Eukarya includes both unicellular and multicellular organisms. It is the only domain of life that contains multicellular species, but a huge number of its members are single-celled. Of the four kingdoms within Eukarya, two are exclusively or predominantly one or the other, while the remaining groups blur the line entirely.
The Four Kingdoms of Eukarya
Eukarya is divided into four kingdoms, each with a different mix of unicellular and multicellular life:
- Protista: Predominantly unicellular. This kingdom includes amoebas, paramecia, diatoms, and many algae.
- Fungi: A mix of both. Yeasts are unicellular, while mushrooms and molds are multicellular. Some species can even switch between the two forms depending on their environment.
- Plantae: Multicellular. All plants, from mosses to oak trees, are made of many cooperating cells.
- Animalia: Multicellular. Every animal, from sponges to humans, is composed of specialized cell types organized into tissues and organs.
So the short answer is that Eukarya is not defined by being one or the other. It spans the full range, from microscopic single cells to blue whales.
Unicellular Eukaryotes Are Enormously Diverse
When people think of single-celled life, bacteria usually come to mind. But unicellular eukaryotes are everywhere and far more complex than bacteria. Every eukaryotic cell, whether it lives alone or as part of a body, contains a nucleus that houses its DNA, along with other internal compartments like mitochondria, an endoplasmic reticulum, and a Golgi apparatus. These membrane-bound structures allow a single eukaryotic cell to carry out sophisticated tasks that bacterial cells cannot.
The protist kingdom alone contains a staggering variety of single-celled organisms. Amoebas like the familiar Amoeba proteus are about 500 micrometers across and move by extending blob-like projections of their cell body. Foraminiferans are unicellular but can grow several centimeters long, sometimes resembling tiny snails. Diatoms encase themselves in intricate glass-like shells made of silicon dioxide. Ciliates like Paramecium are covered in tiny hair-like structures they use to swim. Euglenoids use a primitive eyespot to detect light and two long whip-like tails to steer toward it.
Many unicellular eukaryotes are medically significant. Giardia lamblia is an intestinal parasite, Trichomonas vaginalis causes a common sexually transmitted infection, and trypanosomes carried by tsetse flies cause sleeping sickness. An estimated 1.5 million species of protists exist, many still uncatalogued.
Fungi Sit in Both Categories
Fungi are the clearest example of a kingdom that refuses to fit neatly into either box. Yeasts are fully unicellular and reproduce by budding, where a small new cell pinches off from the parent. Multicellular fungi, on the other hand, grow as networks of thread-like filaments called mycelia. A mushroom is just the above-ground reproductive structure produced by a massive mycelium living underground.
Some fungi are “dimorphic,” meaning they can live as single yeast cells under certain conditions and switch to a multicellular filamentous form when the environment changes. This flexibility makes fungi particularly adaptable and is one reason certain fungal infections are difficult to treat.
How Multicellularity Evolved
All multicellular life descended from unicellular eukaryotic ancestors. The transition happened at least 1.7 billion years ago, and it likely occurred independently in several lineages, which is why animals, plants, and fungi are multicellular in very different ways.
Some living organisms seem to represent intermediate steps in this transition. Certain green algae, like Volvox, form colonies of cells that cooperate but are all essentially the same cell type. These colonies differ from truly multicellular organisms in a key way: true multicellularity involves multiple specialized cell types organized into tissues and organs, with a genuine division of labor. A Volvox colony is like a group of identical workers, while an animal body is like a city with specialists handling different jobs.
Red and green algae illustrate this spectrum particularly well. Within these groups, you can find unicellular species, colonial species, and fully multicellular species. Over evolutionary time, increasing cell specialization drove the transition from loose colonial aggregates to the complex bodies of present-day plants and animals.
What Unites All Eukaryotes
Whether an organism in Eukarya is a single amoeba or a trillion-celled elephant, its cells share the same fundamental architecture. The nucleus stores DNA separately from the rest of the cell, and internal membranes create compartments where different chemical reactions can occur without interfering with each other. Mitochondria generate energy. Many plant and algae cells also contain chloroplasts for photosynthesis.
This internal complexity is what separates eukaryotes from the other two domains of life, Bacteria and Archaea, which are exclusively unicellular and lack these membrane-bound compartments. The compartmentalized eukaryotic cell plan is also what made multicellularity possible in the first place: cells that could already manage complex internal logistics were better equipped to specialize and cooperate as part of a larger body.