Embryogeny and Phylogeny of the Human Posture 2. Anne Dambricourt Malasse
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1.3. Presuppositions
1.3.1. The Neanderthal Man, a stooped posture?
The degree of axial straightening of the Neanderthal can finally be compared to Homo sapiens. The base of the skull was not complete, but it was sufficient to compare the angles of verticalization, with the vertebrae and sacrum. The frontal curvature seemed to be lower than in Homo sapiens because of the particularly pneumatized (“inflated”) supraorbital torus above the orbits, the adult facial block was in front of the frontal lobes and not below them as in the Cro-Magnon Man. The brain was more voluminous, but the impressions of the lobes were different and the prefrontal cortex was especially less developed. The cerebellar lobe was clearly higher than that of the Cro-Magnon Man, it was also smaller in absolute value and in relative value to the brain, which corresponded to a less verticalized clivus and a more open sphenoidal angle. The face was prognathic, the mandible had no chin. The geometric protocols in the median sagittal plane confirmed a general straightening movement, but it was not as pronounced as in Homo sapiens (Figures 13 and 14 in Boule (1911–1913)). The reconstitution of the profile of the spine with isolated vertebrae did not show cervical and lumbar lordosis (forward curvature), so it was not sinusoidal like Homo sapiens. The femurs were curved, more stocky.
The curvature of the femurs gave the impression of a body in a forward bent position, which was accentuated on the La Chapelle-aux-Saints Man due to a congenital dislocation of the left hip, but which really did exist and was systematically found in Neanderthal patients. The general conformation of the skull with high neck and facial prognathism, as well as the slightly flexed profile of the thighs, gave a general cranio-spinal appearance intermediate between that of the great ape and Homo sapiens. Neanderthal Man thus appeared to be less vertical and less cerebralized than the Cro-Magnon Man. The appearance reflected an anatomy and cerebral development of an evolutionary stage preceding that of Homo sapiens. However, Boule did not venture to interpret the brain’s imprints in terms of cognitive capacities beyond what would be possible with analogies with pathological conditions of current Homo sapiens.
1.3.2. Embryologists, geneticists and paleontology at the beginning of the 20th century
Molecular genetics would become a game-changer, but it was still barely studied. Its development would displace comparative embryology from paleontology, and would allow scholars to reason on the systemic and dynamic scale of the organism, and thus reflect on evolutionary processes, with an opening to complex systems science from gametogenesis. Disciplines would not lose sight of morphogenesis with biomathematics and thermodynamics, they would make it possible to preserve the scientific method of the study of the processes of the evolution of forms, the theoretical and thus mathematical formalization of the changes in dynamic trajectories that set up the structures and organs.
Wilhelm Johannsen (1857–1927) was a Danish botanist. In 1909, he created the word gene as opposed to pangene or “gemmules” from Charles Darwin (see Volume 1). The gene is a discrete unit. In 1911, he proposed the terms genotype for germ cell lines, and phenotype for soma cell lines. Mendel’s laws, which had gone unnoticed since 1866, were rediscovered in botany and independently by the Dutchman Hugo de Vries (1848–1935), who created the word mutatio, the German Carl Correns (1864–1933) and the Austrian Erich von Tschermak (1871–1962). In France, genetics was very slow to develop. Lucien Cuénot (1866–1951) was one of the very few geneticists at the French University. Professor of Zoology since 1898 at the Faculty of Sciences of Nancy, his research took up Mendel’s laws and studied them in mammals for the first time, with crosses of gray and albino mice. In 1902, he confirmed the law of transmission for the variability of a characteristic, which was common to both plants and animals. Cuénot then became one of the main actors in the development of genetics.
The nucleus of cells contains macromolecules called chromosomes, themselves composed of these discrete entities called genes, but their behavior during reproduction remained a black box. The first experimental studies aimed to produce mutations, and were carried out in 1908 by the American geneticist Herman Müller (1890–1967). He had the idea of exposing drosophila to X-rays and making them reproduce with unexposed drosophila. This invisible light was discovered at the University of Würzburg in November 1895 by the physicist Wilhelm Röntgen (1845–1923). The results were convincing since some drosophilas were born with malformed wings. X-rays made it possible to infer not only a role of sex cells in heredity, but also to understand that the memory of heredity was on the scale of X-rays, in other words, on the scale of the bonds between molecules, the scale of the forces of attraction that make up these radiations. Physicists did not yet know the structure of chromosomes, nor the nature of X-rays. These were photons, electromagnetic waves. In the future it would be confirmed that it is the segments of chromosomal macromolecules that are altered.
The discovery of hereditary memory on the scale of electronic links brought back the great questions about generation that René Descartes himself pondered, incredulous about the infinite dimension beyond the smallest atoms, as well as Isaac Newton, for whom the beginning of any organization, whatever the scale of space-time, was a question of forces of attraction.
The new doctrine of generation was Hugo de Vries’ “mutationism”, which no longer concerned the organism, but the gene. Genetics gained momentum from the 1920s onward with Ronald Fisher (1890–1962), John Haldane (1892–1964) and Sewall Wright (1889–1988). If mutations in these discrete entities, or genes, had no causal link with an adaptation constrained by the environment, then the Lamarcko-Darwinian doctrine would collapse. If X-ray experiments were to be considered as the model of evolution, the primary cause would be a simple random perturbation of the duplication of electronic bonds between molecules forming chromosomes. The evolution of embryogenesis would be reduced to a sequence of genetic copying errors, which was not consistent with observations in paleontology and comparative anatomy. Flies without wings or with malformed wings are unfit to fly, in nature they starve and do not transmit genetic copy errors, they are abortive.
The mutationist model of the drosophila did not correspond to the level of embryonic organization of the fly, since the fly is not transformed. These mutations did not correspond to the problem of the neuroaxis straightening, because simian embryogenesis was not a variety of the embryogenesis of a prosimian, it was its reorganization with a complexification of the central nervous system.
The German embryologist Hans Spemann (1869–1941) thus resumed van Baer’s research on vertebrate embryogenesis since gastrulation. A Doctor of Medicine at the University of Heidelberg, he continued his research on the embryogenesis of batrachians, in order to isolate and better identify the mechanisms that preside over the differentiation of the three embryonic layers: the ectoderm (future nervous system and part of the dental germs), the mesoderm (from which the dorsal cord is formed) and the endoderm. In 1924, thanks to cell transplants, he revealed a very early territory in the cell cluster derived from the blastula, which he named the “organizing center” with mechanisms qualified as “induction”. The Nobel Prize in Physiology or Medicine was awarded to him in 1935 “for his discovery of the organizing effect in the development of embryos”. Such was the state of the question on the generation of the organizational plan of a species, when human paleontology and prehistory took off. That said, Neanderthal embryogenesis was not a subject for reflection and the orientation of its dorsal chord was not even conceptualized.
1 1 Othniel Charles Marsh (1831–1899), nephew of the philanthropist George Peabody (1795–1869) was one of the first American paleontologists. He was the founder of the Yale Peabody Museum of Natural History (Yale University, New Haven, Connecticut).
2 2 Large marine mammal, belonging to the dugongidae family.
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Asia, The Cradle of Humanity
2.1. Teilhard de Chardin, a destiny from Piltdown to the Muséum
Marcellin Boule was in contact with French naturalists