The domain Eukarya represents all life forms whose cells possess a membrane-bound nucleus and specialized compartments called organelles. This fundamental structure distinguishes eukaryotes from prokaryotes (Bacteria and Archaea), whose cells lack a nucleus and most other internal membrane-bound structures. Eukaryotic organisms range from single-celled microbes to complex multicellular beings, and their cells are significantly larger and more intricate than those of prokaryotes. The evolution of the eukaryotic cell involved endosymbiosis, where ancient prokaryotic cells were engulfed by a host cell, eventually becoming organelles like mitochondria and chloroplasts.
The Three Multicellular Kingdoms
The most familiar types of eukaryotes are grouped into three large, predominantly multicellular kingdoms: Animalia, Plantae, and Fungi. These kingdoms are primarily differentiated by their modes of nutrition and fundamental cell structure.
Organisms within Kingdom Animalia are multicellular and heterotrophic, obtaining nutrients by ingesting other organisms. Animal cells lack rigid cell walls, allowing for the structural flexibility needed for movement and complex tissue organization. Most species exhibit mobility at some point in their life cycle.
Kingdom Plantae consists of multicellular, non-motile organisms that are typically autotrophic, generating food through photosynthesis. Plant cells possess rigid cell walls composed primarily of cellulose, which provides structural support. They rely on chloroplasts, containing chlorophyll, to capture light energy for conversion into chemical energy.
Fungi, in Kingdom Fungi, are mostly multicellular (except for yeasts) and are heterotrophic like animals. Fungi absorb nutrients from the environment after secreting digestive enzymes externally, a method called saprotrophic nutrition. Their cells are encased in a cell wall made of chitin, a tough polysaccharide.
The Problem of Protista
Historically, any eukaryote that did not fit the defining characteristics of an animal, plant, or fungus was placed into a catch-all group called Kingdom Protista. This group was immensely diverse, including organisms such as algae, slime molds, and protozoa. This classification was based on exclusion rather than shared evolutionary ancestry.
Molecular and genetic analyses revealed that Protista was not a natural group because it did not include all the descendants of a single common ancestor. A group that excludes some descendants of a common ancestor is termed paraphyletic, and the traditional Protista was a collection of many distinct lineages. This diversity spanned unicellular forms and simple multicellular colonies, making classification based purely on visible traits problematic.
The realization that some “protists” were more closely related to animals, plants, or fungi necessitated a complete re-evaluation of eukaryotic classification. This resulted in the abandonment of Protista as a formal kingdom and the adoption of a new, phylogeny-based system. This modern classification groups the entire domain Eukarya into a series of supergroups, which are thought to be monophyletic clades.
Eukaryotic Supergroups
The modern classification organizes all eukaryotes, including the three multicellular kingdoms and the former protists, into several major clades known as supergroups. These supergroups are based on shared genetic sequences and common ancestry, rather than morphological similarities. This system provides a more accurate representation of the tree of life, although the exact number and composition of supergroups are still subject to refinement.
One major supergroup is Opisthokonta, which encompasses both Kingdom Fungi and Kingdom Animalia, along with closely related single-celled groups like choanoflagellates. This grouping highlights the shared evolutionary heritage between animals and fungi. Another supergroup is Archaeplastida, which includes Kingdom Plantae and all types of algae whose chloroplasts arose from a single primary endosymbiotic event with a cyanobacterium.
The remaining supergroups primarily contain organisms formerly classified as protists, demonstrating their phylogenetic separation. The Excavata supergroup includes organisms like the parasitic Giardia and photosynthetic Euglena. They are often characterized by an asymmetrical cell shape and a feeding groove, and many members possess highly modified mitochondria.
The Amoebozoa supergroup is characterized by species that use blunt, lobe-shaped cytoplasmic extensions called lobose pseudopodia for movement and feeding. This group includes true amoebas, such as Entamoeba, and the diverse slime molds. Another major lineage is the SAR clade (Stramenopiles, Alveolates, and Rhizaria), a diverse supergroup that includes diatoms, brown algae, and foraminiferans.