Cambrian Explosion: The Case of Mollusks - Evolution News & Views

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Cambrian Explosion: The Case of Mollusks

Tonicella-lineata.jpg

An animal body plan that appeared abruptly in the Cambrian explosion -- the mollusk -- challenges Darwinian assumptions about the history of life. Three classes of mollusks come under consideration in recent evolutionary literature. In each case, no evolutionary progress was demonstrated in the fossil record.

DebatingDDsmall.jpegPhylum Mollusca is a large and diverse collection of invertebrate animals that live in terrestrial and marine habitats, both freshwater and saltwater. The phylum includes snails, slugs, octopi and squid, mussels, clams, limpets, chitons, and many more. Mollusks are complex animals with a gut, a brain, sex organs, a heart, excretion system, sensory organs and methods for locomotion. Some undergo metamorphosis during their development. They vary in size from shell-less chitons less than a millimeter in size to giant squid up to 10 meters long, and live in widely diverse habitats, from deep-sea trenches to tropical forests. About 80 percent of all mollusks are gastropods (snails and slugs). Many mollusks, like the conch and chambered nautilus, manufacture shells of exquisite design, illustrating the Fibonacci series. Cephalopods have some of the most advanced eyes and neural systems of any invertebrates.

The first bivalve mollusk appears abruptly in the early Cambrian. Evolutionists continue to debate the phylogeny of classes within the phylum, the status of certain animals from the Burgess Shale, and the relationship of Kimberella from the Ediacaran period to mollusks (see here for more). Regardless, the complex body plan of even a simple mollusk, with its multiple tissues, organs, and systems, defies the explanatory power of gradual, unguided evolution.

The Stasis of Gastropods

A short news item from the University of Ghent, summarizing a paper in PNAS, describes the sad end of a hunt for evolution in gastropods, a class of mollusks including snails and slugs. Thinking that strata under an African lake would show evolutionary progress, they found, instead, stasis:

Evolutionary biologists at the National Museum of Natural History of the Smithsonian Institution and Ghent University (Belgium) have found an unusual system that allows narrowing the gap in the study of evolutionary processes. Working on living African freshwater snails and their fossil ancestors, they document rapid diversification since massive lake level fluctuations that occurred during the Ice Ages. Despite the ever changing environment in which these snails live, morphological characters studied within a lineage of fossils did not change much over five thousand years. Different morphological characters appear to have behaved differently, with some hardly changing over thousands of years, and others, like body size, changing a lot (the smallest species is less than 1/3 of the size of the largest one). It appears that morphological stasis is a dominant evolutionary pattern, even in young and diversifying clades. (Emphasis added.)

A change in body size is a simple matter, compared to the evolution of a new tissue, organ or function. The hunt produced no evidence of that; “all traits examined experienced morphological stasis within a densely sampled fossil lineage,” the paper says. To rescue Darwinian evolution, they had to bring in the “long questioned suggestion of punctuated equilibrium” -- basically admitting that any evolution had to be found outside their data.

The De-evolution of Chitons

The polyplacophorans (multi-plated chitons) and aplacophorans (unplated chitons) are two classes of marine mollusks. Some aplacophorans that live on the seafloor could be mistaken for roundworms. Closer inspection, however, reveals the characteristic musculature of mollusks in their larval forms -- an example of the larvae being more complex than the adult. An item from the University of Vienna explains. Titled “Evolution is not a one-way road towards complexity,” it describes a surprise case of evolution downward, from complexity to simplicity.

However, new studies on the development of a typical aplacophoran (Wirenia argentea, a species that was collected in 200 m depth off the coast of Bergen, Norway) tell a different story. Although their adult, worm-like body appears rather simple (hence the traditional assumption that they may constitute a basal molluscan group), their small, 0.1 to 0.3mm long larvae undergo a stage in which they show an extremely complex muscular architecture which is largely lost and remodeled during metamorphosis to become the simple muscular arrangement of the adult animal.

The study was published in Current Biology in October. Previously, the simpler aplacophorans were thought to be the primitive ancestors of polyplacophorans. Now, it appears the reverse is true:

While it has been suspected for a long time that aplacophorans and chitons are closely related, it has often been argued that the aplacophoran morphology is closer to the ancestral molluscan condition than the polyplacophoran one. The current data paint a different picture: the fact that the highly complex larval muscular bodyplan is so similar in both groups but is only carried over into the adult stage in one of them -- the chitons -- strongly suggests that the common ancestor of both groups was of similar complexity; thereby implying that the worm-like groups lost these complex traits and became secondarily simplified over evolutionary time.

The aplacophorans, therefore, must have lost the plates of the more complex looking chitons, leaving the adults curled up like worms on the seafloor. This is a good illustration of why you can’t judge a book by its cover. The explanation, though, pushes the complexity back in time to an unknown, theoretical ancestor. Here’s where the Cambrian explosion comes in. Watch for the reverse evolution:

Interestingly, findings from the fossil record support this new developmental evidence. A recently described species from the Silurian -- Kulindroplax perissokosmos -- obviously had a mix of aplacophoran and polyplacophoran characters: while being long, slender, cylindrical in diameter, and covered by spicules -- closely reminding us of today’s aplacophorans -- it had seven shells on its back. Although, at an age of 425 myr, too young to be considered the long-sought ancestor of polyplacophorans, aplacophorans and maybe even all mollusks (the origin of the phylum is known to date back to at least the Cambrian Explosion some 540 myr ago), this relative of the distant past proves that evolution has widely played with the combination of the various morphological character sets in individual molluscan groups. Taking together the data currently available, a coherent scenario emerges that strongly suggests that today’s simple, wormy mollusks evolved from an ancestor that had a much more complex musculature (and probably overall internal anatomy) and was covered with protective shell plates.

Commenting in the same issue of Current Biology, Maximilian J. Telford of University College London expands on the theme. He points out that some annelids (segmented worms, another phylum appearing in the Cambrian explosion) also show evolutionary loss of segments from the earliest segmented species. In addition, their larval forms reveal segment-like structures that disappear during development. His conclusion is doubly surprising:

Each of these studies represent a wonderful use of Haeckelian ontogenetic recapitulation both to reveal hidden phylogenetic affinities and as evidence to resurrect the spirits of long dead ancestors.

This sentence substitutes evolutionary imagination for data (spirits instead of fossils). Also of interest is that Haeckel’s long-discredited recapitulation theory can still be found in journals.

Conclusions

Here we have seen Darwinian evolutionists admitting that mollusks date to the Cambrian explosion. They went looking for evidence of subsequent evolution. Strata under a lakebed in Africa showed simple changes in size, but no “upward” evolution in complexity. Fossils of plated and unplated chitons showed that ancestral forms were more complex than living forms, and larval forms more complex than the adults. Neither case provides empirical evidence that unguided, blind processes of evolution are capable of generating complex specified information.

The conclusions that Stephen Meyer draws in Darwin’s Doubt continue, then, to be supported by new evidence. Mollusks are but one phylum of nearly twenty whose abrupt appearance complete with complex tissues, organs and hierarchical structures defy Darwinian mechanisms.

Science should seek explanations bringing a vera causa for the effect -- a true cause with the necessary and sufficient power to explain the observations. For the Cambrian explosion, including the origin of mollusks, that cause is intelligent design.

Image: Lined Chiton, Tonicella lineata/Wikipedia.