EVOLUTION AND PURPOSE: bipedalism (Evolution)

by David Turell , Monday, April 17, 2017, 18:06 (2559 days ago) @ dhw

Here is a very long essay on the real start of bipedalism 23 million years ago based on skull changes in a monkey species. The point is made that although we resemble apes to some degree we are really a very separate genus.

http://inference-review.com/article/the-last-threshold

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.

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.

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On the Linnaean system of classification, it should be possible to distinguish a structure that elevates Homo sapiens to the rank of genus when compared to the characteristics of the species. Homo anatomy must appear gradually before any sapiens characteristics. For obvious scientific reasons the genus Homo must be rigorously defined. The defining characteristics of the skeleton of Homo sapiens are, in fact, linked to the degree of verticality in the brainstem and spinal cord. Homo corresponds to a verticalization now visible only in sapiens.

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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.

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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).43 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.

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The trajectory of evolution from the first primates to Homo sapiens is defined by the increasing complexity of the nervous system. But this process was not gradual and not limited to the cerebral hemispheres. Neural embryogenesis increased in complexity, while the supporting tissues that would become the skeleton were transformed. Hence the thresholds and angular discontinuities. This is the process at the origin of neural straightening, in particular that of the cerebellum.

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Considering the relevant parameters, it appears that Homo embryogenesis could not have been derived from australopithecine embryogenesis:
The earliest angular values of the embryonic contraction (the accordion phase) of the Homo base are identical with those of australopithecines.

As can be seen in Paranthropus, further evolution of the australopithecine skull base occurred 1.8 million years ago (mya), separately, in eastern Africa (Paranthropus boisei) and South Africa (Paranthropus robustus), exhibiting a maximal embryonic accordion-like compression.

The Homo and sapiens embryonic thresholds are separated by at least 3 million years.

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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.

Comment: this article presents that preparation for H. sapiens began over 23 million years ago in the base of the skull. I note another area of research in my book, The Atheist Delusion , on page 258 which notes lumbar vertebral changes in a monkey 23 million years ago in preparation for upright posture. Pre-planning. God in control.