Evolution and humans: big brain size or use (Evolution)

by David Turell @, Saturday, June 24, 2017, 20:43 (269 days ago) @ David Turell

A review article of Homo brain development from studies in embryogenesis. The entire article should be read for full understanding and traces preliminary changes for upright posture back to about 39 million years ago, involving changes to the base of the skull which are required for bipedalism:


At the end of the twentieth century, the theory that the origins of upright posture originated in climate change was abandoned by Phillip Tobias, a co-author of Leakey’s famous 1964 paper. East and South African habitats, Tobias argued, were a mosaic, ranging from gallery forests to woodlands and savannahs. Ten years later, paleontologists exploring the Ethiopian Rift recognized that Australopithecus afarensis and their quadruped arboreal predecessors, Ardipithecus, also lived in a mosaic environment.

If so, what was the engine driving the development of upright posture?


These embryological details have dramatic consequences. The emergence of upright hominid posture need no longer be linked to habitat changes. Its origin must be attributed to the increasing complexity of the nervous system. The embryonic body plan was reorganized through a series of threshold effects which are still in evidence in every human embryo.


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.


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. (my bold)

Comment: Absolutely, size first, use second! God's preplanning at work! Note my bold. All of my theories come from this article among others. Don't think I've presented this before.

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