Protists—Ancestors and Descendants

How are protists related to other eukaryotes?

Protists were the first eukaryotes. How are they related to other eukaryotic organisms today? As tempting as it might be to look among living protists to find the ancestors to the first plants or the earliest fungi, it would be a scientific mistake to do so. The reason, of course, is that protists living today have been through a process of ­evolution just as extensive as the one that produced every other ­living organism.

Microscopic fossils of eukaryotic cells, like the one shown in Figure 21–3, 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 actual 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 found in every corner of the planet.

Most of the major protist groups have remained unicellular, but two have produced organisms that developed true multicellularity. It is from the ancestors of these groups that plants, animals, and fungi arose.

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. The roots of all eukaryotic diversity, from plants to animals to fungi, are found among the ancestors of the organisms that we call protists.

FIGURE 21–3 Fossil of an Early Eukaryote This 1.5-billion-year-old fossil of Tappania plana indicated to scientists that ancient eukaryotes already had the cytoskeletal structures characteristic of protists today. The bulbous projections on the cell are hypothesized to have functioned in asexual reproduction. (LM 285X)

21.1 Assessment

Review Key Concepts

1. 1. Review What is a protist?

   2. Compare and Contrast Compare the updated classification of protists with the older one.

2. 1. Review Which kingdoms arose from protist ancestors?

   2. Apply Concepts Why is it misguided to try to find our earliest eukaryotic ancestor among modern-day protists?

      

VISUAL THINKING

3. Compare Figure 21–2 with the Tree of Life presented in Chapter 18. What simplification does Figure 21–2 make? How could this simplification be misinterpreted? Explain your answer using your knowledge from Chapter 18 of how cladograms are ­constructed.

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