THE LIVING WORLD
Unit Five. Evolution of Animal Life
19. Evolution of the Animal Phyla
19.3. Six Key Transitions in Body Plan
The evolution of animals is marked by six key transitions: the evolution of tissues, bilateral symmetry, a body cavity, segmentation, molting, and deuterostome development. These six body transitions are indicated at the branchpoints of the animal evolutionary tree in figure 19.3.
Figure 19.3. Evolutionary trends among the animals.
In this chapter, we examine a series of key evolutionary innovations in the animal body plan, shown here along the branches. Some of the major animal phyla are shown on this tree. Lophophorates exhibit a mix of protostome and deuterostome characteristics. The traditional tree shown here assumes segmentation arose only once among the invertebrates, while molting arose independently in nematodes and arthropods. The newly proposed molecular phylogenies assume molting arose only once, while segmentation arose independently in annelids, arthropods,and chordates.
The simplest animals, the Parazoa, lack both defined tissues and organs. Characterized by the sponges, these animals exist as aggregates of cells with minimal intercellular coordination. All other animals, the Eumetazoa, have distinct tissues with highly specialized cells.
2. Evolution of Bilateral Symmetry
Sponges also lack any definite symmetry, growing asymmetrically as irregular masses. Virtually all other animals have a definite shape and symmetry that can be defined along an imaginary axis drawn through the animal’s body.
Radial Symmetry. Symmetrical bodies first evolved in marine animals exhibiting radial symmetry. The parts of their bodies are arranged around a central axis in such a way that any plane passing through the central axis divides the organism into halves that are approximate mirror images.
Bilateral Symmetry. The bodies of all other animals are marked by a fundamental bilateral symmetry, a body design in which the body has a right and a left half that are mirror images of each other. This unique form of organization allows parts of the body to evolve in different ways, permitting different organs to be located in different parts of the body. Also, bilaterally symmetrical animals move from place to place more efficiently than radially symmetrical ones, which, in general, lead a sessile or passively floating existence. Due to their increased mobility, bilaterally symmetrical animals are efficient in seeking food, locating mates, and avoiding predators.
3. Evolution of a Body Cavity
A third key transition in the evolution of the animal body plan was the evolution of the body cavity. The evolution of efficient organ systems within the animal body was not possible until a body cavity evolved for supporting organs, distributing materials, and fostering complex developmental interactions.
The presence of a body cavity allows the digestive tract to be larger and longer. This longer passage allows for storage of undigested food and longer exposure to enzymes for more complete digestion. Such an arrangement allows an animal to eat a great deal when it is safe to do so and then to hide during the digestive process, thus limiting the animal’s exposure to predators.
An internal body cavity also provides space within which the gonads (ovaries and testes) can expand, allowing the accumulation of large numbers of eggs and sperm. Such storage capacity allows the diverse modifications of breeding strategy that characterize the more advanced phyla of animals. Furthermore, large numbers of gametes can be stored and released when the conditions are as favorable as possible for the survival of the young animals.
4. The Evolution of Segmentation
The fourth key transition in the animal body plan involved the subdivision of the body into segments. Just as it is efficient for workers to construct a tunnel from a series of identical prefabricated parts, so segmented animals are assembled from a succession of identical segments. Segmentation was assumed to have evolved only once among the invertebrates in the traditional taxonomy, as it seemed such a significant alteration of body plan.
5. The Evolution of Molting
Most coelomate animals grow by gradually adding mass to their body. However, this creates a serious problem for animals with a hard exoskeleton, which can hold only so much tissue. To grow further, the individual must shed its hard exoskeleton, a process called molting or, more formally, ecdysis.
Ecdysis occurs among both nematodes and arthropods. In the traditional taxonomy these are treated as two independent evolutionary events. The new phylogenies suggest ecdysis evolved only once. This would imply that arthropods and nematodes, both of which have hard exoskeletons and molt, are sister groups, and that segmentation rather than ecdysis must have evolved several times among the invertebrates, rather than once.
6. The Evolution of Deuterostome Development
Bilateral animals can be divided into two groups based on differences in the basic pattern of development. One group is called the protostomes (from the Greek words protos, first, and stoma, mouth) and includes the flatworms, nematodes, mollusks, annelids, and arthropods. Two outwardly dissimilar groups, the echinoderms and the chordates, together with a few other smaller related phyla, comprise the second group, the deuterostomes (Greek, deuteros, second, and stoma, mouth). Protostomes and deuterostomes differ in several aspects of embryo growth and will be discussed later in the chapter.
Deuterostomes evolved from protostomes more than 630 million years ago, and the consistency of deuterostome development, and its distinctiveness from that of the proto-stomes suggests that it evolved once, in a common ancestor to all of the phyla that exhibit it.
Characteristics of the major animal phyla are described in table 19.2.
TABLE 19.2. THE MAJOR ANIMAL PHYLA
Key Learning Outcome 19.3. Six key transitions in body design are responsible for most of the differences we see among the major animal phyla.