Protist Phylogenetic Tree

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson we will discuss the somewhat messy, polyphyletic protist family tree. We will talk about how protists evolved and mention several broad groups of protists.

Protist Phylogeny

Do you have a junk drawer in your house? Perhaps it has spare keys, ribbon, notepads, rubber bands, electronic chargers, batteries, and a few old business cards, or something along those lines. What do all of those items have in common? They're small, but functionally, they aren't much alike at all. They're all there because they don't have anywhere else to go.

The kingdom Protista is the junk drawer of the modern classification system. Protists are single-celled organisms, but don't have much else in common. Protists are polyphyletic, meaning that they developed from more than one ancestral lineage. Polyphyletic groups drive systematicists (people who study the interrelationships of organisms) a little crazy. Polyphyletic groups mess up our tidy little organizational system.

In contrast, the kingdom Animalia is believed to be monophyletic, having a single ancestor. If you look back far enough, the kingdom Animalia only branched off of the tree of life once, about 670 million years ago. Every descendant of that branch is also classified as an animal. Protists, on the other hand, probably arose uniquely several times, and not every descendant of every branch is considered to be a protist.

Protists show up on several places on the eukaryotic branch of the tree of life.

History of Classification

Why the mess? The word 'protist' was coined in 1866 to describe all single-celled organisms. In the early-to-mid 1900s, though, this group was divided into prokaryotes and eukaryotes.

  • Prokaryotes have simple circular DNA, no nuclear membrane, and few or no organelles (subunits dedicated to a particular function). Bacteria and archaea are classified as prokaryotes.
  • Eukaryotes have a nucleus and are vastly more complex. Protists are eukaryotes, but so are many multicellular animals, like humans.

In 1969, Robert Whittaker introduced the 5-kingdom system you may have seen: Plantae, Animalia, Fungi, Monera (bacteria), and Protista. Protists had their own kingdom, though organisms classified as protists aren't necessarily more closely related to each other than to other groups.

Evolution of Eukaryotes

Scientists have puzzled over how eukaryotes may have evolved. In the 1960s and 1970s, evolutionary biologist Lynn Margulis introduced the theory of endosymbiosis, the idea that at one point, a prokaryotic cell ate another prokaryotic cell, but instead of destroying each other, the cells went on to have a happy symbiotic relationship. The one that ate the other gained energy, and the one eaten gained protection and a permanent home. It would be like if Pac-Man ate one of the dots but the dot stayed in him and simply generated extra energy, enabling Pac-Man to do more things.

Not only protists, but plants, animals, and fungi eventually sprung from this fortuitous relationship.

How We Classify Protists

Protists are classified by their molecular relatedness, by how they move, their mitochondria, pigmentation, what they eat, and whether they have body armor. Some protists obtain energy through photosynthesis, some are predators, some are decomposers, and some are parasites.

Keep in mind that the accepted phylogeny of protists is constantly being updated as we learn more. Sometimes group names will be different depending on which source you find. However, hopefully these general classifications will help.

Photosynthetic Protists

The group of photosynthetic protists includes Pyrrhophyta, Euglenophyta, Chrysophyta, Rhodophyta, Phaeophyta, and Chlorophyta. Have you ever used a product called 'diatomaceous earth' for insect control or as an abrasive? Diatomaceous earth is the fossilized remains of diatoms, photosynthetic protists that have a silicon shell. These shells form beautiful geometric shapes that can be viewed under a microscope. It is the shape of these tiny shells that allow them to be a gentle abrasive, and they are sometimes used in toothpaste or metal polish.

Diatom drawings

Heterotrophic Protists

The group of heterotrophic (non-photosynthesizing), motile protists with flagella includes Sarcomastigophora and Ciliophora. Some people think that members of this group are ancestors of the kingdom Animalia. This is in part because so many animals have motile sperm, and many flagellate protists will congregate.

The group of heterotrophic protists that are motile but don't have a permanent locomotor apparatus includes Rhizopoda, Actinopoda, and Foraminifera. These protists move around with a blob called a pseudopod. If you know what an amoeba looks like, you've likely seen a pseudopod. Scary (but rare) cases of disease commonly called 'brain amoeba' are caused by an amoeba called Naegleria fowleri.

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