by AMY MAXMEN plus NATURE magazine
Priapulid worm Priapulus caudatus in a Petry dish. The specimen was found in the intertidal of the Russian coast of the Barents Sea. IMAGE/Flickr/Dmitry Aristov
A study on the development of priapulids or ‘penis’ worms throws doubt on a feature that has been thought for more than 100 years to define the largest branch of the animal tree of life. Members of this branch — the protostomes — have historically been defined by the order in which they develop a mouth and an anus as embryos. But gene-expression data suggest that this definition is incorrect, researchers report this week in Current Biology.
Evolutionary biologists will need to rename the protostomes. To do that, “we need to rethink how our earliest ancestors developed,” says Andreas Hejnol, an evolutionary developmental biologist at the University of Bergen in Norway and lead author on the report.
Deep roots
Tiny differences in how embryos develop have defined branches of the animal tree of life because they can lead to monumental changes in adults. For example, a major step in evolution occurred when an embryonic ball of cells formed two indentations as opposed to one, giving way to a separate mouth and an anus rather than the single opening that creatures such as jellyfish and sea anemones have. In 1908, animals with a mouth and anus were divided into two groups. In the protostomes (from the Greek for ‘mouth first’), the mouth formed first, and the anus second. In the other, the deuterostomes (‘mouth second’), the mouth formed after the anus.
Scientific American for more