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 Gerald Maurice Edelman (July 1, 1929 – May 17, 2014). Edelman's 1987 book Neural Darwinism introduced the public to the theory of neuronal group selection (TNGS), a theory that attempts to explain global brain function.
TNGS (also referred to as the theory of neural Darwinism) 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, which describes the columnar structure of the cortical groups within the neocortex, and argues for selective processes operating among degenerate primary repertoires of neuronal groups. The development of neural Darwinism was deeply influenced by work in the fields of immunology, embryology, and neuroscience, as well as Edelman's methodological commitment to the idea of selection as the unifying foundation of the biological sciences.
Introduction to neural Darwinism
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 into a detailed model of real-time neural and cognitive function that is biological in its orientation. It is built from the bottom up utilizing the variation that shows up in nature. This is in contrast to computational and algorithmic approaches 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 (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 (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 (1992), A Universe of Consciousness – How Matter Becomes Imagination (2000) with Giulio Tononi, Wider Than The Sky – The Phenomenal Gift of Consciousness (2004), and Second Nature – Brain Science and Human Knowledge (2006).
Neural Darwinism is an exploration 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. In the course of laying out the case for neural Darwinism, or more properly TNGS, Edelman delineates a set of 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 capable of supporting primary and higher-order consciousness.
Population thinking – somatic selective systems
Edelman was inspired by the successes of fellow Nobel laureate 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 adaptation 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 adaptation is preexisting diversity". – 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
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.
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.
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 produced in the aftermath of a set of whole genome duplication events at the origin of vertebrates 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
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 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.
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. In this arena, redundancy fails - a response might be designed to the wrong problem.
Variation fuels degeneracy; degeneracy provides somatic selective systems with more than one way to solve a problem and the propensity to reuse a solution on other problems. This property of degeneracy makes the system more adaptively robust in the face of unforeseen contingencies: When one particular solution fails unexpectedly, there are other unaffected pathways that can be engaged in pursuit of the same end. 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
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.