Scientific Method / Science & Exploration

Which worms are we related to again?

A new sequencing effort regroups the chordates and reveals an entirely new …

A little while back, as part of a larger essay, I mentioned a finding that the common understanding of the relationships among the deuterostomes might be a bit more complex than expected. This is specifically of interest to us humans, because deuterostomes contain vertebrates, but they also include a number of things with different forms of nerve cords as well as the radially-symmetric echinoderms. The earlier finding had suggested that organisms with a nerve cord and a vaguely brain-like structure at one end (cephalochordates) might actually be more closely related to echinoderms like sea urchins than they are to vertebrates, a finding that runs a bit contrary to expectations.

The new study added 35 new genes and two new species to the analysis, and rebuilt the trees. A more traditional deuterstome tree came out of the analysis, though one surprise that came out of the previous was retained. Although us vertebrates are closely related to the cephalochordates, there's another group that's even closer. This group, called the urochordates, have a nerve cord during their larval stages, but get rid of it when they reach adulthood. So, at least at the phylum level, our closest relatives may be sedentary and radially symmetric as adults.

The report also looked into a bizarre creature called Xenoturbella, which I had never heard of prior to this paper. The animal lives in oceanic mud, and has an extremely simple body plan, with a single-opening gut, no brain, and very few internal organs. Scientists have yet to even identify a gonad, so they have no idea how these things reproduce, meaning we're a long way from studying them in the lab. They were once identified as molluscs based on DNA sequences, until it was determined that they actually ate molluscs, leading to contaminated samples. New sequences from these things make it clear that Xenoturbella is a deuterostome, but branched off very early in the lineage, and we truly haven't identified anything else like it. The authors recommend giving it its own phylum, one that's loosely grouped with the echinoderms.

It's important to note that this analysis is based on a decent number of genes, but the final word will probably have to wait until we have complete genomes from a number of these organisms. It's entirely possible that I'll be writing a further update in the future, explaining why this study got a few things wrong. The tentative nature of science strikes again.

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