Animal of the week, February 1, 2017—Is it a mouth, is it an anus?

It's so hard to tell these days with all the crap some people speak. But is seems that the problem of telling mouthpiece from arsehole stems back over half a billion years in our evolutionary history.

This week's Animal of the Week Saccorhytus hit the headlines for being perhaps the earliest known example of the major group of animals that includes humans—the deuterostomes.

S Conway Morris / Jian Han

These tiny blighters lived among grains of sand 540 million years ago in what is now China, but was then under the sea. Despite their tiny size, they had muscles, skin, and features that place them firmly in the deuterostomes, which include all animals with backbones, alongside echinoderms (starfish and that lot), and a few other things I won't go into now. What it didn't have, which is somewhat unusual among complex animals, is an anus.

Saccorhytus likely swallowed mouthfuls of sand and algae, squashed out the water through cone like structures that scientists believe might have been precursors of gills, and then spat out waste and unwanted particles.

I won't go into the developmental biology and phylogenetics now, because I'll probably get it wrong and trying to do so will only bring back distant unhappy memories of undergraduate hangovers. But I will mention that the name deuterostome has something to do with developing the anus first and the mouth second. Saccorhytus seems to have made do with just one. When you're a millimetre long, why bother with both?


Researchers' analyses suggest that Saccorhytus was actually most closely related to a long extinct major lineage of deuterostomes and was not a direct ancestor of either humans and other vertebrates or sea cucumbers and other echinoderms, but it does give us the best indication of what the earliest members of this major grouping might have looked like—but then, perhaps it was actually very specialised and unlike other early deuterostomes in its adaptations to living a tiny life.

The mind does boggle at the number of other microscopic fossils that might have been overlooked previously and could offer clues about the evolution of major animal lineages, and anuses. It seems like 2017 might be the year to take a long hard look and really reflect on how we've ended up with the aresholes we've got.