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A Remarkably Candid Statement About an Unsolved Evolutionary Puzzle

Paul Nelson
Photo: Sphaerechinus granularis, a sea urchin, via Wikimedia Commons.

According to current systematic theory, everyone reading this right now belongs to the taxonomic category Deuterostomia. This refers to the “second opening”: the group was originally defined with respect to the embryological appearance of the anus (first opening) versus mouth (second opening), a trait no longer considered diagnostic. Deuterostomia is still around as a systematic grouping, however, and it is showing signs of strain. At present, three phyla belong to Deuterostomia: chordates (that’s you), echinoderms (e.g., sea urchins), and hemichordates (acorn worms).

 A Long-Standing Mystery

The origins of Deuterostomia represent a long-standing mystery:

Deuterostomes are the major division of animal life which includes sea stars, acorn worms, and humans, among a wide variety of ecologically and morphologically disparate taxa. However, their early evolution is poorly understood, due in part to their disparity, which makes identifying commonalities difficult, as well as their relatively poor early fossil record. [Emphasis added.]

A new review article, from a team at Harvard, the Smithsonian, and the University of Oklahoma, carefully evaluates the conflicting evidence about deuterostome affinities. See Karma Nanglu et al., “Worms and gills, plates and spines: the evolutionary origins and incredible disparity of deuterostomes revealed by fossils, genes, and development,” Biological Reviews 98 (2023): 316-351.

Leaving a Muddle

Their message is — no clarity, many possibilities, leaving a muddle. What really struck me was this paragraph:

In many ways, despite hundreds of years of zoological effort and two decades since the publication of the new animal phylogeny (Halanych et al., 1995; Aguinaldo et al., 1997), we remain in an intellectual wild west with regard to deuterostome origins. No hypothesis, no matter how far-fetched it may seem, can be entirely discarded. No theory, no matter how enticingly logical, can claim to have emerged victorious among its competitors. The deuterostomes continue to elude a single, clean narrative to describe their early evolution, a state that is both fascinating and frustrating in equal measure.

The philosopher in me then asks — at what point does the category Deuterostomia become unreal? That is, become a term without a corresponding referent out there in nature; something whose origins no one should spend any more time trying to explain, because there is actually nothing there to explain.

When does an unsolved problem morph into a non-problem? Turn around, and walk out of that alley. There’s a brick wall at the end.