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

2. Lesson Overview Lesson Overview Protist Classification—The Saga Continues 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.

3. Lesson Overview Lesson Overview Protist Classification—The Saga Continues 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.

4. Lesson Overview Lesson Overview Protist Classification—The Saga Continues Multiple Kingdoms? 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.

5. Lesson Overview Lesson Overview Protist Classification—The Saga Continues What “Protist” Means Today Biologists assembling the Tree of Life favor the classification shown in the cladogram.

6. Lesson Overview Lesson Overview Protist Classification—The Saga Continues 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.

7. Lesson Overview Lesson Overview Protist Classification—The Saga Continues Protists—Ancestors and Descendants 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.

8. Lesson Overview Lesson Overview Protist Classification—The Saga Continues 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.

9. Lesson Overview Lesson Overview Protist Classification—The Saga Continues Protists—Ancestors and Descendants The roots of all eukaryotic diversity, from plants to animals, are found among the ancestors of protists.