Tuesday, September 20, 2022

Self-Organization in Evolutionary Biology

 



Evolution is neither linear nor does it depend on external factors.

Evolution proceeds by quality 'jumps' when certain internal thresholds are reached.

Complex systems have principles of self-organization, processes at the origin of the organization of dynamic structures and their reproductive properties, from the smallest cellular unit to the most complex interactions within the organism and then between organisms for the same unit of time and space.



Prologue:

The Last Threshold

Anne Dambricourt Malassé

 

MAN HAS BEEN a part of the animal kingdom for more than three million years. Yet many cultures have seen man as quite separate, or excluded man entirely, from nature. We see this among monotheistic societies that nurture theological explanations for the origin of our species. Prior to the Scientific Revolution, theological considerations served largely to constrain inquiries in this area. The naturalist movement that emerged during this period enabled some finally to break free of scriptural assumptions.

Jean-Baptiste Lamarck held the chair in invertebrate zoology at the Muséum national d’Histoire naturelle (MNHN) in Paris, when he published Recherches sur l’organisation des corps vivants (Research on the Organization of Living Bodies) in 1802. This small, but revolutionary, book contained the first theory of evolution applied to the origins of Homo sapiens. In this view the progressive straightening of the vertebral column seen in the first hominids was driven by climatic changes that forced some apes to adopt walking over arborealism. This idea was then popularized by Charles Darwin in The Descent of Man, published in 1871. It is now the consensus in paleoanthropology.

A very different reality is reflected in the collections of the MNHN, on which I have based my work. On the contrary, straightening, with its correlated anatomical and psychomotor changes, is an intrauterine process that took place over the course of more than forty million years. It resulted from a growth in complexity of the embryonic nervous system and its rotational dynamics, and led to a succession of threshold effects incompatible with the nested hierarchy of Linnaean classification.

The identification of the first hominids is flawed.

(…)

NEURAL STRAIGHTENING FROM Australopithecus to Homo is linked to the development of the nervous system. This explains the straightening of not only the base of the skull but also the whole vertebral column. The neural tube is formed by fibers lengthening along its path above the notochord. Complex movements at stage nineteen of embryonic development, corresponding to the axis of rotation, can be seen above the point at which the dorsal cord terminates. Hominization begins precisely at the cephalic limit of the dorsal cord.

Remarkable.

The sole vertebrate embryo in which the dorsal cord extremity is almost verticalized is that of Homo sapiens. This is a process that began around thirty-nine million years ago in an Asian species of prosimian that underwent a contraction in the base of its skull and a declination of its brain stem. This produced the first degree of neural straightening and cranio-facial contraction in the simians. Twenty-three million years ago, at least one African species of small gibbon-like simians underwent further contraction and declination.

This produced the second degree of neural straightening. The embryonic dorsal cord was almost vertical among many species of great apes, remaining so until adulthood. This was presumably the case, at least, with respect to Australopithecus (4.5–1.977 mya). Thereafter the process accelerated, at an unprecedented rate. The lowered cerebellum and straightened brain stem is that of Homo sapiens, which Linnaeus named in 1758 and which emerged in East Africa 160,000 years ago. The evolutionary trajectory follows the straightening of the dorsal cord, but during the first stages of verticalization there was no dramatic accompanying increase in brain volume. Cranial volume is thus no longer the benchmark, or rubicon. The benchmark is, in fact, the straightening of the skull base.

(…)

WHEN AUSTRALOPITHECINE EMBRYOGENESIS emerged around 4 mya, the new organism was no longer a subspecies of great ape. It did not share a great ape’s neural embryogenesis, still visible today in gorillas, chimpanzees, and orangutans. The same is also true for Homo embryogenesis. There is no bone conformation that can be nested within previous embryonic organizations. The shapes of the structures are reorganized and innovations have appeared. This dynamic is typical of emergence in complex systems.

Sapiens embryogenesis is no longer that of any fossil Homo species. The axial skeleton reveals a new threshold of neural straightening and craniofacial contraction, with an increased complexity and reorganization of the central nervous system. The cerebellum is shifted even further toward the front and down. The occluso-postural equilibrium and the psychomotor development visible in, for example, Neanderthals, has also changed.

As Buffon and Cuvier observed, there is no unique pattern of anatomical organization among primates. The idea that there have been gradual transformations within the limits of a such an organization is absurd. This would mean that Homo sapiens has gone through all the strata of evolution—prosimian, ape, great ape, hominin. Homo sapiens is not a prosimian, nor is it a great ape. The conclusion that Homo sapiens is a primate indicates that prosimian innovations have been preserved. But it does not signify evolution within the limits of the same embryonic organization. Sapiens never passed through the stages of phylogenesis.

Linnaean classification is invisible in our embryogenesis.

If the word Homo must designate the paleoanthropes, then our embryogenesis is no longer Homo, but sapiens. This is true for each of the five great embryogeneses, or fundamental ontogeneses, since the emergence of Simian; each emerged from the preceding one, each time more complex in its neural organization. Numerous subspecies appear within the limits of the new axial embryogenesis, with different modes of locomotion and ways of life. Buffon, Cuvier, de Quatrefages, and Broca were right.

 

Conclusion

IT IS NOTHING short of remarkable that the ability to create second-degree stone tools emerged from the threshold of embryonic neural verticality. This is not an arbitrary boundary for distinguishing between Homo and other hominins, as is the case with the notion of a cerebral rubicon. The threshold is objective and allows for the deduction of a reorganization of the nervous system and its component neuronal networks with the sensors necessary for controlling the body’s equilibrium. In Homo sapiens, the connections between the cerebellar and cerebral neocortex are known, and it appears they participate in high-level cognitive functions, for example memory, dexterity, language, and reflection. Gestures such as walking and grasping become conscious with psychomotor development.

The great novelty here is the sudden change in posture of the cerebellum, and a new neuronal complexity; the cerebellum had to control its own balance. A new loop of complexity must have developed between the neocortex of the cerebellum and the brain. These connections could then have favored the development of new reflective cognitive capacities associated with movements, those of the hands in particular.

New manual chains of operation reflect a symbolic and conceptual level of thought attributed to the brain of the genus Homo. My suggestion is that the emergence of these capacities should be broadened to encompass the hominin stage, denoted by the verticalization of the cerebellum, such as for Australopithecus, Kenyanthropus, and Paranthropus. Although their brains were smaller than that of Homo habilis, they may have been capable of conceptual and creative innovations. Passing those first thresholds made possible the creative expression of ideas and concepts.

Crossing the final threshold, Sapiens, amplified those cognitive properties.




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