Simple Fractal Branching Explains the Ediacaran Rangeomorphs - Evolution News & Views

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Simple Fractal Branching Explains the Ediacaran Rangeomorphs

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As explained in Stephen Meyer's book Darwin's Doubt, the Ediacaran animals (if they can be called animals at all) bear no evolutionary connection to the complex body plans that "exploded" onto the scene with great profusion in the Cambrian fossil record. That assessment is reinforced by a new analysis of some frond-shaped Ediacarans called Rangeomorphs by Simon Conway Morris and Jennifer H. Hoyal Cuthill in the Proceedings of the National Academy of Sciences.

The take-home message of Conway Morris and Cuthill's detailed classification is that the rangeomorphs are very simple things. Composed of tubular units, they can be described as fractal patterns resulting from simple branching rules. (This is not news, just confirmed to a greater degree of detail.) The creatures had no complex organs: "No evidence exists for either motility or active feeding (such as musculature, filter feeding organs, or a mouth)." Most likely they were soft-bodied colonies of cells, extending into the water to collect organic molecules by osmosis.

Using a mathematical model they designed, the authors were able to reproduce many of the fossil forms. This means that the various frond-like shapes that might have been classified as separate species (they considered 11 "species" in their study) essentially collapse into variations on the same theme:

Collectively, these reconstructions demonstrate how different body shapes and symmetry patterns [characteristics relied upon by some previous comparative studies] emerge from variations on a shared branching pattern.... We demonstrate that rangeomorph morphologies can be reconstructed by applying approximately self-similar branch production rules (as described above) at increasingly fine scales, although these may be modified by different parameter values to give subtle variations in the level of self-similarity. (Emphasis added.)

They picture the fronds adapting and diversifying, competing for resources in a Darwinian manner, feeling "strong selective pressure for greater height." But fractal growth is not Darwinian. The emergence of self-similar patterns is like crystal growth. It doesn't have to be specified in genes. The authors themselves point out that very little new genetic information would be required to produce the variations:

Therefore, superficially similar branching arrangements are not necessarily indicative of close phylogenetic affinity. Indeed, one of the most striking features of such fractal patterns is that highly detailed structures can be described mathematically by quite simple rules. Correspondingly, the genetic and developmental programs of self-similar biological structures may be comparatively simple (and conceivably require a relatively small number of genetic changes to evolve) because emergent structural properties do not, in themselves, require genetic specification.

In a commentary on this paper for PNAS, Marc Laflamme of the University of Toronto portrays the Ediacarans as important players in the transition from multicellular life to complex life in an evolving ocean. A closer look, though, shows that he agrees with statements made by Conway Morris and Cuthill: that (1) the Ediacarans had a unique morphology unrelated to anything that followed; (2) they do not represent transitional forms to Cambrian animals, and (3) they went extinct before the Cambrian explosion.

The fossil record of this revolution showcases the transition from microscopic single cells into large, multicellular and morphologically complex organisms. Typifying this transition is the Ediacara biota, a group of globally distributed soft-bodied organisms whose affinities are fiercely debated and whose disappearance from the fossil record before the Cambrian explosion is equally perplexing. Given the variation in shape, biological architecture, growth strategies, and body symmetries seen within the diverse Ediacara biota, it is most likely that these organisms represent an assortment of higher-level clades, many of which went extinct with the advent of bilaterian animals. Among the extinct clades, the Rangeomorpha ... are particularly unusual in possessing repeating and apparently fractal branching architecture that is not known in any modern organisms.

Conway Morris and Cuthill think the appearance of the Cambrian animals spelled doom for the rangeomorphs. Notice, however, that they do not portray the rangeomorphs as transitional in any way:

The appearance of rangeomorph fossils occurred after a move away from anoxic, sulfidic, and ferruginous oceans, toward conditions more favorable for aerobic macroorganisms. Their disappearance coincides with the Cambrian explosion in metazoan diversity, a dramatic increase in competition, and, crucially, decreased availability of organic carbon in ocean water. These potentially interacting factors suggest that the Ediacaran to Cambrian transition was a bad time to be a sessile, soft-bodied osmotroph. The unique rangeomorph fronds were fractal, surface area specialists of the Ediacaran. At the Cambrian explosion, the ecological and geochemical conditions to which the rangeomorphs were optimized ceased to exist, and their extraordinary body plan was lost from life's repertoire.

The "transition," in their view, was not one of ancestry, but more like one team leaving the field as a completely different team entered. The paper mentions the Cambrian explosion several times, but does not offer any new hypothesis about the origin of all the unprecedented, complex body plans and tissue types detailed by Meyer.

This paper adds nothing new to challenge Chapter 4 of Darwin's Doubt, where Meyer discusses the Ediacaran fossils in detail. We can view it as reinforcing of what Meyer contends: the Ediacarans -- fascinating as they are -- do not help resolve the enigma of the Cambrian explosion. They do not represent a "long fuse" to the explosion. In fact, since they appear rather suddenly in the record themselves, it doesn't help evolutionists to explain one explosion by adding another explosion.

When reading through papers like this, one must keep in mind that the authors are true believers in a traditional scheme of evolution, and generally unsympathetic to the intelligent-design position. This makes them "hostile witnesses" to the veracity of the design hypothesis presented by Meyer. If they had better evidence, or a better mechanism for accounting for the sudden appearance of the Cambrian body plans by unguided processes, they would hasten to present it.

Meanwhile, we must not let them get away with tossing a "word salad" to distract readers from the difficulties with the Darwinian view. When Conway Morris and Cuthill speak of "adaptive radiation" of "fractal morphologies," we should be aware that the variations involve trivial parameter changes on simple fractal rules, not major innovations. When Laflamme speaks of "the advent of bilaterian animals," he leaps over a grand canyon of difficulties with a religious-sounding term. Only with a leap of faith can one continue to cling to purposeless, aimless natural processes when confronted with the observable reality of the Cambrian explosion.

Image: Holotype of Charnia masoni/Wikipedia.