21.1 Protist Classification —The Saga Continues Lesson Overview 21.1 Protist Classification —The Saga Continues

THINK ABOUT IT Some of the organisms we call “protists” live quietly on the bottom of shallow ponds, soaking up the energy of sunlight. Others swim vigorously in search of tiny prey. Some, such as diatoms, sparkle in coastal waters. Still others drift in the human bloodstream, destroying blood cells and killing nearly a million people a year. What kind of life is this, capable of such beauty and such destruction?

The First Eukaryotes What are protists?

The First Eukaryotes What are protists? Protists are eukaryotes that are not members of the plant, animal, or fungi kingdoms.

The First Eukaryotes More than a billion years ago, the first eukaryotes appeared on Earth. Single-celled eukaryotes are still with us today and are often called “protists”—a name that means “first.” Traditionally, protists are classified as members of the kingdom Protista. Protists are eukaryotes that are not members of the plant, animal, or fungi kingdoms.

The First Eukaryotes Although most protists are unicellular, quite a few are not. Brown algae called kelp are the largest protists. They contain millions of cells arranged in differentiated tissues. Kelp are considered protists because they are related more closely to certain unicellular protists than to members of any other kingdom. Otters wrap themselves in giant kelp to keep from drifting out to sea while they sleep.

The “Protist” Dilemma Biologists have discovered that “protists” display a far greater degree of diversity than any other eukaryotic kingdom. Euglena, brown algae, diatoms, and slime molds are examples of protists.

The “Protist” Dilemma In addition to their diversity, biologists also found that many “protists” are far more closely related to members of other eukaryotic kingdoms than they are to other “protists.” By definition, the members of a living kingdom should be more like one another than like members of other kingdoms. This is not true of protists, which means that reclassification is necessary. In the past, scientists sorted protists into three groups: plantlike protists, animal-like protists, and funguslike protists. However, this solution began to fail as biologists learned that many protists do not fit into any of these groups. Biologists also discovered that many of the animal-like and funguslike protists are so similar that they belong in a single group, not two.

Multiple Kingdoms? The most recent studies of protists divide them into six major clades, each of which could be considered a kingdom.

Multiple Kingdoms? This cladogram represents an understanding of protist relationships supported by current research.

Multiple Kingdoms? Surprisingly, the plant, animal, and fungi kingdoms fit right into these six clades. Animals and fungi actually emerge from the same protist ancestors. Protists were the first eukaryotes, and evolution has had far more time to develop differences among protists than among more recently evolved eukaryotes like plants and animals. By finding the fundamental divisions among protists, we also identify the most basic differences among all eukaryotes.

What “Protist” Means Today Biologists assembling the Tree of Life favor the classification shown in the cladogram.

What “Protist” Means Today Even though the biologist building the Tree of Life prefer a different classification, the word “protist” remains in common usage, even among scientists. Bear in mind that “protists” are not a single kingdom but a collection of organisms that includes several distinct clades.

Protists—Ancestors and Descendants How are protists related to other eukaryotes?

Protists—Ancestors and Descendants How are protists related to other eukaryotes? Today’s protists include groups whose ancestors were among the very last to split from the organisms that gave rise to plants, animals, and fungi.

Protists—Ancestors and Descendants Microscopic fossils of eukaryotic cells, like Tappania plana shown, have been found in rocks as old as 1.5 billion years. Genetic and fossil evidence indicates that eukaryotes evolved from prokaryotes and are more closely related to present-day Archaea than to Bacteria. The split between Archaea and Eukarya may have come as early as 2.5 billion years ago. Since that time, protists have diversified into as many as 300,000 species.

Protists—Ancestors and Descendants Most of the major protist groups have remained unicellular, but two have produced multicellular organisms. Plants, animals, and fungi arose from the ancestors of these multicellular groups.

Protists—Ancestors and Descendants The roots of all eukaryotic diversity, from plants to animals, are found among the ancestors of protists.