The evolution of segmentation is a controversial topic in biology. Biologists have struggled to answer even the most basic questions. Were our ancestors segmented or not? Has segmentation evolved only once, or has it evolved multiple times independently? What is a segment, after all?
The basis of such confusion is partly biological. Biology is complicated, and there aren’t always simple answers. Studying evolution itself is challenging. Comparative data is hard to get, phylogenetic trees are not yet resolved, and the fossil record is sparse.
But part of this confusion regarding segmentation is also semantic. When a biologist says segmentation, it’s often unclear what they mean exactly.
Segmentation can stand for trunk segments, like in arthropods and annelids. But it can also be used for the repeated vertebra, the sequential mesodermal somites, or the branchial arches of chordates. Segmentation means the pattern—the repeated nature of these traits—but it also entails the process by which these structures become segmented. Worst of all, biologists frequently refer to “segmented animals” as a uniform group, without considering the large differences between their segmental traits. What a mess!
The concept of segmentation is just too abstract. And now I understand the root of it. Segmentation is a zombie noun. It’s a nominalization:
Nouns formed from other parts of speech are called nominalizations. Academics love them; so do lawyers, bureaucrats and business writers. I call them “zombie nouns” because they cannibalize active verbs, suck the lifeblood from adjectives and substitute abstract entities for human beings:From Helen Sword at Opinionator, The New York Times
Maybe the first step to solving the segmentation conundrum in biology is to avoid this zombie noun as much as possible, and be explicit about which segmental traits are being discussed.