The "Flat Animal": Is It a Cambrian Ancestor? - Evolution News & Views

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The "Flat Animal": Is It a Cambrian Ancestor?

Trichoplax_mic.jpg

Placozoans ("flat animals") are free-living marine invertebrates with a very simple, blob-like structure. Possessing no organs, their relationship to other animal phyla is a matter of debate. The only known examples are lumped into a single species, Trichoplax adhaerens. Since they do not appear in the fossil record, their origins can only be inferred from genomics and comparative anatomy. The results have been troublesome for evolutionary theory.

The most complete examination of cell types of Trichoplax appeared in Current Biology last month. Smith et al. realize that the evolutionary position of this species is important:

Results of recent genome projects have renewed interest in the long-standing question of how the earliest-diverging animals are related. Genomic sequences reveal that the basal lineages Porifera, Placozoa, Ctenophora, and Cnidaria share a large fraction of the genetic toolkits involved in cell fate, patterning, differentiation, and cell-cell communication in bilaterians. Whether the homologous toolkits serve similar functions and how the gene products are parsed into cell types and organ systems in early-diverging taxa remain largely unknown. Moreover, adequately detailed anatomical understanding to properly exploit the genome data is lacking in some early-branching groups. (Emphasis added.)

The team from the Schwann-Schleiden Centre for Molecular Cell Biology in Göttingen, Germany, found six (not four, as previously thought) distinct cell types: dorsal epithelial cells, ventral epithelial cells, fiber cells, gland cells, crystal cells and lipophil cells, all arranged in functional patterns. The epithelial cells have cilia and microvilli with which the animals glide over algae, absorbing nutrients.

We find two previously undetected cell types, lipohil [sic] and crystal cells, and an organized body plan in which different cell types are arranged in distinct patterns. The composition of gland cells suggests that they are neurosecretory cells and could control locomotor and feeding behavior.

Placozoans are not mentioned by Steve Meyer in Darwin's Doubt, so where do they fit in the scheme of animal phyla? Nobody knows for sure. Some had suggested they might be remnants of Ediacaran animals, like Dickinsonia (see Meyer, pp. 80-86), because of their simple shapes lacking clear bilateral symmetry.

Hopes for making placozoans a transitional form leading up to the Cambrian explosion, though, were dashed by examining their genes. Back in 2008, the Joint Genome Institute (JGI) announced, "Genome of Simplest Animal Reveals Ancient Lineage, Confounding Array of Complex Capabilities." Lo and behold, here was a so-called "simple" organism with the developmental toolkit for a nervous system, even though it doesn't have one! JGI said that Trichoplax "appears to harbor a far more complex suite of capabilities than meets the eye." Its genes also contain 80 percent of the introns found in humans.

Even the arrangement of genes is conserved between the Trichoplax and human genomes. This stands in contrast to other model systems such as fruit flies and soil nematodes that have experienced a paring down of non-coding regions and a loss of the ancestral genome organizations.

With its pancake shape, gutless feeding, and genomic primitiveness, the rich array of metabolic capabilities begs additional consideration. While it has been observed to motor around via cilia, eat by mounting its prey, and reproduce by fission (pulling itself into pieces) -- it may in fact have a secret sex life.

With so much complexity in such a simple-looking thing, evolutionists have turned the phylogenetic tree on its head. Now, many view Trichoplax as a descendent of more complex animals, like comb jellies. Those animals (Phylum Ctenophora) possess a nervous system, a gut, and complex swimming behaviors. Nature describes their complex lives:

So what are these mysterious animals that have turned the table on textbook ideas? Comb jellies are fabulous marine predators that propel themselves through the water column by means of blocks of cilia -- the shimmering combs that give them their name. They catch their prey using innervated tentacles seamed with sticky cells called colloblasts and swallow it through their mouth, which opens into a sac-like gut. They have a nerve net with regional specializations, such as a sensory organ located at one pole of the body that is used for light reception and gravity sensing. Notably, sponges and placozoans lack all of these features, so the proposal that the jellies evolved first seems odd.

"Odd" as it sounds, the article does not dispute the new theory. It celebrates it! "Heretical hypotheses such as this strike a blow against the anthropocentric view that complex animals emerged gradually along one lineage only, culminating in humans, and that complex organ systems did not evolve twice." Darwinists should embrace this new heresy!

According to the new heretical orthodoxy, placozoans did not evolve upward from simple colonies of cells, but "devolved" downward from more complex animals. A 2009 item from Ludwig-Maximilians-Universität in Munich agreed: "Since the comb jellies already have nerve and muscle cells, this would suggest that these features developed several times independently in animal history, or that they were lost in sponges and placozoans."

This puts Darwinists into another rendition of Darwin's Dilemma: either placozoans developed a complex genetic toolkit on their own, or they are stripped-down remnants of more complex animals. Either way, they cannot be assigned a place on a theoretical fuse leading to the Cambrian explosion. Smith et al. keep out of that controversy, noting in conclusion only that "The improved structural methods and markers for specific cell types introduced in this study provide tools that will prove useful in future studies of placozoans."

Image source: Wikipedia.