4 The paleontological point of view

There is not as much paleontology as one would expect by the title of the chapter. Mostly paleontology is an excuse to further discuss developmental biology, except for subsection 4.1

4.1 Tetrapod origin

If we turn to paleontology, we find a description of tetrapods appearance into three main steps. Appearance of chordates, segmentation of lateral fins, appearance of tetrapods.

That’s the shortest version one can get except “pouf they appeared”. Interesting nevertheless the second step, the “segmentation of lateral fins“. This is one of three hypothetical, not exclusive, working models. Not to be used as a granted fact (see below).

Good news, bad news.

Good news are that Fleury abandonned the idea it appeared in one of his conferences announcement, and promoted in fora, that the tetrapods may have appeared suddenly, with all there attributes, specifying suddenly as “in a single generation“.

But he still think that:

These early tetrapods have well formed complex limbs apparently almost “right away”.

Almost right away being an estimate of the time-lapse between Haikouichthys ercaicunensis, presenting a single median fin-fold and tail, to the tetrapodomorph Tiktaalik roseae; almost right-away corresponds to 100 millions years. At least we are not anymore at the “single generation“level.

Progressive modifications are problematic for a model which is based on a suddenly appearing bauplan.


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3.3 The limb field

This is the last part of “The genetics of vertebrate development”. Not much about most of the features of vertebrates were presented and this last part is about just “the limb field”. There is a lot of interesting papers out there one could read to learn about the subject.
It would be really sad if all you got is what Fleury present in cteappv.

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3.2 Limb patterning in tetrapods

Geoffroy de Saint Hilaire would have be happy to read the mix Fleury made of his observations of structural homologies between tetrapods and insects and genetic homologies, but he probably have laughed with the persistence of his own idea of analogy between ribs and insects appendages armed with current knowledge of embryology (no ectoderm participating in the building of ribs, but then Fleury seems to mess all the time with the embryonic tissues). And he probably have being delighted with the actual knowledge on deep homology1, more then reading cteappv.

Fleury is concerned with limbs positioning, FGF role in limb development, limb positioning and specification, and autopod patterning. He struggles to express the specification of limbs in terms where the role of morphogenic genes is minimized and FGFs role maximized2. Interspersed to his presentation there are a few quite particular points of view, which have more to do with his misunderstanding of empirical approaches in general than with the actual subject.

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3.1 The homeobox genes in vertebrates

The author focus on Hox genes in mammals (rather than homeobox genes in vertebrates), and what he delivers is disappointing in regard with the title.

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3 The genetics of vertebrate development

In the third section of the paper Fleury starts to display his incredulity of genetic explanations of developmental morphogenesis, concluding by p16, col2, §2:

The review presented here highlights that inductive cascades of unidirectional gradients of scalar quantities cannot explain morphogenesis.

The review is incomplete and often bogus,which explain why Fleury is reaching this conclusion.

Certainly a lot is to be discovered in the field of developmental biology, but also a lot is known and one can’t just ignore it just because it doesn’t fit his point of view.

For those unfamiliar with biology I will try to point gross errors (factual or conceptual) and propose more appropriate readings.

The rhetoric of Fleury’s argumentation will be discussed as well, as they are one of the most interesting elements of the paper and may help understanding his point of view more clearly than his knowledge of the subject.

An overview of what will be discussed.

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Introduction

Introduction

I plainly agree with Vincent Fleury that animal embryos are material objects and their development is according laws of condensed physics. But I wouldn’t call them “things” but rather objects and I would extend these considerations to all living organisms, not just animals, and at every phase of their lives, not just development.

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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.

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