Earlier, we commented on news from scientists with the French National Centre for Scientific Research (CNRS). They sought to answer among other fascinating questions, "How has evolution produced a structure as complicated as a vertebrate?" Vertebrates appear initially in evolutionary history in the Cambrian explosion when, to judge from the fossil record, most known animal body plans spring into existence. In dispelling the enigma of the Cambrian event, this research does not seem to hold out much promise, if indeed that was their intent. See our previous post, "To Explain the Origin of Animal Body Plans, Here’s What May Be the Most Inadequate Proposal Yet."

Their news release concluded:

These findings thus offer an explanation for the coupling of cell differentiation and morphogenesis (acquisition by the embryo of its shape), so that a well-formed animal containing territories with different and physically separated functions, emerges "naturally". Understanding this process fills a conceptual gap between a shapeless mass of cells and an "animal archetype", and sheds new light on how vertebrates have emerged during evolution.

One of the scientists involved with the study complained to us about our analysis. As readers will confirm, we aim to please. After a review of the paper, published in European Physical Journal E, we can report that our original coverage was entirely correct.

In their paper, the authors spent the bulk of their time discussing legitimate scientific observations about chick embryos, using the chicken as a model vertebrate. They show how the cells migrate into positions that create stretching forces that aid in the placement of developing organs. Their experiments with rubber sheets and with actual chick embryo tissues do bear some relationship, in terms of the physical forces at work and how buckling occurs.

They recognize that much more than buckling is involved in morphogenesis of the final product:

Whenever the surface buckles, it does so along the pattern of rings (possibly deformed by advection). That fixes a few general primitive lines in the animal body, segregating the corresponding territories. These general lines correspond to the overall body plan. Of course, many other biological features will contribute to the exact final form of the animal (we think especially of cell migration which is not considered here). [Emphasis added.]

Furthermore, they recognize the need for coded genetic instructions for these physical forces:

A natural question is how genetic transcription correlates with the cell domains. Inspection of general databases shows readily that many genetic markers have domains of expressions sharply correlated to the cell domains evidenced here.

In conclusion, we have shown the existence of annular domains of different cell sizes, with cell size increasing step-wise radially. These boundaries form a 2D template for the animal, whose instruction or "coding" is the contrast of stiffness between rings. Since the 2D body plan locks the fold furrows, the animal pattern is robust against minor spatial or temporal fluctuations in the magnitude of the traction force, as typically may occur during embryogenesis.

It’s good to see, also, their epistemic modesty about how much their experiments reveal, and their recognition that further research will be required to form conclusions:

We leave for further studies several important other phenomena observed in the course of this work: tensions exerted by the folds, feedback of the folding process onto the boundaries of the cellular domains, possible interdigitation of the domains correlating with brain segmentation. A major issue to be solved in the near future is whether the correlation between cell domains, buckling pattern, and cadherin expressions is causal, and if so in what direction.

Nevertheless, there are indications in the paper suggesting that their motivation is to shed light on the origin of animal body plans in general, in terms of simple mechanical forces (which we concluded is Lamarckian and oversimplified).

• Their first reference is to Darwin’s Origin of Species. Since the reference is to the whole book, not a particular passage, they are evidently speaking of Darwin’s theory as a whole. From there, they mention subsequent historical findings — all naturalistic. Although they suggest that Darwin’s ideas were inadequate, they never repudiate them, but rather consider them partially successful.

• Their work is to demonstrate mechanical forces that generate body plans, not intelligent causes.

• They believe the mechanical forces are simple. "The work presented here," they say, "suggests a simple mechanism for segregating 3D cellular territories by folding, which ‘automatically’ segregates differentiated territories into spatially segregated domains."

• They locate the coding in the physics, not in the genetic information: "Simple physical phenomena like buckling, and shearing in rotatory flows, may play important roles in embryo formation, especially in vertebrates, and provide a mechanism for rapid formation of a body at a global scale."

• They fail to mention a mechanism for transferring the coding in the physical forces into genetic information.

We stated earlier:

If the paper merely describes the observable, measurable forces that cells in a chicken embryo produce as they differentiate under genetic control, there would be no problem. But if they think this explains the origin of new body plans in the Cambrian explosion (as the news release seems to imply), that’s another matter.

We welcome any opportunity the authors may find to explain in detail why (1) how these changes in body plans could become encoded in the DNA, (2) why their theory is not Lamarckian.

Image: � mindgamesru_ / Dollar Photo Club.