leads to emergence of complex body structures.

Later, In Bright Air, Brilliant Fire, Edelman describes what he calls Darwin's Program for obtaining a complete understanding of the rules of behavior and form in evolutionary biology.[29] He identifies four necessary requirements:

  • An account of the effects of heredity on behavior – and behavior, on heredity.
  • An account of how selection influences behavior – and, how behavior influences selection.
  • An account of how behavior is enabled and constrained by morphology.
  • An account of how morphogenesis occurs in development and evolution.

It is important to notice that these requirements are not directly stated in terms of genes, but heredity instead. This is understandable considering that Darwin himself appears to not be directly aware of the importance Mendelian genetics. Things had changed by the early 1900s, the Neodarwinian Synthesis had unified the population biology of Mendelian Inheritance with Darwinian Natural Selection. By the 1940s, the theories had been shown to be mutually consistent and coherent with paleontology and comparative morphology. The theory came to be known as the Modern Synthesis on the basis of the title of the 1942 book Evolution: The Modern Synthesis by Julian Huxley.[30]

The Modern Synthesis really took off with the discovery of the structural basis of heredity in the form of DNA. The Modern Synthesis was greatly accelerated and expanded with the rise of the genomic sciences, molecular biology, as well as, advances in computational techniques and the power to model population dynamics. But, for evolutionary-developmental biologists, there was something very important missing... – and, that was the incorporation of one of the founding branches of biology, embryology. A clear understanding of the pathway from germ to zygote to embryo to juvenille and adult was the missing component of the synthesis. Edelman, and his team, were positioned in time and space to fully capitalize on these technical developments and scientific challenges – as his research progressed deeper and deeper into the cellular and molecular underpinnings of the neurophysiological aspecsts of behavior and cognition from a Darwinian perspective.

Edelman reinterprets the goals of "Darwin's program" in terms of the modern understanding about genes, molecular biology, and other sciences that weren't available to Darwin. One of his goals is reconciling the relationships between genes in a population (genome) which lie in the germ line (sperm, egg, and fertilized egg); and the individuals in a population who develop degenerate phenotypes (soma) as they transform from an embryo into an adult who will eventually procreate if adaptive. Selection acts on phenotypes (soma), but evolution occurs within the species genome (germ).

Edelman follows the work of the highly influential American geneticist and evolutionary biologist Richard Lewontin (1929–present), drawing particular inspiration from his 1974 book, The Genetic Basis of Evolutionary Change.[31] Edelman, like Lewontin, seeks a complete description of the transformations (T) that take us from:[32]

  • Genome-germ (zygotes) – the paternal and maternal gene contributions are recombined in the fertilized egg, along with the maternal endowment of proteins, and mRNAs, and other developmental components, but the individuals newly formed diploid genetic complement is not in control of the zygote yet; it needs to be activated, or bootstrapped, into the zygotes ongoing maternally-inherited metabolism and physiology. Shortly after recombination the zygote proceeds through transformation (T1) to the point where genetic control of the zygote has been handed off to the individual,
  • Phenotype-soma (embryo) – the embryo, which transforms (T2) according to the rules that govern the relationship between the genes, cellular behavior, and the epigenetic contingencies of nature, into
  • Phenotype-soma (adult) – an adult, who procreates (T3) with another individual to bring together a new genetic recombination by each introducing a gamete in the form of
  • Genome-germ (gametes) – sperm and egg, which contain the haploid genetic contribution of each parent which is transformed (T4)...
  • Genome-germ (zygotes) -into a diploid set genes in a fertilized egg, soon to be a newly individual zygote .

Lewontin's exploration of these transformations between genomic and phenotypic spaces was in terms of key selection pressures that sculpt the organism over geological evolutionary time scales; but, Edelmans approach is more mechanical, and in the here and now – focusing on the genetically-constrained mechano-chemistry of the selection processes that guide epigenetic behaviors on the part of cells within the embryo and adult over developmental time.

Mechano-chemistry, mesenchyme, and epithelia – CAMs & SAMs in morphoregulatory spacetime[edit]

Mesenchymal-epithelial transitions – Epithelia to mesenchyme (EMT) and Mesenchyme to epithelia (MET) transisitions utilizing CAMs and SAMs to form epethelia; and, growth factors and inducers to mediate the transition to mesenchyme as the CAMs and SAMs are withdrawn or localized on the cell membrane.[33][b]

Edelman's isolation of NCAM lead him to theorize on the role of cell adhesion molecules (CAMs) and substrate adhesion molecules (SAMs) in the formation of the animal bodyplan in both realtime and over evolutionary time. Topobiology is primarily dedicated to this issue that is foundational to the understanding of Neural Darwinism and the formation of the primary repertoire of TNGS.

In his Regulator Hypothesis, Edelman hypothesizes about the role of cell surface molecules in embryogenesis and how shifting expression of these molecules in time and place within the embryo can guide the development of pattern.[35] Later, he will expand the hypothesis into the Morpho-regulatory Hypothesis.[36] He describes the embryonic cell populations as either organized as mesenchyme or epetheilia.

Edelman characterizes the two population types as follows:

  • Epithelia – a population of cells that are organized into coherent tissues, that have well established CAM patterns; as well as a stable pattern of substrate adhesion between the cells and the extracellular matrix.
  • Mesenchyme – a population of cells that are loosely associated and migratory, that have retracted (or localized) their CAM and SAM molecules such that they can follow homophilic and heterophilic gradients within other cell populations of the embryo.

He envisages a CAM, and SAM, driven cycle where cell populations transform back and forth between mesenchyme and epethelia. via epithelial-mesenchymal transformations,[37] as the development of the embryo proceeds through to the fetal stage. The expression of the CAMs and SAMs is under genetic control, but the distribution of these molecules on the cell membrane and extracellular matrix is historically contingent upon epigenetic events, serving as one of the primary bases for generating pre-existing diversity within the nervous system and other tissues.

"The key idea may be summarized roughly as follows: These molecules link cells into collectives whose borders are defined by CAMs of different specificity. The binding properties of cells linked by the CAMs are dynamically controlled by the cells themselves as a result of signals exchanged between collectives. Cell binding in turn regulates cell motion and further signalling, and thus the ensuing forms. Control of the expression of CAM genes by the specific biochemistry affecting CAM regulatory genes at particular morphologic sites assures constancy of form in a species. But because the main function of CAMs is to regulate dynamic cellular processes and not specify cell addresses exactly, variability is also introduced during development."[38] - Gerald M. Edelman (1987)

The developmental genetic question[edit]

There are many developmental questions to be considered, but Edelman is able to succinctly summarize the problem in a way that will show a clear explanatory path forward for him. The developmental genetic question defines the problem - and, the theoretical approach for him.

"How does a one-dimensional genetic code specify a three-dimensional animal?"[39] – Gerald M. Edelman, from the glossary of Topobiology

By 1984, Edelman would be ready to answer this question and combine it with his earlier ideas on degeneracy and somatic selection in the nervous system. Edelman would revisit this issue in Topobiology and combine it with an evolutionary approach, seeking a comprehensive theory of body plan formation and evolution.

The regulator hypothesis[edit]

In 1984, Edelman published his Regulator Hypothesis of CAM and SAM action in the development and evolution of the animal body plan.

"According to the regulator hypothesis, the genes for adhesion molecules (CAMs) are expressed in schedules that are prior to and largely independent of those for cytodifferentiation. The expressed CAMs act as regulators of the overall patterns of those morphogenetic movements that are essential for inductive sequences or early milieu-dependent differentiations. It is proposed that, during evolution, natural selection eliminates those organisms in which variants of CAM gene expression or of morphogenetic movements or of both result in interruptions in the inductive sequence. Under this assumption, more than one (but not all) combinations of these two variables will lead to stabilization of the order of inductive sequences and of the body plan in a variety of species. Moreover, small variations in the pattern of action of regulatory genes for CAMs in those organisms that are not selected against could lead to large changes in animal form within relatively short periods of evolutionary time."[40] – Gerald M. Edelman (1984)

Edelman would reiterate this hypothesis in his Neural Darwinism book in support of the mechanisms for degenerate neuronal group formation in the primary repertoire. The Regulator Hypothesis was primarily concerned with the action of CAMs. He would later expand the hypothesis in Topobiology to include a much more diverse and inclusive set of morphoregulatory molecules.

The evolutionary question[edit]

Edelman realized that in order to truly complete Darwin's Program, he would need to link the developmental question to the larger issues of evolutionary biology.

"How is an answer to the developmental genetic question (q.v.) reconciled with the relatively rapid changes in form occurring in relatively short evolutionary times?"[41] – Gerald M. Edelman, from the glossary of Topobiology

The morphoregulator hypothesis[edit]

Shortly after publishing his Regulator Hypothesis, Edelman expanded his vision of pattern formation during embryogenesis - and, sought to link it to a broader evolutionary framework. His first and foremost goal is to answer the developmental genetic question followed by the evolutionary question in a clear, consistent, and coherent manner.

"The Morphoregulatory (MR) Hypothesis – A hypothesis linking control of epigentic primary processes to a set of genetic elements (morphoregulatory, historegulatory, and selector genes) in order to account for morphogenesis. The linkage occurs via morphoregulatory proteins acting in CAM cycles and SAM modulatory networks. If confirmed this hypothesis would provide the basis for an answer to the developmental genetic question (q.v.)."[42] – Gerald M. Edelman, from the glossary of Topobiology

Building a theory of global brain function[edit]

Atalanta Fugiens (Michael Maier) Emblem 21 – The alchemist were seeking to discover and understand transformations of the spirit via the transformations of matter. A cross-cultural database of observation and ritual accumulated from antiquity.[43] From the Renaissance, thru to the Enlightenment period, the early founders of Chemistry and Physics were alchemists. Their quest for knowledge was rooted in the hedonic needs of the spiritual quest but newly articulated critiques of authority gave rise to the ideas of empiricism[44] and skepticism[45] as the knowledge of the ancient authorities was called into question. That, in turn, allowed for a culling of rituals and an elimination of unsubstantiated data within the alchemical data base – eventually resulting in the birth of the physical sciences as the material quest was separated from the spiritual quest. The scientific method devised, does not resolve the truth but rather eliminates false conceptions, leaving behind only those models that are consistent with reality as experienced. What "Science" as a cultural endeavor achieves is not the "truth", but a better approximation to what we are actually experiencing.[46][47] This in turn allows us to take what is potential in the material world and transform it into the manifest. [c]

"Man has throughout the ages been seeking something beyond himself, beyond material welfare – something we call truth or God or reality, a timeless state – something that cannot be disturbed by circumstances, by thought or by human corruption."[50] – Jiddu Krishnamurti (1969)

Edelman's work recognizes the limitations we are faced with in the quest that the enigmatic 20th century writer, speaker, and philosopher Jiddu Krishnamurti (May 11. 1895 – February 17. 1986) speaks of. Krishnamurti would say that knowledge is limited or bound by the range of one's experience. Beyond that range, the universe is mysterious but, at any moment, it could surprise us with something new and novel. Indeed, he would also intimate that what we do know, constrains the way we perceive the world – and, we can become trapped by the finiteness of our own experience and knowledge-base. He would point out that there is a difference between thought and awareness. Thought is comparing things as they are to what you believe they should be. Awareness is seeing things as they are – unlabeled by preconceptions and expectations. As a mystic, he seeks Freedom from the Known in his quest to know the universal.

For Edelman, adaptive pattern recognition will bypass these grand conceptions and focus on survival using the known in the face of the unknown – for the time-being. The "known" of humans is primarily linguistic in nature. We are story-telling creatures with a long pre-literate history of ancestor knowledge transmission in the oral tradition within culture. But, linguistic thought lies on prelinguistic substrate that is deep and well developed in mammals prior to the advent of language. Language is both constrained by prelinguistic cognitive structures – and, enjoys a limited access to them in their entirety at this moment in time.

Since language is the province of consciousness, Edelman will divide the problem in two – 1) Primary consciousness as the process of creating a unitary Remembered Present which assimilates and accommodates novelty into learning from earlier experience but has no sense of past or future, only a self-centered sense of events in spacetime; and, 2) Higher-order consciousness where the brain becomes massively reentrant and developes a narrative sense of past, present, and future that is reflective, historical, and cultural in nature.

Necessary criteria for a selectionist theory of higher brain function[edit]

Edelman's first theoretical contribution to Neural Darwinism came in 1978, when he proposed his Group Selection and Phasic Reentrant Signalling: A Theory of Higher Brain Function.[4] Edelman lays out five necessary requirements that a biological theory of higher brain function must satisfy.[51]

  • The theory should be consistent with the fields of embryology, neuroanatomy, and neurophysiology.
  • The theory should account for learning and memory, and temporal recall in a distributed system.
  • The theory should account how memory is updated on the basis of realtime experience.
  • The theory should account for how higher brain systems mediate experience and action.
  • The theory should account for the necessary, if not sufficient, conditions for the emergence of awareness.

Cognizing self and non-self – Topobiological neural maps & posture[edit]

Ієрархія кіркової репрезентації
Somatosensory and somatomotor isocortical mapping of the somatic division.[d]
Mapping the sensorimotor-sheets at the periphery – Peripheral nervous system (PNS) diagram

Edelman's background made him well aware that the most fundamental distinction an organism can make is between self and non-self. He considers the key problem for cells, organized into mesencyhme and epithelia within a multicellular animal, is how to organize themselves such that they can act as a unified whole, or "self" within a broader ecology when survival necessitates it. With his understanding of duality of vertebrate anatomical architecture, he focuses on how the nervous system might give rise to a unified sense of "self" from its structure and dynamic interactions with the environment.

Edelman describes the central nervous system as a set of topological maps of the body periphery and muscle groups that become increasingly plastic as we move up from the base of the spine all the way to the midbrain and thalamus.[52][53] From there the thalamus maps to the neocortex via the Thalamo-cortical system, a key point of focus for Edelman's TNGS theory.

The maps are able maintain their topological integrity due the way traveling nerve bundles in development largely preserve their spatial relationships they migrate to contact other cell groups in the body. The maps are formed in development via the neural transmission from the periphery, generated by sensory stimulation from the environment in conjunction with internally generated periodic oscillations, entraining downstream neurons and propagating inward to developing regions of the central nervous system. Topobiological patterning resulfts from stimulating the release of enhancing and/or inhibiting growth factors and synaptic reinforcement amongst the cells and neurons involved – thereby, consolidating and stabilizing the maps.

A second critical component in establishing a sense of self vs. non-self is the motor-ensembles, along with their proprioceptors, which underlie the posture of the organism. Cognition has three primary components – sensorysheet derived stimulation, motor-ensemble driven postural feedback, and the hedonic evaluation of the situation. Posture is how the organism orients itself in the environment based upon its hedonic state and the ecological context. Posture is mediated by the motor ensembles responding to the hedonic evaluation of perceptual experience, and re-sensed via proprioception.

The motor ensembles are topobiologically mapped, just as the sensory maps of the periphery – indeed, the sensory maps are intimately tied to the motor groups that move them, providing an internally-generated parallel signal that can be used, in conjunction with the corresponding sensory signal, to abstract multidimensional perceptual features from the two dimensional sensory sheets relative to the bodies movement in the environment.[54]

These systems of embodied interactive topobiological maps and postural motor ensembles interact at all levels of the CNS, are increasingly plastic as we move up the CNS to the cortex, and are ultimately tied to hedonic pathways emanating from the subcortical and brainstem centers that receive feedback from the viscera and play an important role in an evolving sense of "self" coalescing around a hedonic center as the system adapts to everything else that is non-self.

Perceptual Categorization[edit]

Illustration by Ernst Mach, Inner perspective

For Edelman, the world of signals that the sensory sheets experience is effectively infinite and unknowable ahead of time. The nervous system starts off knowing practically nothing about the world other than the phylogenetically selected ethological patterns of behavior that have emerged over time within a given species. Edelman describes the world of sensory signals emanating from an "object" that hit the sensory sheets as polymorphous – and, of ambiguous importance until selection in the context of hedonic feedback elevates them to prominence in the neural processing of the environment.

"An individual animal endowed with a richly structured brain must also adapt without instruction to a complex environment to form perceptual categorizations or an internal taxonomy governing its further responses to its world"[55] – Gerald M. Edelman (1987)

Edelman defines Perceptual Categorization as "the selective discrimination of an object or event from other objects or events for adaptive purposes".[56]

The problem of animal nervous systems – finite structures in an infinite world[edit]

In Edelman's view, animal nervous systems operate fundamentally in terms of adaptive pattern recognition rather than logic. They are finite structures with a finite resolution on reality and their experience. They cannot know an effectively infinite reality in its totality, therefore they do not attempt to do so because it would be impossibly costly in terms of time and resources.

Animal nervous systems are not logic devices, nor are they truth-seeking devices. Instead, animal nervous systems evolved a strategy of adaptive pattern recognition that allows for the environment to be cognitively sampled and approximated, or "imagined", based upon the finite nature of its experience.

The nervous system is capable of constructing a seemingly infinite variety of cognitively degenerate approximations that are of greater or lesser degree of correspondence to the actual features of reality that they are experiencing.

They are not primarily interested in those models with the closest correspondence to reality, but rather select those models meeting their primary adaptive and/or hedonic needs at any particular point in time and space.

The limits of cognitive models – language is a late-comer[edit]

In The Remembered Present, Edelman reasons that the neuroanatomical structures giving rise to emotive precept, thought, category, concept,[e] narrative, and ecological cognitive modeling was well established in mammals prior to humans developing language.

"The conceptual categorization that emerges prior to language is obviously richer than perceptual categorization but is also enormously enhanced by language. Nonetheless, concepts are about the world..."[57] – Gerald M. Edelman (1989)

Edelman's observes that although language is an enhancer on cognition and behavior, is also a late-comer in the evolutionary scheme of things – and, that much of cognition is mediated by a neuronal group architecture consisting of a nested hierarchy of reentrant sensorimotor and higher-order topobiological maps, operating in conjunction with hedonic feedback systems, that is capable of carrying out a pre-linguistic "state of being aware of things in the world – of having mental images in the present."[58]

Biological consciousness – The problem of novelty and recognition[edit]

One specific goal of the theory is to demonstrate how the subject of consciousness can approached scientifically and in a manner that is consistent with the underlying principles of Darwinian biology. Neural Darwinism ties the process of consciousness directly to the cognitive architecture of the organism and the need to assimilate previously un-encountered phenomena into the organisms repertoire of adaptive responses.

Edelman seems to take two approaches to the issue, consciousness as a state, and consciousness as a process. He seeks to ground the process in the anatomy and physiology of the organism – specifically to neuronal groups within the nervous system and how they perform the task of perceptual categorization from an initially nebulous wave of world signals being received by the sensory sheets in conjunction with hedonic feedback systems when confronted with novelty.

Consciousness as a state[edit]

Edelman acknowledges that human consciousness appears to have evolved a broader range of potential than our animal cousins and he takes time to divide the process of consciousness into Primary and Higher-order so that he can address the uniqueness of linguistic consciousness vis a vis the remembered present of mammalian consciousness more broadly.

"I have made a distinction, which I believe is a fundamental one, between primary consciousness and higher-order consciousness. Primary consciousness is the state of being aware of things in the world – of having mental images in the present. But it is not accompanied by any sense of a person with a past and future. ...higher-order conciousness involves the recognition by a thinking subject of his or her own acts or affections. It exhibits direct awareness – the non-inferential or immediate awareness of mental episodes without the involvement of the sense organs or receptors."[58] – Gerald M. Edelman (1992)

Edelman associates primary consciousness with a state of awareness that is driven by hedonic need, but has little or no explicit sense of an autobiographical self. This type of consciousness is an awareness that is entirely in the moment.

Consciousness as a process[edit]

In addition to being a state of awareness, in Edelman's view consciousness operates as a sub-component process of cognition more broadly. Consciousness functions to deal with novel aspects of our experience which have not been previously encountered and adapted to. Edelman reasons that consciousness is dedicated to the assimilation of novelty and attending to hedonically-salient ecological con-specifics.

Once the novelty has been adapted to and assimilated as habituated reflex, or the basic hedonic need met, the novelty ceases to be novel – and, consciousness is freed up to attend to further novel aspects of the organisms perceptual experience and the ongoing adaption to its environment.

By restraining the definition of consciousness, as a process, in this manner, the term acquires a very specific, finite, and biologically definable form as a psycho-physiological process with an underlying anatomical architecture and evolutionary history, allowing it to be scientifically tested, categorized, and examined. This in turn, should allow conscious states to be characterized as the anatomical-physiological configuration associated with the assimilation of novelty at any particular point in time. If we consider conscious states as occurring in phases, then we can consider the neurodynamics occurring within time frames instead.

The thalamo-cortical system is of particular importance since the neocortex is thought to play an important role in higher order consciousness – and, this is the primary sensory gateway that connects the neocortex to the rest of the sub-cortical system. The connections from the paleo- and archi-cortex provide important input as well. The neocortex itself, will send out a shower of inhibitory outflow that modulates and refines sub-cortical motor actions as it travels down the central nervous system to the distal neuro-muscular motor groups along each tract. In The Remembered Present and later[7][8][9] Edelman argues that thalamocortical and corticocortical reentrant signaling are critical to generating and maintaining conscious states in mammals.

TNGS – The theory of neuronal group selection[edit]

Infinite observer paradox of the homunculus looking at the output of the brain. In this mode of explanation, you need a homunculus to explain each humunculus in an infinite regress. This is known of as the Homunculus Argument in the philosophy of mind.
19th century engraving of Homunculus from Goethe's Faust part II – Goethe's character provided the inspiration for Wilder Penfield and Edwin Boldrey to introduce the term Homunculus into the lexicon of neuroscience[59] in their 1937 publication in Brain – "Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation".[60] The sensorimotor homunculus is a representation of the topobiological maps and the relative representation of each region to the others.

Edelman's motivation for developing the Theory of Neuronal Group Selection (TNGS) was to resolve "a number of apparent inconsistencies in our knowledge of the development, anatomy, and physiological function of the central nervous system."[61] A pressing issue for Edelman was explaining perceptual categorization without reference to a central observing homunculus or "assuming that the world is prearranged in an informational fashion."[61]

To free himself of the demands, requirements, and contradictions of information processing model; Edelman proposes that perceptual catergorization operates by the selection of neuronal groups organized into variant networks that are differentially amplified of their responses in conjunction with hedonic feedback over the course of experience, from within a massive population of neuronal groups being confronted by a chaotic array of sensory input of differing degrees of significance and relevance to the organism.

Edelman outright rejects the notion of a homunculus, describing it as a "close cousin of the developmental electrician and the neural decoder", artifacts of the observer-centralized top-down design logic of information processing approaches. Edelman properly points out that "it is probably a safe guess that most neurobiologists would view the homunculus as well as dualist solutions (Popper and Eccles 1981) to the problems of subjective report as being beyond scientific consideration."[62][63]

Organization of the TNGS theory[edit]

Neural Darwinism organizes the explanation of TNGS into three parts – Somatic Selection, Epigenetic Mechanisms, and Global Functions. The first two parts are concerned with how variation emerges through the interaction of genetic and epigenetic events at the cellular level in response to events occurring at the level of the developing animal nervous system. The third part attempts to build a temporally coherent model of globally unitary cognitive function and behavior that emerges from the bottom up through the interactions of the neuronal groups in real-time.

Edelman organized key ideas of the TNGS theory into three main tenets:

  • Primary Repertoire – Developmental formation and selection of neuronal groups;
  • Secondary Repertoire – Behavioral and experiential selection leading to changes in the strength of connections between synaptic populations that bind together neuronal groups;
  • Reentrant Signaling – The synchronous entrainment of reciprocally connected neuronal groups within sensorimotor maps into ensembles of coherent global activity.

The primary repertoire is formed during the period from the beginning of neurulation to the end of apoptosis. The secondary repertoire extends over the period synaptogenesis and myelination, but will continue to demonstrate developmental plasticity throughout life, albeit in a diminished fashion compared to early development.

The two repertoires deal with the issue of the relationship between genetic and epigenetic processes in determining the overall architecture of the neuroanatomy – seeking to reconcile Nature, Nurture, and variability in the forming the final phenotype of any individual nervous system.

"A central feature of the theory of neuronal group selection is that the mechanisms leading to the formation of both the primary and secondary repertoire are epigenetic: while bounded by genetic constraints, events occurring at both developmental and experiential stages of selection lead to increases with time in both the heterogeniety and spatial diversity of cells and cellular structures. Such events depend upon the prior occurrence of other events in time courses that are long compared with those of intracellular events, and the cells involved exhibit interactive and cooperative spatial orderings that could not have been stored directly in the genetic code."[64] – Gerald M. Edelman (1987)

There is no point-to-point wiring that carries a neural code through a computational logic circuit that delivers the result to the brain because

  • firstly, the evidence does not lend support to such notion in a manner that is not problematic,
  • secondly, the noise in the system is too great for a neural code to be coherent,
  • and third, the genes can only contribute to, and constrain, developmental processes; not determine them in all their details.

Variation is the inevitable outcome of developmental dynamics.

Reentrant signalling is an attempt to explain how "Coherent temporal correlations of the responses of sensory receptor sheets, motor ensembles, and interacting neuronal groups in different brain regions occur".[65]

Primary repertoire- Developmental selection[edit]

Human Neural Developmental Timeline

The first tenet of TNGS concerns events that are embryonic and run up to the neonatal period. This part of the theory attempts to account for the unique anatomical diversification of the brain even between genetically identical individuals. The first tenet proposes the development of a primary repertoire of degenerate neuronal groups with diverse anatomical connections are established via the historical contingencies of the primary processes of development. It seeks to provide an explanation of how the diversity of neuronal group phenotypes emerge from the organism's genotype via genetic and epigenetic influences that manifest themselves mechano-chemically at the cell surface and determine connectivity.

Edelman list the following as vital to the formation of the primary repertoire of neuronal groups but, also contributing to their anatomical diversification and variation:

  • Cell division – there are repeated rounds of cell division in the formation of neuronal populations
  • Cell death – there is extensive amounts of pre-programmed cell death that occurs via apoptosis within the neuronal populations.
  • Process extension and elimination – the exploratory probing of the embryonic environment by developing neurons involve process extension and elimination as the neurons detect molecular gradients on neighboring cell surface membranes and the substrate of the extracellular matrix.
  • CAM & SAM Action – The mechanochemistry of cell and surface adhesion molecules plays a key role in the migration and connectivity of neurons as they form neuronal groups within the overall distributed population.

Two key questions with respect to this issue that Edelman is seeking to answer "in terms of developmental genetic and epigenetic events" are:[66]

  • "How does a one-dimensional genetic code specify a three-dimensional animal?"
  • "How is the answer to this question consistent with the possibility of relatively rapid morphological change in relatively short periods of evolutionary time?"
The neuronal group – Intrinsic and extrinsic connections[edit]

These neuronal groups will be one of the primary units of selection in the formation of the primary repertoire in TNGS.

Secondary repertoire – Experiential selection[edit]

The second tenet of TNGS regards postnatal events that govern the development of a secondary repertoire of synaptic connectivity between higher-order populations of neuronal groups whose formation is driven by behavioral or experiential selection acting on synaptic populations within and between neuronal groups. Edelman's notion of the secondary repertoire heavily borrows from work of Jean-Pierre Changeux, and his associates Philippe Courrège and Antoine Danchin – and, their theory of Selective Stabilization of Synapses.[67]

Synaptic modification[edit]

Once the basic variegated anatomical structure of the primary repertoire of neuronal groups is laid down, it is more or less fixed. But given the numerous and diverse collection of neuronal group networks, there are bound to be functionally equivalent albeit anatomically non-isomorphic neuronal groups and networks capable of responding to certain sensory input. This creates a competitive environment where neuronal groups proficient in their responses to certain inputs are "differentially amplified" through the enhancement of the synaptic efficacies of the selected neuronal group network. This leads to an increased probability that the same network will respond to similar or identical signals at a future time. This occurs through the strengthening of neuron-to-neuron synapses. These adjustments allow for neural plasticity along a fairly quick timetable.

The postsynaptic rule[edit]
Presynaptic modifications[edit]
Transmitter logic[edit]


The third, and final, tenet of TNGS is reentry. Reentrant signalling "is based on the existence of reciprocally connected neural maps."[65] These topobiological maps maintain and coordinate the real-time responses of multiple responding secondary repertoire networks, both unimodal and multimodal – and their reciprocal reentrant connections allow them to "maintain and sustain the spatiotemporal continuity in response to real-world signals."[65]

The last part of the theory attempts to explain how we experience spatiotemporal consistency in our interaction with environmental stimuli. Edelman called it "reentry" and proposes a model of reentrant signaling whereby a disjunctive, multimodal sampling of the same stimulus event correlated in time that make possible sustained physiological entrainment of distributed neuronal groups into temporally stable global behavioral units of action or perception. Put another way, multiple neuronal groups can be used to sample a given stimulus set in parallel and communicate between these disjunctive groups with incurred latency.

The Extended Theory of Neuronal Group Selection – The Dynamic Core Hypothesis[edit]

In the aftermath of his publication of Neural Darwinism, Edelman continuted to develop and extend his TNGS theory as well as his Regulator Hypothesis. Edelman would deal with the morphological issues in Topobiology and begin to extend the TNGS theory in The Remembered Present. Periodically over the intervening years, Edelman would release a new update on his theory and the progress made.

In The Remembered Present, Edelman would observe that the mammalian central nervous system seemed to have two distinct morphologically organized systems – one the limbic and brainstem system which is primarily dedicated to "appetitive, consumatory, and defensive behavior";[68] The other system is thalamocortical reentrant system, along with the "primary and secondary sensory areas and association cortex"[68] which are "linked strongly to exteroceptors and is closely and extensively mapped in a polymodal fashion."[68]

In 1993, Edelman released Bright Air, Brilliant Fire where he elaborated further on his concepts of the Thalamocortical system and the cortical appendages that integrated memory, and realtime function of the cortex with the subcortical architecture of the mammal. He extends the TNGS theory with his Dynamic Core Hypothesis.

The Thalamocortical System[edit]

The Thalamocortical System

The thalamus is the gateway to the neocortex for all senses except olfactory. The spinothalamic tracts bring sensory information from the periphery to the thalamus, where multimodal sensory information is integrated and triggers the fast response subcortical reflexive motor responses via the amygdala, basal ganglia, hypothalamus and brainstem centers. Simultaneously, each sensory modality is also being sent to the cortex in parallel, for higher-order reflective analysis, multimodal sensorimotor association, and the engagement of the slow modulatory response that will fine-tune the subcortical reflexes.

Thalamus – Polymodal sensory integration[edit]

Lawrence 1960 22.4

Cerebral cortex – The isocortical neocortex[edit]

The Cortical Appendages – The organs of succession[edit]

Edelman realized the limits of his TNGS theory and his attempts at replication automata were inadequate to the task of explaining the realtime sequencing and integration of the neuronal group output of the cortex with the subcortical organization of the organism. "Neither the original theory nor simulated recognition automata deal in satisfactory detail with the successive ordering of events in time mediated by the several major brain components that contribute to memory, particularly as it relates to consciousness."[69] This problem lead him to focus on what he called the organs of succession; The Cerebellum, Basal Ganglia, and Hippocampus.

Cerebellum – Archi-, paleo-, and neocerebellar nuclei[edit]

Neuron counts of cerebral cortex and cerebellum – The cerebellum is a key player in integrating the output of the thalamocortical system with the subcortical system. Notice the greater than four-fold abundance of neurons in the cerebellum relative to the neocortex.

The Basal Ganglia – Archi-, paleo-, and neostriatum[edit]

The Hippocampus – Archicortex[edit]

Superior-pattern-processing-is-the-essence-of-the-evolved-human-brain-fnins-08-00265-g0002[70] – Structural features of the brains of mammals are conserved from rodents to humans. The upper drawings show the hippocampal formation of an adult human, a kitten and a young mouse. The lower two drawings show the cellular organization of the cerebral cortex of an adult human and an adult mouse, both of which exhibit six cell layers. All of the drawings are adapted from Santiago Ramon y Cajal (DeFelipe and Jones, 1988). CA, cornu ammonis; DG, dentate gyrus; SUB, subiculum.

Edelman's collaborators, accomplices, and disciples[edit]

Bernard Baars – Global Workspace[edit]

Giulio Tononi – Information Integration Theory and PHI[edit]

The Neurosciences Institute – Building Brain-based Devices[edit]

Despite Edelman's criticism of computational approaches to biology, he has no qualms about applying biology to computation. Edelman's research institute, The Nuerosciences Institute (NSI) in La Jolla, California, has been developing brain-based devices that utilize the conceptions of Neural Darwinism to operate and "program" themselves from the bottom-up.

Darwin III and Nomad[edit]

Darwin III and Nomad were two early versions of brain-based devices that Edelman discusses.

The Scripps Research Institute (TSRI) - The Skaggs Institute for Chemical Biology[edit]


An early review of the book Neural Darwinism in The New York Review of Books[71] by Israel Rosenfield invited a lively response on the part of the neurosciences community.[72] Edelman's views would be seen as an attack on the dominant paradigm of computational algorithms in cognitive psychology and computational neuroscience – inviting criticism from many corners.

There would be copious complaints about the language difficulty. Some would see Edelman coming across as arrogant, or an interloper into the field of neuroscience, from neighboring molecular biology. There were legitimate arguments raised as to how much experimental and observational data had been gathered in support of the theory at that time. Or, if the theory was even original or not.

But more often, rather than dealing with Edelman's critique of computational approaches, the criticism would be centered around whether Edelman's system was a truly proper Darwinian explanation. Nonetheless, Neural Darwinism, both the book and the concept, received fairly broad critical acclaim.

One of the most famous critiques of Neural Darwinism would be the 1989 critical review by Francis Crick, Neural Edelmanism.[73] Francis Crick based his criticism on the basis that neuronal groups are instructed by the environment rather than undergoing blind variation. In 1988, the neurophysiologist William Calvin had proposed true replication in the brain,[74] whereas Edelman opposed the idea of true replicators in the brain. Stephen Smoliar published another review in 1989.[75]

England, and its neuroscience community, would have to rely on bootleg copies of the book until 1990, but once the book arrive on English shores, the British social commentator and neuroscientist Steven Rose was quick to offer both praise and criticism of its ideas, writing style, presumptions and conclusions.[76] The New York Times writer George Johnson published "Evolution Between the Ears", a critical review of Gerald Edelman's 1992 book Brilliant Air, Brilliant Fire.[77] In 2014, John Horgan wrote a particular insightful tribute to Gerald Edelman in Scientific American, highlighting both his arrogance, brilliance, and idiosyncratic approach to science.[78]

It has been suggested by Chase Herrmann-Pillath that Friedrich Hayek had earlier proposed a similar idea in his book The Sensory Order: An Inquiry into the Foundations of Theoretical Psychology, published in 1952.[79] Other leading proponents of a selectionist proposals include Jean-Pierre Changeux (1973, 1985),[67][80] Daniel Dennett, and Linda B. Smith. Reviews of Edelman's work would continue to be published as his ideas spread.

A recent review by Fernando, Szathmary and Husbands explains why Edelman's Neural Darwinism is not Darwinian because it does not contain units of evolution as defined by John Maynard Smith. It is selectionist in that it satisfies the Price equation, but there is no mechanism in Edelman's theory that explains how information can be transferred between neuronal groups.[81] A recent theory called Evolutionary Neurodynamics being developed by Eors Szathmary and Chrisantha Fernando has proposed several means by which true replication may take place in the brain.[82]

These neuronal models have been extended by Fernando in a later paper.[83] In the most recent model, three plasticity mechanisms i) multiplicative STDP, ii) LTD, and iii) Heterosynaptic competition, are responsible for copying of connectivity patterns from one part of the brain to another. Exactly the same plasticity rules can explain experimental data for how infants do causal learning in the experiments conducted by Alison Gopnik. It has also been shown that by adding Hebbian learning to neuronal replicators the power of neuronal evolutionary computation may actually be greater than natural selection in organisms.[84]

See also[edit]


  1. ^ Work by Rodney Porter with the enzyme papain resulted in cleavage of the antibody into Fab and Fc fragments, while work by Gerald Edelman lead to the reduction of the disulfide bridges so as to separate the molecule into light- and heavy-chain fragments. Together, this work allowed the antibody structure to be sequenced and reconstructed, resulting in the awarding of the Nobel Prize in Physiology or Medicine in 1972.
  2. ^ This figure is from Hill & Wang 2020, The importance of epithelial-mesenchymal transition and autophagy in cancer drug resistance. The connection between the cellular processes of cancer and the cellular processes of embryogenesis are not far fetched. These are the very processes that were undergoing selection at the origin of animals as they transitioned from unicellular organisms to multicellular organisms.[34][12] Cancer is an example of cells that have escaped the developmental constraints of the multicellular organism and reverted to their ancestral pattern of immortal reproduction in the absence of the inhibitory mechanisms on reproduction that proper bodyplan formation requires. One could say that cancer is a problem of morphology.
  3. ^ Culture is ancestor knowledge transmission. Adaptive pattern recognition in humans is as much social as it is individual. Newborn human babies are symbolically enculturated through the process of socialization where, reciprocal co-engagement between caregivers and children entrain their emotional responses, cognition and posturing to the shared emotive and cognitive evaluation of the community. Cognitively, there are two fundamental sides to a symbol, the somatic which is tuned to the environment and is often visual, auditory, or postural in nature; and, the visceral which is tied to hedonic value and is reflected in consciousness as feelings or emotions.[48][49] Emotion could be seen as a form of non-linguistic thought conveying the awareness of hedonic evaluation to consciousness.
  4. ^ Careful observation of the body part distribution and area will reveal that the cortex is primarily dedicated to two major efforts: 1) to a refined, integrated and reflective sensory analysis that modulates the reflexive responses of subcortical sensory processes brought about as early mammals completed their adoption to land and adaptively reorganized their primary senses into a second level of integration within the nervous system; and 2) the modification of pre-mammalian motor routines into distinctly mammalian postures as the limbs are brought under the body carriage and the head-neck/pharyngeal-arch region is remodeled relative to that of their amniote ancestors. Inhibitory outflow from the cortex will play an important role in modulating subcortical, brainstem, and spinal motor routines as mammals adapt to the new body postures. One of the major modifications of the spinal motor-pattern will be the transition form side-to-side undulatory motion (rooted in the fin ancestry of the limbs) to the vertical undulatory motion (associated with bringing the limbs under the body carriage in early mammals).
  5. ^ Conceptualization is prelinguistic and the number of concievable concepts exceeds the number of available words in any language, which is why new words are being coined all the time. Language does not create concepts but rather helps us to label and describe our experiences as feelings, sensations, perceptions, categories, concepts, narratives, and cognitive appraisals. Language is not required for conceptualization, but rather allows concepts to become socially transmitted and become a form of cultural cognition. In the process, it sets certain constraints on the way that concepts can be expressed through language.
    Concepts are the building blocks of cognitive modeling. Linguistic cognitive models have this in common:
    • A set of fundamental assumptions about the world, the basic precepts of reality,
    • A circle of reasoning and insight through application of the assumptions, guided by instinct, intuition, logic, or rationality,
    • A limit of validity determined by the legitimacy of the underlying assumptions,
    • A set of paradoxes and contradictions associated with exceeding the limit of validity for one or more of the primary assumptions. The contradiction can occur internally via logical paradox, or conflicts between the model's predictions and the empirical reality of experience. Both arise when the limit of validity for a fundamental assumption has been exceeded.
    • And, eventually, a paradigm shift resulting from the modification, addition, or abandonment of fundamental assumptions that leads to a better set of assumptions with a wider range of validity.
    This is an adaptive process that can be run from scratch or iterated, but it is also very uncharacteristic of an artifact designed to capture reality as it "is".


  1. ^ Edelman & Porter 1972.
  2. ^ Edelman 1987b.
  3. ^ Mountcastle & Edelman 1978, p. 7-50, An Organizing Principle For Cerebral Function: The Unit Module And The Distributed System.
  4. ^ a b Mountcastle & Edelman 1978, p. 51-100, Group Selection and Phasic Reentrant Signalling: A Theory of Higher Brain Function.
  5. ^ Darwin 1859.
  6. ^ Edelman 1988.
  7. ^ a b Edelman 1989.
  8. ^ a b Edelman 1992.
  9. ^ a b Edelman & Tononi 2000.
  10. ^ Edelman 2004.
  11. ^ Edelman 2006.
  12. ^ a b Tauber & Chernyak 1991.
  13. ^ a b Dehal & Boore 2005.
  14. ^ a b Edelman 1987a.
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Further reading[edit]

External links[edit]