In response to: “The Last Threshold” (Vol. 3, No. 1)

To the editors:

I am very pleased to offer a critical reading of Anne Dambricourt’s essay “The Last Threshold,” especially as my own research on the Homo sapiens of the past one hundred thousand years takes a biodynamic and architectural approach close to that defended and presented here.

“Linnaean classification is invisible in our embryogenesis.” This sentence illustrates the importance of the subject at hand and, paradoxically, the ambiguity between taxonomy—i.e., the classification of organisms, both fossil and living, vertebrates and invertebrates—and the plans of organization of fossil species in vertebrate paleontology, which is a matter of macroevolution. Dambricourt’s theory is saltationist, and opposed to gradualism. I will return to this in more detail below.

But first of all, what is the theory? The point of departure is a phenomenon called occipital rocking in hominids. Dambricourt uses the term cranio-facial contraction (or cranio-facial plicature). Seen from the perspective of skeletal anatomy, one can understand how the bony parts of the base of the skull rotate. Most paleoanthropologists, however, do not fully understand this dynamic mechanism. Only dental-facial orthopedists, a few orthodontists, and postural engineers come to know the stages of human craniofacial and dental development, through their defects.

As it happens, since her degree from the Institute of Human Paleontology in 1983, Anne Dambricourt has addressed the external structures of skeletal organization, such as craniofacial form and function and the chewing apparatus. But she has also studied the internal structure: the deep structures of the craniofacial morphogenetic organization, in particular the sphenoidal and cerebellar fossae, the position of the semicircular canals, the verticality of the central nervous system during embryogenesis and perinatal and adult ontogenesis. Most of all, she has scrutinized the mechanism of embryogenesis over the course of human evolution more closely than any other paleoanthropologist.

The craniofacial contraction is initially embryonic, and is linked to the magnitude of neural rotation and ultimately to the duration of neurogenesis. In those species, from prosimians to Homo sapiens, on which Dambricourt bases her theory, there is a succession of six evolutionary stages, called fundamental ontogeneses, which emerge one after another according to the theory of punctuated equilibrium. This theory, developed by Stephen Gould and Niles Eldredge in their 1972 work Punctuated Equilibrium: An Alternative to Phyletic Gradualism, is a form of saltationism, in which there are evolutionary leaps after periods of stasis. It is thus opposed to the gradualism of the synthetic theory of evolution that posits slow and progressive evolutionary changes.

In France, Gould and Eldredge’s theory was adopted, despite criticism, by Jean Chaline and his team at the University of Dijon, and by Marchand and Dommergues in particular, who supported Dambricourt’s innovative work first on the mandible and then on the craniofacial contraction. Punctuated equilibria in, for example, developmental heterochronies, which have been analyzed in a large number of species, show that the theory still has good days ahead.

It was also necessary to introduce Dambricourt’s complex concept by giving a short history, beginning in the eighteenth century, of scientific interest in the evolution of the species sapiens.

The Fundamentals of the Paradox

Dambricourt has been working on this issue in hominids for thirty-five years. Her reflections on the origin of man thus take on a double dimension, both intellectual history and thoughts on how certain theories are verified and analyzed, through dimensional and architectural studies. Her analysis proves to be an exciting quest for the latest member of the hominid lineage, Homo sapiens, and in particular for his consciousness, his intelligence, his know-how, even his culture...

In reality, human evolution is a co-evolution of the brain and the cerebellum, seen through craniofacial analysis. This field of research is rarely explored in human paleontology, especially analysis of the cerebellum during embryogenesis, which appears, according to Dambricourt, as the ultimate evolutionary vector. This new, internal, brain-cerebellum interpretation provides new tools for determining and redefining the status of our species Homo sapiens.

The history of science and of historiography should be compulsory parts of evolutionary science. Without them, the numerous theories—such as Lamarckism, fixism, Darwinism, and so on—entertained by naturalist scholars are often misunderstood by students in vertebrate paleontology and paleoanthropology, if they are known at all.

To this end, the historical introduction is absolutely necessary, not only so that the reader could reencounter the spirit of the Enlightenment embodied by Linnaeus, Buffon, Lamarck, and Jussieu, but also, and above all, to offer a chronological understanding of both the lineage of the theories and their taxonomic ambiguity. Nor should we forget the essential factor linking them: the classification of Homo sapiens within the family of hominids and then within the order of primates.

Linnaeus’s zoological classification in 1758 was revolutionary in its hierarchy of taxa, and it is striking how long-lasting its influence has been. Concerning the Swedish zoologist, Dambricourt pauses for a moment in her classification of the primates to note:

In the case of primates, in fact, Linnaean classification groups together several anatomical configurations without taking into account their more extreme variations. It is not a suite of species that diverges, by small gradations, from lemur towards Homo sapiens. No primatologist working today would consider Homo sapiens an augmented prosimian, which defines “primate morphology.” And yet they would also place Homo sapiens within the order of primates. Morphological discontinuity is a fact. Why this paradox?

In the course of her essay, Dambricourt evokes many authors, from Daubenton and his famous 1764 work on the movement of the base of the skull, through observation of the morphology and displacements of the occipital hole, to the most recent current discoveries, by way of Buffon, Lamarck, Cuvier, Armand de Quatrefages, Charles Darwin, Paul Broca, Jean Piveteau, Camille Arambourg, Teilhard de Chardin, Henri Vallois, G.G. Simpson, John Haldane, Yves Coppens Tobias, Ernst Mayr, Théodore Dobzhansky, and Brigitte Senut. It is noteworthy that Jean Chaline—a supporter of Gould and Eldredge—was one of Dambricourt’s first scientific supporters.

Some among them, such as Cuvier, Buffon, Lamarck, and Darwin, were primarily anatomists, in particular, comparative anatomists. Paul Broca, however, when he created the Société d’Anthropologie de Paris in 1897 broke new ground in the quantified description of human morphology, or anthropometry.

As always, Dambricourt remains committed her conviction that Darwin and his theory of the origin of species do not suit the evolution of Homo sapiens:

Darwin and his contemporaries were themselves part of the Lamarckian revolution. A function gives rise to an organ and then the organ, including the shape of its bone tissues, is gradually modified. Those changes are then, through use, transmitted to later generations. Darwin did not invent the idea of a common ancestor for the chimpanzee and Homo sapiens, nor that evolution was a result of behavior, nor that modifications adapted by choice are conserved. The concept of natural selection is merely a formulation of the Lamarckian idea of the conservation and transmission of modifications, constrained by climatic changes.

Dambricourt’s Lamarckianism challenges gradualism as applied to modern man, because, she insists, “the plan of anatomical organization, or the internal structure, remains an obstacle to Linnaean classification.” On this point Dambricourt is not wrong.

I agree with her reasoning. The craniofacial and dental architectonic analysis that I carry out on hundreds of sapiens individuals, of all ages, points me towards the same conclusion. It makes no difference whether the cephalometry is performed at the facial, basi-cranial or occlusal level. Cranio-facial folding (occipital contraction) follows the trajectory of the cerebellum and/or the other way round. The latter is related to the degree of verticalization and also to the closing or opening of the sphenoidal angle which induces, in Homo sapiens, the advance or retreat of the mandible.

In all taxa that precede modern man, dental articulation is in a state of labidontia (edge-to-edge incisors), and so the occlusal dynamic is different. In other words, plans of organization defined from fossil skulls are insufficient, and can even be deceptive. Linnaean classification only takes into consideration the cranial capacity of the complete object, and its morphological and biometric description, therefore only its external characteristics.

The deep structures visible in imaging are not analyzed by paleoanthropologists. They have not mastered the kinetic development of the skull, the trajectory of the bones that make up the craniofacial puzzle, nor the failure of growth, or developmental anomalies, let alone the rhythms of growth. In the end, classification appears problematic and vague. Examples are numerous; see the specimen from Djebel Irhoud in Morocco, long considered Neanderthal on the basis of external characteristics such as the low vault and the supra-orbital torus. It turned out to be sapiens.

She adds:

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.

Between a Threshold of Verticality and an Internal Dynamic

Embryogenesis is not taught to paleontologists and paleoanthropologists because there are no fossil embryos. Dambricourt instead turned to the comparative anatomy of the great apes. The seventy-one radiographed great ape skulls from the Comparative Anatomy Laboratory (thirty-eight Pan, twenty-six Gorilla and seven Pongo) enabled her to consider the internal dynamic of the embryo.

Intrauterine growth in several primates shows five morphogenetic groups including monkeys, great apes, australopithecines, plus fossil species of the genus Homo, all adapted in an earlier time. According to Bolk’s 1924 fetalization theory, all skulls (posterior base) of mammalian embryos pass through these stages of growth: a folding at the beginning of the embryonic stages, disappearance of the folding at the fetal stage, and finally, at birth, a flattened base. Unfortunately, this beautiful demonstration, which seemed related to the irregular rhythms of the base of the skull and the face, does not work because the cranial base of a large number of vertebrates remains flat at all stages of growth.

How then to explain the mechanism of verticalization of the central nervous system and its trajectory during morphogenesis in human paleontology? This is a question which no paleontologist can answer, for the question is not asked in this way in paleontology, given the absence of fossilized embryo remains. All we have are the endocranial walls of fossil specimens—these can retain a meningeal imprint—or endocranial castings that reconstruct the lobes and their cerebral capacities.

Dambricourt’s discussion of the embryogenesis of the nervous system is, perhaps, too complex for the average reader. It involves morphogenesis of the ancient nervous system, in particular, the dorsal chord, which began around 600 million years ago in the first vertebrates, which were close to fish. In them, the movement of the neural tube would not have made its first contractions.

What does appear to be absolutely correct is precisely the increasing angularity of the contraction process in verticalization; Homo sapiens is ultimately the only vertebrate embryo in which the extremity of the dorsal cord is almost verticalized. The process of contraction of the base and the lowering of the brainstem began about forty-five million years ago, with an Asian species of prosimian, and produced the first stage: monkeys. This first stage was followed by five others, ending with the embryonic contraction, the lowering of the cerebellum, and the straightening of the brain stem in Homo sapiens.

Dambricourt demonstrates that there was a hominid stage which combined at least two embryogeneses, that of the australopithecines and that of Homo. In other words, the internal structure of the australopithecines and that of the genus Homo is as different as that between Homo sapiens and the other species of the genus; no species preceding Homo sapiens has an embryonic folding and an associated axial verticality. Our organizational plan could be unique.

In any case, this argument is verifiable from the hundreds of individuals of modern men for the past 100,000 years, in their sphenoidal angle, skull in flexion, psalidontia, maxillo-mandibular dysmorphoses, morphology of the chin and forehead, and so on.

Among the methods used by Dambricourt to support her hypothesis is the medical imaging of the semicircular canals, developed in collaboration with Martin and de Kerviler in 1999. This technique begun by Delattre and Fenart in 1960 consists of knowing how the basilar rocking (slope of the clivus) is linked to changes in the position of the canals and the cochlea. Paleontologists applied this method to fossil hominid skulls in the 1990s in order to find a relationship with the sphenoidal dynamic.

For Dambricourt and her colleagues, it was necessary to find a correlation between the angular deviation, the degree of rotation of the sphenoid, and the frontalization of the petrous bones after birth, among Pan individuals. From this first test, other protocols were defined that looked at the semicircular canals, the sphenoidal angle, and the angle of frontalization of the petrous bones. The results confirmed embryonic origin and have now opened the field to further medical imaging studies.

We find

a much more open angle in Homo sapiens; the size does not change in the chimpanzee from one-year-old to adult, likewise in the gorilla fetus, the sphenoidal angle opens from 138° in the one-year-old child to reach an average of 160° in the adult; the value of the sphenoidal angle between the great ape and the man is accompanied by a frontalization of the petrous bones.

If we consider only these results, it seems that the embryonic dynamics in Homo sapiens attest to a new postural dynamic in human phylogenesis.

It is a fact that the binominal nomenclature published by Linnaeus in 1758 in Systema naturae concerns only living species of zoology and botany, apart from minerals, and while the binomial system of the species can be used to classify fossils, there is no real need for the paleontologist to make any changes.

Dambricourt brings together all the elements of an internal postural dynamic. She should use them to propose a new phylogenetic classification, to be published in an international journal of the Nature or Science type.

Djillali Hadjouis

Anne Dambricourt replies:

The critical analysis in my essay could only benefit from Djillali Hadjouis’s vast expertise in animal paleontology, in paleoanthropology, in paleopathology, in cranio-sacral biodynamics, as well as in epistemology and the history of science. It is a complex argument—one could hardly think otherwise—and actually requires such broad knowledge to reveal the originality of such an approach.

It is clear that our upright anatomy is the result of an evolution of embryogenesis and of the complex movements involved in it, over the course of forty-five million years. This evolution has proceeded through successive thresholds, as the complexity of the embryogenesis requires, and has proceeded always in the same direction. I have thus posited that this embryonic structure, the dorsal cord six hundred million years ago, provides a normal for the vertebrate embryo; the choice of geometry allows setting a point in the internal space of the organism that is common to living and fossil vertebrates.

My essay follows the historical narrative of the research on the straightening of the nervous system, from Daubenton’s premises to my predecessors, many of whom are now gone.

Hadjouis explains why it has been so difficult to change the view of paleontologists, after thirty-five years of research, abnormally long in the sciences. The issue is not the discovery of thresholds, but rather the delay in recognizing their existence and thus in recognizing ourselves as the last threshold; this delay is even less understandable in that it concerns our conscious being, here and now.

This lack of interest comes from my discipline; in other fields there has been no problem with accepting this, such as in the fields of dento-maxillo-facial orthopedics and in osteopathy, where evolution is permanently visible. In my entire career, only when we touch on man do I find scholars who are truly interested in the origins of the straightening of our body, in a global and systemic approach. We have attained the last spatial threshold. There is no reason that at the scale of gametes, this process that has been accelerating for four million years should suddenly cease.

Hadjouis is an exception.

There is another story, that of the paleontologist Jean Chaline, the former director of the geoscience laboratory at the University of Bourgogne. We got to know each other after my 1987 thesis and, after the discovery of the embryonic origins of human verticality and of the thresholds. With Chaline, his colleagues Didier Marchand and Bruno David (the current president of the National Museum of Natural History), we published external—not internal—studies of the skull, as well as my curve for the cranio-facial contraction through successive thresholds, a curve of only the cephalic part of the neural straightening. Chaline has always been engaged, he has been the paleontologist most invested in spreading this discovery, through his technical articles for scientists and popular books for the wider public. Experts called upon to approve it at the Centre national de la recherche scientifique (CNRS) in 1992, did so, including, for example, the mathematician, René Thom, the paleoanthropologist Phillip Tobias, and Chaline. 

The journal Nature does not seem to me right for this type of discovery, which unfolded over the long term. It is the sum of work accumulated over many years, a process with periods and collaborations; foundational articles exist, some with handwritten annotations by René Thom.

The lack of interest in my discipline for the entire internal part of the axial skeleton which protects the nervous system has led to an impasse; the internal movements are probably too complex to visualize. And hominization becomes an empty word; how can one be interested in the cognitive capacity of long-lost hominids without beginning with the neural straightening and the instability of a cerebellum that must then become more complex?

We few have chosen for the near future a new mode of understanding—with the collaboration of complementary disciplines like posturology and of course osteopathy—our evolving identity: global and systemic, and reflected in our neural straightening.

Djillali Hadjouis is a paleoanthropologist, associate researcher at the CNRS, and director of research with the Centre National de Recherches Préhistoriques Anthropologiques et Historiques of Algiers.

Anne Dambricourt Malassé is a paleoanthropologist at the CNRS and a founding vice-president of the Federation of Research on Human Evolution, Osteopathy and Posture.

Translated from the French by the editors.