Microscopic missing link: Stephanopogon

OK, this is silly. I keep (reading statements by Intelligent Design apologists blandly claiming that complex life appeared “suddenly” — that’s suddenly in geological time, mate! — and that scientists have no clue how it happened. Or that “we” have no clue, which is closer to the truth, because the writer never looked at the wealth of evidence.

Currently I’m reading a 20-year-old textbook about invertebrate life. I’m reading it because many years have passed since I took biology. Not only have I forgotten many of the details, but knowledge has grown manyfold since I was an undergrad. So I’m taking a private refresher. It’s an old book because new ones are expensive, while old ones are practically free: an old textbook costs pennies or a few dollars at a thrift store. And it’s working! I’m finding out new things, learning new details, and every few pages saying, “Aha! That’s more evidence for evolution.” Here’s just one of them from the chapter on protozoans: a transitional form between flagellates and ciliates. (The image is Stephanopogon paramesnili, from PlanktonNet.)

From the 1987 edition of Living Invertebrates, by Vicki Pearse et al.:

A diagnostic characteristic of all ciliates is the presence of two different kinds of nuclei [a micronucleus and a macronucleus]… (page 51)

Stephanopogon formerly was listed as a primitive and exceptional ciliate with only one kind of nucleus…. This benthic marine protozoan looks like a ciliate, but recent studies have shown that it lacks typical ciliate infraciliature and pellicle structure and that it divides like a flagellate It has been removed from the ciliates and placed among the flagellates as a separate flagellate order or phylum, Pseudociliata. (page 51, sidebar)

The main link is to the “recent” (1982) study, “Stephanopogon, a Phylogenetically Important “Ciliate,” Shown by Ultrastructural Studies to Be a Flagellate” by Diana L. Lipscomb and John O. Corliss.

That article was cited in a very interesting paper from 2003, “Bridging Morphological Transitions to the Metazoa,” in the journal of Integrative and Comparative Biology:

Our inability to answer many questions regarding the development of metazoan complexity may be due in part to the prevailing idea that most eukaryote “phyla” originated within a short period of geologic time from simple unicellular ancestors. This view, however, is contradicted by evidence that larger groups of eukaryotes share characters, suggesting that these assemblages inherited characters from a common ancestor. —Ruth Ann Dewell, et al.

Follow the “Morphological Transitions” link to read more about the researchers’ methods and conclusions. I hope to write more about it later.


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