Abstract The origin of tetrapods is a c…

Abstract

 

The origin of tetrapods is a complex question that webs together genetic, paleontological, developmental and physical facts. 

A very interesting question of evolutionary biology indeed, mobilizing the efforts of many.

 

Basically, the development of embryos is described by a complex mix of mechanical movements and biochemical inductions of genetic origin. 

It is difficult to sort out in this scientific question what are the fundamental features imposed by conservation laws of physics, and by force equilibria, and what can be ascribed to successive, very specific, stop-and-go inductions of genetic nature.

Is it a difficult question ? From zygotes you can get individuals with no appendages or with four, six, eight, ten or many (myriapods). It is quite easy to identify what differentiate them: genomes. Genomes which specify the entire spectrum of physico-chemical properties of every part of the developing embryo, as well of the formed individual throughout its existence, in interaction with the environment. And these physico-chemical properties specify every aspect of cellular displacements from zygote to death, including embryogenesis.
That’s the easy part, easy enough for everybody to understand, even if he isn’t a specialist.
It’s much more difficult to understand the mechanisms in detail. And that’s the fun part.

 

A posteriori, evolution selects the parameters of this process as found in the observed species. 

There is a huge misunderstanding of the term “evolution”. Fleury seems to be allergic to the notion of Natural Selection and, as he say, is preoccupied by Evolution not Natural Selection, ignoring the mechanisms known to operate in biological evolution.
The persistence of such an error in the abstract of the paper calls a question: did an evolutionary (or evo-devo) biologist reviewed the paper? I hope the answer is no. I hope that no biologist reviewed the paper.

Whether there is a general law to animal formation seems out of the question. 

By experience: there will be a phenomenological description, disconnected from the mechanisms driving the object of the description.

 

However, several concepts developed in biology, like the concept of “organizer” seem questionable from a physics point of view, since the entire deformation and force field should be the “organizer” of development, and one can hardly ascribe such a role to a single small area of the embryo body. 

One thing isn’t negotiable when it come to scientific understanding of the world, or a bit of it, say embryogenesis: facts. Fleury have read the Spemann & Mangold paper [118]1, but he don’t wish to accept the properties of “a single small area of the embryo“. Nobody ascribed their role to organizers, the concept emerged by observation and experimentation, it’s about facts.
Wishful thinking and gratuitous statements as “force field should be the “organizer” of development” in order to fit a model unsupported by experimentation should be banned, don’t they?
That’s the second point where one may ask the question: was the paper reviewed by a biologist? Let me make the question a little bit more general and harsh: did the paper was reviewed by a competent scientist? (of any discipline)
Any model of embryogenesis must account for observed (repeatedly observed) facts, including the Spemann-Mangold organizer (or his analogs in non avian species), otherwise it is flawed. One can accept using incomplete models if they offer some added value, keeping in mind that they are to be completed. What would be the added value of the model proposed by Fleury? As far as I know none for the moment since the first publication of his hypothesis [2] in Organogenesis four years ago (he doesn’t even cite his seminal paper! and he is right, it’s such a mess). He does cite it in fact [131]

 

In the same spirit, the concept of “positional information” encapsulated in concentration of chemicals seems questionable since the deformation and force fields in embryonic tissues are tensors. 

In the same spirit, deformation and force fields in embryonic tissues are the result of genome’s expression, repeatedly demonstrated to be affected by positional information cues, including morphogenic factors and interaction with adjacent cells. Naturally present during embryogenesis of experimentally introduced into the system. How could experimental data be questionable?

 

Finally, the concept of a development organized in space along three orthogonal (“Cartesian”) axes associated to chemical gradients seems also questionable, since early embryo development is driven by complex vortex fields, with hyperbolic trajectories which span the entire embryo. Such hyperbolic trajectories are best understood by a description in terms of dipolar components of the morphogenetic forces, whose pro jections along orthogonal axe have no specific meaning except as a mathematical tool. 

One can use whatever mathematical tool best describe the observed phenomenon and various coordinates settings can describe the same phenomenon. That’s quite convenient. Nevertheless, the description of the developmental process along the antero-posterior, dorso-ventral and proximo-distal axes is not just another mathematical tool but is based on one hand on the final morphology of the system and on the other to the presence of gradients of morphogenic factors along those three axes, orienting the system.

 

I review here the present state of description of several aspects of tetrapods morphogenesis and evolution, from the point of view of physics. It is getting clear that several basic features of tetrapods body are a direct consequences of fundamental laws of physics. Several lines of work reviewed here show that the topology of the tetrapods may be directly related to the structure of the earliest movements in embryos. 

It would be surprising that such a natural object as an embryo wouldn’t behave in harmony with the fundamental laws of physics, and much more surprising that movements in embryos have nothing to do with their topology ; and this is true for all embryos, not only the ones of tetrapods. What it is not clear is how just the fundamental laws of physics could account for tetrapod’s topology without the specifications carried by their genomes.

By experience: there will be a phenomenological description, disconnected from the mechanisms driving the object of the description.

 

The bio-mechanical approach leads to important consequences for the constraints on evolution of the craniates. Such consequences have received a controversial welcome in the last decade, although they may encapsulate the true origin of craniates, esp. simians, and eventually homo. 

Some background may be necessary here for international readers who ignore the specifics of the Anne Dambricourt-Malassé (ADM) affair in France and who is one of the elements which experienced the “controversial welcome“. In fact, what was harshly criticized, not as harshly as I would like it to be, is not the observations published as “bio-mechanical approaches” but the interpretations: clearly anti-Darwin and teleological (inspired by the jesuite teleologist Teilhard de Chardin). And a lot of buzz was made due to the connexions of ADM (quite loose but existent within the frame of the anti_darwin movement3) with the Discovery Institute and the Université Interdisciplinaire de Paris which acts as the local (French) antenna of the John Templeton Foundation promoting the Science & Religion (a neo-creationist approach) movement in Europe.


First impressions from the abstract:
Fleury is going to

  1. question experimental observations that don’t fit to his model instead of adjusting the model to account for facts,
  2. he will try to minimize the role of genetic information, the specifier of biological objects, to promote his neoplatonian point of view.

The whole encapsulated in a misrepresentation of biological evolution.

One may wonder what kind of review process may lead to the acceptance of such papers (the present one and the one published by Organogenesis2 four years ago). For Organogenesis the acceptance of the paper was really astonishing as it contains factual errors perceptible by a Modest Biologist4, which gone unnoticed by the editor in chief, an embryologist!
The situation is different for EPJ-AP, where the editorial staff must be more physics-oriented, but still, how is it possible to accept such a rejection of experimental data as the one displayed about the Spemann-Mangold organizer?
Not the best advertising for the journal.

 


1. References present in the paper will be given in blue, new ones in red, footnotes in black.

2. An Elasto-Plastic Model of Avian Gastrulation, Organogenesis, Vol. 2, 1 pp 6-16, Open Access.

3. She signed the infamous Scientific Dissent from Darwinism

4. That’s how Jean Staune, perpetual Secretary General of the Université Interdisciplinaire de Paris calls me😉 [salut Jean]

One Response

  1. […] display of the sense of “archetype” by Darwin, the denial of experimental data (see Abstract) and a non displayed connection to Biological Structuralism, the reader may expect a rather nasty […]

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