The largest branch of animals in the tree of lifes are the protostomes – historically defined by the order in which they develop a mouth and an anus as embryos. But new gene-expression data conducted on “penis worms” suggests otherwise.

The mouth and the anus

Biologists will have to rename and rethink the protostomes, explains Andreas Hejnol, an evolutionary developmental biologist at the University of Bergen in Norway and lead author on the report: “we need to rethink how our earliest ancestors developed”, he explains.

Even the tiniest differences in embryonic evolution can lead to monumental changes in adults; the best example is when an embryonic ball of cells formed two indentations instead of one, thus leading to the separation of the mouth and the anus, rather than the single, all purposes hole jellyfish and sea anemones have. Back in 1908, more than 100 years ago, animals with both anus and mouth were divided into two different groups: the ones in which the mouth is formed first (protostomes), and the ones in which the anus is formed first.

Today, the protostomes include most invertebrates, while the other group, called deurostomes include most vertebrates, including human – yes, our anus is formed before our mouth. Because analyzing how the embryon is formed and divided is technically challenging and quite costly, researchers have analyzed only a fraction of animals in each group, but this theory has beeon confirmed by other DNA sequencing data and the division between protostomes and deurostomes remained in place.

Shaking things up

Everything seemed to be ok, but this theory was shaken up by the most unlikeliest of creatures: Priapus caudatus, commonly referred to as the “penis worm”. The animal was believed to be a primitive protostome, but Hejnol and his team revelead that it acually develops like a deuterostome.

“Here is an animal that is the poster child for early protostomes, and it develops just like a deuterostome,” says Mark Martindale, a developmental biologist at the University of Hawaii in Honolulu. “We’ve been using the name protostome for 100 years, and now it’s clear that it doesn’t mean anything.”

The problem is that they are not only simply placed on the wrong side of the tree. Other similarities, mostly DNA sequences indicate they very much belong to the protostomes.

“This is a big achievement because for years, people have wanted to do work like this in priapulids, but it’s technically very difficult,” says Detlev Arendt, an evolutionary developmental biologist at the European Molecular Biology Laboratory in Heidelberg, Germany.

So where does this leave us? Obviously, if the research is correct, and there is no reason to doubt this, we need a new classification, and a new class of species; unfortunately… biologists don’t have one ready yet.

“At the turn of the twentieth century, embryologists drew what they saw. But their microscopes stunk and they didn’t know about genes,” Martindale says. “Now we’re finally able to look closer, and we’re finding that they’re often wrong. But if you think about the tools they had, maybe it’s more surprising that anything is right at all.”