Unit Four. The Evolution and Diversity of Life


15. How We Name Living Things


15.9. Domain Eukaria


For at least 1 billion years, prokaryotes ruled the earth. No other organisms existed to eat them or compete with them, and their tiny cells formed the world’s oldest fossils. The third great domain of life, the eukaryotes, appear in the fossil record much later, only about 1.5 billion years ago. Metabolically, eukaryotes are more uniform than prokaryotes. Each of the two domains of prokaryotic organisms has far more metabolic diversity than all eukaryotic organisms taken together.


Three Largely Multicellular Kingdoms

Fungi, plants, and animals are well-defined evolutionary groups, each of them clearly stemming from a different single-celled eukaryotic ancestor. They are largely multicellular, each a distinct evolutionary line from an ancestor that would be classified in the kingdom Protista.

The amount of diversity among the protists, however, is much greater than that within or between the three largely multicellular kingdoms derived from the protists. Because of the size and ecological dominance of plants, animals, and fungi, and because they are predominantly multicellular, we recognize them as kingdoms distinct from Protista.


A Fourth Very Diverse Kingdom

When multicellularity evolved, the diverse kinds of single-celled organisms that existed at that time did not simply become extinct. A wide variety of unicellular eukaryotes and their relatives exists today, grouped together in the kingdom Protista solely because they are not fungi, plants, or animals. Protists are a fascinating group containing many organisms of intense interest and great biological significance.


Symbiosis and the Origin of Eukaryotes

The hallmark of eukaryotes is complex cellular organization, highlighted by an extensive endomembrane system that subdivides the eukaryotic cell into functional compartments called organelles (see chapter 4). Not all of these organelles, however, are derived from the endomembrane system. Mitochondria and chloroplasts are both believed to have entered early eukaryotic cells by a process called endosymbiosis (endo, inside) where an organism such as a bacterium is taken into the cell and remains functional inside the cell.

With few exceptions, all modern eukaryotic cells possess energy-producing organelles, the mitochondria. Mitochondria are about the size of bacteria and contain DNA. Comparison of the nucleotide sequence of this DNA with that of a variety of organisms indicates clearly that mitochondria are the descendants of purple bacteria that were incorporated into eukaryotic cells early in the history of the group. Some protist phyla have in addition acquired chloroplasts during the course of their evolution and thus are photosynthetic. These chloroplasts are derived from cyanobacteria that became symbiotic in several groups of protists early in their history. Figure 15.11a shows how this could have happened, with the green cyanobacterium being engulfed by an early protist. Some of these photosynthetic protists gave rise to land plants. Endosymbiosis is not strictly an ancient process but still happens today. Some photosynthetic protists are endosymbionts of some eukaryotic organisms, such as certain species of sponges, jellyfish, corals (the green structures inside the coral in figure 15.11& are endosymbiotic protists), octopuses, and others. We discussed the theory of the endosymbiotic origin of mitochondria and chloroplasts in chapter 4, and we will revisit it in chapter 17.



Figure 15.11. Endosymbiosis.

(a) This figure shows how an organelle could have arisen in early eukaryotic cells through a process called endosymbiosis. An organism, such as a bacterium, is taken into the cell through a process similar to endocytosis but remains functional inside the host cell. (b) Many corals contain endosymbionts, algae called zooxanthellae that carry out photosynthesis and provide the coral with nutrients. In this photograph, the zooxanthellae are the greenish-brown spheres packed into the tentacles of a coral animal.


Key Learning Outcome 15.9. Eukaryotic cells acquired mitochondria and chloroplasts by endosymbiosis. The organisms in the domain Eukarya are divided into four kingdoms: fungi, plants, animals, and protists.


Inquiry & Analysis

What Causes New Forms to Arise?

Biologists once presumed that new forms—genera, families, and orders—arose most often during times of massive geological disturbance, stimulated by the resulting environmental changes. But no such relationship exists. An alternative hypothesis was proposed by evolutionist George Simpson in 1953. He proposed that diversification followed new evolutionary innovations, “inventions” that permitted an organism to occupy a new “adaptive zone.” After a burst of new orders that define the major groups, subsequent specialization would lead to new genera.

The early bony fishes, typified by the sturgeon (see lower right), had feeble jaws and long, sharklike tails. They dominated the Devonian (the Age of Fishes), to be succeeded in the Triassic (the period when dinosaurs appeared) by fishes like the gar pike, with a shorter, more powerful jaw that improved feeding and a shortened, more maneuverable tail that improved locomotion. They were in turn succeeded by teleost fishes like the perch, with an even better tail for fast, maneuverable swimming, and a complex mouth with a mobile upper jaw that slides forward as the mouth opens.

This history allows a clear test of Simpson's hypothesis. Was the appearance of these three orders followed by a burst of evolution as Simpson predicts, the new innovations in feeding and locomotion opening wide the door of opportunity? If so, many new genera should be seen in the fossil record soon after the appearance of each new order.

If not, the pattern of when new genera appear should not track the appearance of new orders.

The graph shows the evolutionary history of the class Osteichthyes, the bony fishes, since they first appeared in the Silurian some 420 million years ago.

1. Applying Concepts. What is the dependent variable?

2. Interpreting Data. Three great innovations in jaw and tail occur during the history of the bony fishes, producing the superorders represented by sturgeons, then gars, and then teleost fishes. In what period did each innovation occur?

3. Making Inferences. Do bursts of new genera appear at these same three times, or later?

4. Drawing Conclusions. Do the data presented in the graph support Simpson's hypothesis? Explain.

5. Further Analysis. If you were to plot on the gra] the rate at which new families of fishes appeared, what general pattern would you expect to see, relative to new orders, if Simpson is right? Explain.



Test Your Understanding

1. The wolf, domestic dog, and red fox are all in the same family, Canidae. The scientific name for the wolf is Canis lupus, the domestic dog is Canis familiaris, and the red fox is Vulpes vulpes. This means that

a. the red fox is in the same family, but different genus than dogs and wolves.

b. the dog is in the same family, but different genus than red foxes and wolves.

c. the wolf is in the same family, but different genus than dogs and red foxes.

d. all three organisms are in different genera.

2. The evolutionary relationship of organisms, and their relationships to other species, are its

a. taxonomy.

b. phylogeny.

c. ontogeny.

d. systematics.

3. Organisms are classified based on

a. physical, behavioral, and molecular characteristics.

b. where the organism lives.

c. what the organism eats.

d. the size of the organism.

4. Organisms that are closer together on a cladogram

a. are in the same family.

b. comprise an outgroup.

c. share a more recent common ancestor than those organisms that are farther apart.

d. share fewer derived characters than organisms that are farther apart.

5. The six kingdoms of organisms can be organized into three domains based on

a. where the organism lives.

b. what the organism eats.

c. cell structure.

d. cell structure and DNA sequence.

6. All of the extremophiles belong to the domain of

a. Bacteria.

b. Archaea.

c. Prokarya.

d. Eukarya.

7. Bacteria are similar to Archaea in that they

a. arose through endosymbiosis.

b. are multicellular.

c. live in extreme environments.

d. are prokaryotes.

8. It is theorized that the ancestral organisms that gave rise to the plants, animals, and fungi originated in the kingdom

a. Bacteria.

b. Archaea.

c. Protista.

d. All of the above, each giving rise to one of the three kingdoms listed.

9. One difference between the kingdom Protista and the other three kingdoms in the domain Eukarya is that the other kingdoms are mostly

a. prokaryotic.

b. multicellular.

c. eukaryotic.

d. unicellular.

10. It is thought that the mitochondria and chloroplasts of eukaryotic cells came from

a. the development of the internal membrane system.

b. protists.

c. mutation.

d. endosymbiosis of bacteria.