But many would argue that’s not true of the rank of phylum, particularly for animals. “In terms of the taxonomic hierarchy, there’s some sense in which the phylum and the species both have rigorous currency,” said Derek Briggs, a paleontologist at Yale University, “whereas everything in between there’s a kind of construct.”

To Briggs and others, the delineations of phyla may be crude, but they are clear: They distinguish sets of organisms with “fundamentally different body plans.” The definitions of body plans can draw on details of anatomy and tissue organization; they can also refer to one or more stages in an organism’s life cycle or development. Individual characteristics may belong to multiple phyla, but the full set of characteristics is supposed to define each phylum uniquely. For example, among other defining traits, the anemones and jellyfish that make up the phylum Cnidaria are radially symmetrical, have an opening that serves as both mouth and anus, and capture prey with specialized stinging cells; the roundworms of the phylum Platyhelminthes have three distinct tissue layers as embryos, are bilaterally symmetrical, and lack a body cavity; the insects, spiders and crustaceans in the phylum Arthropoda have segmented exoskeletons and molt between developmental stages.

This idea that distinctive body plans could serve as an organizational scheme for life is actually older than the term phylum. The French zoologist Georges Cuvier sorted animal life into four “embranchments” based on comparative anatomy in 1817; similarly, the German scientist Karl Ernst von Baer identified four “animal types” based on embryology in 1828. The term phylum was coined by Ernst Haeckel in his Generelle Morphologie der Organismen, published in 1866. Haeckel specifically noted five phyla of animals — coelenterates, echinoderms, “articulates” (a group including annelids and arthropods), mollusks and vertebrates — but he used the term the way modern scientists might use the terms clade or monophyletic group to refer to a set of organisms that all descend from a common ancestor.

Over time, the number of animal phyla has expanded to about 35. Yet there has never been a solid definition for what makes a group a phylum as opposed to a subphylum, a class or any other taxonomic rank. There have been many arguments about whether groups like vertebrates or nematodes are distinctive enough to be their own phyla, or whether phyla like arthropods, tardigrades, velvet worms and annelids should be lumped in with others (as they were in Haeckel’s day).

Ultimately, the decision to name a particular clade a phylum is “completely anthropogenic,” Hejnol said. He also noted that the distinctions are biased to favor human perspectives on what looks different, because they tend to emphasize qualities obvious to our eyes over less visible ones, such as genomic characteristics.

But perhaps a bigger problem than the artificiality of the boundaries between phyla is that they also tell us little about the range in diversity within a phylum. Some, like the phylum Placozoa, have almost no morphological diversity: All placozoans look so much alike that researchers haven’t yet decided whether there are only a handful of species or more than a hundred of them.

Other phyla contain creatures so divergent from their phylum’s nominal body plan that they’re almost impossible to recognize as members of that category. Tetraplatia, for example, is “a weird jelly that’s more or less shaped like a worm,” said Allen Collins, the director of the National Oceanic and Atmospheric Administration’s National Systematics Lab. If it were part of an older lineage, Tetraplatia might be considered to belong in its own order, class or phylum rather than being lumped in as a bizarre hydrozoan beside the jellies, corals and anemones of the phylum Cnidaria.

Or take rhizocephalan barnacles. The adult females are internal parasites of crabs: They grow inside their hosts in a form that resembles a branching mass of roots. They look about as different from other arthropods as you could imagine. Nothing about the phylum label Arthropoda suggests that it contains such a weird departure from the rest — or that maybe it shouldn’t.

Indeed, the irony is that no matter what strange new forms evolution may invent in eons to come, no new phyla can be created to house them — because future organisms must fall under the same phylum as their ancestors, and the only firm taxonomic rule defining a phylum is that it cannot be nested inside another phylum. “That shows you the artificiality of it,” Jenner said, “because that parasitic barnacle may be as different from [its] arthropod ancestor as any other thing, but you cannot make it its own phylum.”

This points to the paradox inherent in the phylum concept: In theory, phyla mark the morphological uniqueness of distinct body plans. But phyla can’t really signify morphological uniqueness because the hierarchical system requires shoving all of life’s divergent forms into ranks that treat them as equals, no matter how different they may really be. “Mother Nature can’t be straitjacketed,” Jenner said. “And if she makes a phylum-level difference — a ‘hopeful monster’ that looks nothing like its brethren — then yes, we should be able to say this is literally a phylum-level difference. But we can’t.”

Blowing Up the Cambrian Explosion

In reality, phyla are defined by more than body plans. They’re often considered to be distinctive groups of organisms that arose within a particular stretch of 5 to 20 million years during the early Cambrian Period, which started more than 500 million years ago. The sudden burst of diversity during this time is often referred to as the “Cambrian explosion,” and as James Valentine, professor emeritus of biology at the University of California, Berkeley, once explained, it’s thought to have occurred because the lack of animal biodiversity up to that point was unique in the history of life. Some say that abrupt climatological or geological shifts were important, too — but whatever the exact trigger, the way that evolution altered species back then was seemingly different from the way it alters them now.

But more recent data have countered this idea that there was something special about the diversification of life a half-billion years ago. The discovery of new fossils that sit on the “stems” of currently recognized phyla show that the so-called body plans actually arose stepwise over time. Their apparent morphological distance from one another could therefore be purely an artifact of fossilization and extinction, without being representative of unique biological processes. Some biologists, like the paleogenomics researcher David A. Gold at the University of California, Davis, have proposed that what happened in the early Cambrian was less an “explosion” than the ignition of a “long fuse” of biological innovation. Others argue for a series of pulses of diversification.

One thing is clear: The phyla didn’t all pop into existence at the same time. Cnidarians, for example, had already split into the lineages we recognize as classes before echinoderms came onto the scene. As a 2019 paper in Nature Ecology and Evolution pointed out, there is more divergence between those cnidarian classes — which include the Scyphozoa (jellyfish), the Cubozoa (box jellies), the Anthozoa (sea anemones, corals, sea pens), and the highly diverse Hydrozoa — than between humans and sea urchins.