Unit Five. Evolution of Animal Life


19. Evolution of the Animal Phyla



Animals are the most diverse in appearance of all eukaryotes. Polistes, the common paper wasp you see here, is a member of the most diverse of all animal groups, the insects. It has been a major challenge for biologists to sort out the millions of kinds of animals. This wasp has a segmented external skeleton and jointed appendages, and so, based on these characteristics, is classified as an arthropod. But how are arthropods related to mollusks such as snails, and to segmented worms like earthworms? Until recently, biologists grouped all three kinds of animals together, as they all share a coelom body cavity, a character assumed to be so fundamental it could have evolved only once. Now, molecular analyses suggest this assumption may be wrong. Instead, mollusks and segmented worms are grouped together with other animals that grow the same way you do, by adding additional mass to an existing body, while arthropods are grouped with other molting animals. These animals increase in size by molting their external skeletons, an ability that seems to have evolved only once. Thus we learn that even in a long-established field like taxonomy, biology is constantly changing.


19.1. General Features of Animals


As best as biologists can determine, we and all other animals evolved from a kind of protist called a choanoflagel- late. From these early animal ancestors, a great diversity of animals has since evolved. While the evolutionary relationships among the different types of animals are being debated (see section 19.2), all animals have several features in common (table 19.1): (1) Animals are heterotrophs and must ingest plants, algae, animals, or other organisms for nourishment. (2) All animals are multicellular, and unlike plants and protists, animal cells lack cell walls. (3) Animals are able to move from place to place. (4) Animals are very diverse in form and habitat. (5) Most animals reproduce sexually. (6) Animals have characteristic tissues and patterns of embryonic development.




Heterotrophs. Unlike autotrophic plants and algae, animals cannot construct organic molecules from inorganic chemicals. All animals are heterotrophs—that is, they obtain energy and organic molecules by ingesting other organisms. Some animals (herbivores) consume autotrophs, other animals (carnivores) consume heterotrophs; others, like the bear to the right, are omnivores that eat both autotrophs and heterotrophs, and still others (detritivores) consume decomposing organisms.

Multicellular. All animals are multicellular, often with complex bodies like that of this brittlestar (right). The unicellular heterotrophic organisms called protozoa, which were at one time regarded as simple animals, are now considered members of the large and diverse kingdom Protista, discussed in chapter 17.

No Cell Walls. Animal cells are distinct among those of multicellular organisms because they lack rigid cell walls and are usually quite flexible, like these cancer cells. The many cells of animal bodies are held together by extracellular lattices of structural proteins such as collagen. Other proteins form a collection of unique intercellular junctions between animal cells.

Active Movement. The ability of animals to move more rapidly and in more complex ways than members of other kingdoms is perhaps their most striking characteristic, one that is directly related to the flexibility of their cells and the evolution of nerve and muscle tissues. A remarkable form of movement unique to animals is flying, an ability that is well developed among vertebrates and insects like this butterfly. The only terrestrial vertebrate group never to have evolved flight is amphibians.

Diverse in Form. Almost all animals (99%) are invertebrates, which, like this millipede, lack a backbone. Of the estimated 10 million living animal species, only 42,500 have a backbone and are referred to as vertebrates. Animals are very diverse in form, ranging in size from organisms too small to see with the unaided eye to enormous whales and giant squids.

Diverse in Habitat. The animal kingdom includes about 55 phyla, most of which, like these jellyfish (phylum Cnidaria), occur in the sea. Far fewer phyla occur in freshwater and fewer still occur on land. Members of three successful marine phyla, Arthropoda (insects), Mollusca (snails), and Chordata (vertebrates), dominate animal life on land.

Sexual Reproduction. Most animals reproduce sexually, as these tortoises are doing. Animal eggs, which are nonmotile, are much larger than the small, usually flagellated sperm. In animals, cells formed in meiosis function directly as gametes. The haploid cells do not divide by mitosis first, as they do in plants and fungi, but rather fuse directly with each other to form the zygote. Consequently, with a few exceptions, there is no counterpart among animals to the alternation of haploid (gametophyte) and diploid (sporophyte) generations characteristic of plants.

Embryonic Development. Most animals have a similar pattern of embryonic development. The zygote first undergoes a series of mitotic divisions, called cleavage, and, like this dividing frog egg, becomes a solid ball of cells, the morula, then a hollow ball of cells, the blastula. In most animals, the blastula folds inward at one point to form a hollow sac with an opening at one end called the blastopore. An embryo at this stage is called a gastrula. The subsequent growth and movement of the cells of the gastrula differ widely from one phylum of animals to another.

Unique Tissues. The cells of all animals except sponges are organized into structural and functional units called tissues, collections of cells that have joined together and are specialized to perform a specific function. Animals are unique in having two tissues associated with movement: (1) muscle tissue, which powers animal movement, and (2) nervous tissue, which conducts signals among cells. Neuromuscular junctions, where nerves connect with muscle tissue, are shown here.



Key Learning Outcome 19.1. Animals are complex, multicellular, heterotrophic organisms. Most animals also possess internal tissues.