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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 him to conceive of a simiiforme lineage as early as the Eocene (thus before the Oligocene, which saw the appearance of simians or monkeys):

If, even before the Oligocene, the group of Tarsiers, which seems to be interposed between the Lemurs and the Simians, was approaching maturity, it must be assumed that, from that time on, the group of the superior primates¹ was in full growth. This gives depth to our views on the group to which we belong: a Tarsier found, almost entirely formed, in the Phosphorites may mean that, somewhere on Earth, at that same time, Anthropoids were already sketched out [...]. They come, independently of each other, from a still unknown group of very small animals with a large brain which must have lived in the Paleocene, or even an earlier epoch. (Teilhard de Chardin, Sur l’origine tarsienne de l’Homme, 1921, author’s translation)

This theory of a third phylum of large-brained Simiiformes, already differentiated at the end of the Secondary era, was the reasoning of Teilhard with two evolutionary levels, that of the trunk or the succesion of morphotypes illustrated by generalized (not specialized) species, and that of the branches, or the specialization of species which have diverged from the trunk. The trunk was the dynamic of encephalization, while the divergences were the specializations that stabilized the organism and slowed down the complexification dynamics of the central nervous system. The trunk concerned the generalized species which would transmit this process of complexification of neurogenesis and followed an acceleration from the final Paleocene. And this trunk was the ancestral lineage of Man which became its apex and the current process of neural complexification.

The theory of a Paleocene simiiforme phylum has never been corroborated.

Teilhard’s paleontological study nevertheless marked a major milestone in paleoprimatology, and the American George Gaylord Simpson (1902–1984), one of the great figures of 20th century in paleontology, paid tribute to him by giving his name, Teilhardina, to the oldest fossils of primates that lived in Asia, Europe and North America, from the Upper Paleocene to the Lower Eocene (between 56 and 47 million years ago). Today, the oldest primates are dated to 55 million years and were discovered in central China in 2003 in Hubei and in 2004 in Hunan. The fossils are kept at the Institute of Vertebrate Paleontology and Paleoanthropology of China, in Beijing. The first fossil from Hubei is an almost complete skeleton of very small size (7 cm and 20 cm with the tail) named Archicebus achilles (Ni et al. 2004). The orbits are small and the appendicular skeleton is that of an arboreal primate. The second fossil Teilhardina asiatica from Hunan has preserved only fragments of the skull and mandible.

As long as the chordal skeleton is not straightened and since locomotion is not the mechanical cause of straightening, these locomotor adaptations do not allow us to speak of simiiforme features. It is the simians, characterized by the straightened embryonic cord, that have inherited these arboreal characters. It would be interesting to compare the axial endoskeleton of Archicebus with the current species of Loris in Southeast Asia. The last study in 2019 confirms, using dental characteristics, the distinction between these two Chinese fossils (Morse et al. 2019).

Pierre Teilhard de Chardin defended his thesis entitled “Les mammifères éocènes inférieurs français et leurs gisements” (“French Lower Eocene mammals and their deposits”), which earned him international renown, at the Sorbonne University in April 1922. He was 41 years old and received the prize of the Société géologique de France (Geological Society of France) and the Roux prize of the Académie des sciences. He was elected President of the Société Géologique de France in 1926.