The question of how many phyla exist in the tree of life does not have a single, fixed answer, but rather a spectrum of estimates depending on the domain of life being discussed. Biological classification is a constantly evolving science, and the number of recognized phyla changes based on new discoveries and genetic analysis. The total count varies dramatically between macroscopic Eukaryotes, like animals and plants, and microscopic Prokaryotes, such as bacteria and archaea. The number reflects the deep and ancient evolutionary divisions that separate life forms.
What is a Phylum?
A phylum represents a major, foundational grouping in the Linnaean system of biological classification, positioned directly beneath the Kingdom rank and above Class. This taxonomic level is designed to group organisms that share a fundamental, ancient body plan or structural organization, reflecting a very early divergence in evolutionary history. For instance, all members of a phylum might share a specific type of symmetry or a particular method of embryonic development.
The definition of a phylum historically relied on easily observable features, known as the phenetic definition, which focused on morphological and developmental similarity. Modern classification also incorporates the phylogenetic definition, requiring that all members of a phylum share a higher degree of evolutionary relatedness to each other than to any other group. For plants and fungi, the term “division” is often used interchangeably with phylum.
The Current Count in Eukaryotic Life
The number of phyla within the eukaryotic domain, which includes animals, plants, fungi, and protists, is relatively small and stable. The Kingdom Animalia contains the largest set of divisions, with approximately 35 to 40 accepted phyla. This number reflects the diversity of body plans that evolved during the Cambrian explosion, such as the segmented bodies of Arthropoda and the water vascular systems of Echinodermata.
Examples of these major animal groupings include the Phylum Chordata, which encompasses all vertebrates, and the Phylum Arthropoda, the largest phylum by number of species, including insects, spiders, and crustaceans. The other eukaryotic kingdoms contain much smaller phylum counts. The Kingdom Plantae is divided into about 14 phyla, while the Kingdom Fungi contains an estimated 8 to 19 phyla, illustrating the narrower range of body plans in these domains.
Why Prokaryotic Phyla Are Harder to Count
The count of phyla explodes when considering the domains Bacteria and Archaea, collectively known as prokaryotes, where the total number is far less certain. Classification in these domains cannot rely on body plan, as most prokaryotes are single-celled organisms that lack complex morphology. Instead, scientists rely heavily on molecular data, primarily sequencing the gene for the small subunit ribosomal RNA (16S rRNA), which acts as a molecular clock to estimate evolutionary distance.
This molecular approach has revealed a vast, hidden microbial diversity, with estimates for the total number of bacterial phyla ranging from around 41 formally accepted groups to potentially over 1,300, depending on the definition and database used. A significant challenge is that approximately 72% of these widely recognized phyla are “candidate phyla,” meaning they have been identified solely through genetic sequencing from environmental samples. These candidate groups have no cultured representatives, making their physical and metabolic characteristics unknown and their taxonomic status tentative. The constantly changing nature of this field means the number of prokaryotic phyla is a dynamic figure that will likely continue to rise.