Embryogeny and Phylogeny of the Human Posture 2. Anne Dambricourt Malasse

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СКАЧАТЬ of the external appearance of the skull of the tarsier. All of the skulls had a long muzzle aligned in the extension of the spine, except the tarsier, which gave the impression of a globular neurocranium, a very short face and a great occipital hole (foramen magnum) tilted forward and downward. The approach to simians was built on these adult exocranial appearances, without considering their embryonic morphogenetic trajectories.

      The internal base and the sphenoidal angle were ignored because observation and measurements were not conceivable; they would destroy the integrity of the fossils, which were rare. As for Broca’s measuring instruments, they could only be used for very large skulls. Therefore, the sphenoidal angle and the straightening of the base would not be studied by paleontologists and would not enter into phylogenetic criteria. The logic of comparative embryonic chronology, which should have developed after Haeckel, was not to have a future in paleontology. Today, nomenclators (classifiers) no longer speak of prosimians, and they distinguish at least two lineages, one that brings together species with long snouts and small orbits, the other that brings together species with very large orbits and seemingly have short snouts. The first are the Strepsirrhini (e.g. Cuvier’s Adapis and lemurs), the second are the Haplorrhini with the tarsier and a fossil species like Omomys (North America). The nomenclators then explained that the simians were also Haplorrhini. In other words, the classifiers invented a category based only on adult exocranial features, without being interested in the orientation of the clivus, which was not taken into consideration. The origin of its straightening was left aside by paleontologists and therefore ignored, only the orientation of the occipital hole counted, as in Daubenton’s study.

      However, these analogies with simians were only made due to the disproportionate development of the orbital cavities; the skull disappeared under two enormous eyes, while in the median sagittal section the face was no less prognathic than a Loris and the primordial cranium (the base) was flat. It is the development of the embryonic optic capsules that disorganizes the shape of the forehead and thus gave the impression that there is no muzzle (see Chapter 5). These differences have nothing in common with the embryonic mechanisms that straighten the dorsal cord and separate the simians, or Simiiformes, from the prosimians, which in fact include Tarsiiformes and their ancestors.

      If Georges Cuvier were still alive, he would have adopted the same posture as that of Etienne Geoffroy Saint-Hilaire who could not distinguish between analogy and homology. Consequently, as we have seen, the latter did not conceive of the need to take into account, in the nomenclature, the primary reality of the organizational plans structured from embryogenesis. Yet, these plans did not fit into each other, they emerged from each other. Georges Buffon would have said that the Haplorrhini category was a view of the mind, that it had no basis in reality. There was no cranio-spinal embryogenesis common to tarsiers and simians.

2.3. Teilhard de Chardin sets out the main phylogenetic principles

As long as the embryonic organization of primates was not established, the place of tarsiers in human phylogenesis would continue to divide nomenclators. Teilhard considered that he had solved this primordial question with his thesis. He cleared a fragment of mandible and skull (maxilla and orbits) from a gangue of sediment preserved in Montauban. These remains resembled the tarsier and no such specimen was more complete: the rare fossils were fragmented. The size was very small in comparison, but the fossil resembled it with the oversized development of the orbits. Teilhard then made the very first comparison between Eocene primates from North America and Europe (see the geological scale, Figure 2.1). He concluded his study by considering this fossil a representative of the tarsier lineage and he put forward a reasoning that would mark the Teilhardian phylogenetic logic.

      Fossils are discovered much later than the emergence of the type, because it takes time for generations to reproduce, to expand geographically before reaching a sufficient population density for the probability of finding a fossil to begin to be relevant. In other words, and this is a matter of common sense, the collected fossilized species are already specialized and not generalized and confused with the ancestral morphotype. As a consequence, the probability of finding the earliest example of a stem population is almost zero, so much so that Teilhard called this the “blank of birth” in the paleontological archives of a new lineage.

      Teilhard claimed his conclusions without waiting for his thesis. He published them as early as 1921 in the “Annales de paléontologie” founded by Marcellin Boule and presented to the Institut français d’Anthropologie (French Institute of Anthropology) on March 16, 1921, under the presidency of Boule, who gave him the full weight of his authority. This Institute was a learned society founded in 1910 on the initiative of the ethnologist Paul Rivet:

      The question arises as to whether the Tarsiidae (in the new sense) should not be more completely removed from the group of lemuroid forms, to make a sub-order between Lemuroidea and simians. Their line of evolution now appears so defined, so independent of that of the other Primates, that one is tempted to give them this exceptional place. But then one sees, once again, how much we must project far back in the past, the date of the dispersion of the Primates [...]. The small Malaysian Tarsier, which zoologists did not know whether they should designate as a Lemur or a Monkey for a long time, appears decidedly today as an independent and isolated type in nature; it is the only survivor of an important group which covered North America and Europe before the Oligocene. (Teilhard de Chardin, Sur l’origine tarsienne de l’Homme, 1921a, author’s translation)

This conclusion is today accepted by the international community. The oldest lineages of primates dating back to the Eocene can be divided into two phylums: on the one hand, that of the current families of Lorisidae, Galagidae and Lemuridae, united in Strepsirrhini, and, on the other hand, that of Tarsiidae. Both phylums derive from an even older primate population that dates back at least to the Paleocene (59 million years ago) (see geological time scale, Figure 2.1).

Figure 2.1. Geological time scale of Tertiary and Quaternary eras, International chronostratigraphic chart 2017 (source: Institut Cartogràfic i Geològic de Catalunya). For a color version of this figure, see www.iste.co.uk/dambricourt/embryogeny.zip

      Teilhard continued his reasoning: first, the fossil families of Strepsirrhini were already all specialized, and, second, the tarsier type had been stabilized since the Eocene (35 million years ago); thus, there was necessarily a third non-specialized lineage able to transform into Simiiformes. The species of this lineage were said to be “generalized”. This notion of generalized lineage at the origin of the lineage of simians (Simiiformes) was absolutely crucial, and it would be taken up and developed by Teilhard’s successors, such as Jean Piveteau.

      Hominization was possible because species did not specialize. The criterion that Teilhard used to identify the evolution of these “generalized” species was the organ that compensated for a lack of specialization. This organ was the central nervous system with the development of cognitive abilities. In other words, the guiding thread was encephalization. Obviously, the reference is Homo sapiens, which did not have specialized subspecies, but a plasticity adaptive to all environments, including the most extreme in latitude, altitude and depth. Sapiens did not need wings, fins or gills, it compensated with its technical intelligence. Later, Teilhard would be surprised to note that the cranial globular shape of the tarsier did not correspond to an encephalization, because the cerebral hemispheres were as smooth as those of a Strepsirrhini. In reality, this shape was not due to the brain but to the exuberant lateral extension of the orbits. His hypothesis of a third Eocene phylum prompted СКАЧАТЬ