Chapter 32 Introduction to Animal Evolution

26.16 Our changing view of biological diversity

26.1 Some major episodes in the history of life. Note that molecular evidence puts origin of animals at 1 bya.

26.2 Clock analogy for some key events in evolutionary history

32.4 A traditional view of animal diversity based on body-plan grades

Kingdom Animalia – general characteristics (and yes, there are exceptions)  Multicellular, heterotrophic eukaryotes.  Feed by ingestion.  Lack cell walls. Bodies held together by structural proteins embedded in tissues (collagen) and between cells.  Presence of nervous and muscle tissue  Store carbohydrate reserves as glycogen  Reproduce sexually with characteristic development of the early 2N embryo.  Transformation and development of the zygote controlled by special regulatory genes (Hox genes).

I. What is an animal? A. Structure, nutrition, and life history define animals 1. Animals are multicellular, heterotrophic eukaryotes. - Animals must take in organic molecules by ingestion; they eat other organisms or organic material that is decomposing. 2. Animal cells have no cell walls. - Bodies are held together by proteins, especially collagen. 3. Animals have two unique types of tissues: a. Nervous tissue and b. Muscle tissue

4. Most animals reproduce sexually. a. Dominant stage is typically diploid. b. Motile sperm fertilizes a larger, non-motile egg (both 1n). c. The resulting zygote (2n) goes through embryonic development as follows: i. The zygote undergoes cleavage, a series of mitotic cell divisions. ii. Cleavage results in the formation of a blastula, a hollow ball of cells. iii. Further development results in the formation of a gastrula, a two-layered, cup-shaped cluster of cells. - This is the stage where tissue differentiation occurs: - Endoderm = digestive tract - Ectoderm = skin, nerves

iv. Many embryos develop directly into adults (sexually mature organisms) v. Some develop into larvae (sexually immature organisms) and later undergo metamorphosis to the adult stage

32.1 Early embryonic development

47.8 Cleavage in a frog embryo

47.6 Cleavage in an echinoderm (sea urchin) embryo

Sea urchin development, from single cell to larva

5. Growth from embryo to adult is modulated (controlled and organized) by Hox genes.  Many genes are the same or similar in all animals. The sequence in which they are turned on and off during development causes embryos to develop into different animals. Thus, for example, the same genes that give rise to dolphins and humans are in both organisms, but the sequence and time in which they are turned on creates either the dolphin or human. This is controlled by the Hox genes.

Animals probably evolved from colonial, flagellated protists, like this choanoflagellate colony. xyYBuI_U

One hypothesis for the origin of animals from a flagellated protist.

II. Two views of animal diversity -The phylogenetic tree of animals has about 35 phyla. - Continuous remodeling of the tree 1. The traditional view of animal diversity is based on anatomy and embryology. 2. Newer trees are based on molecular evidence.

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This section will focus on the four main branch points in this phylogenetic tree. 1. Porifera (Sponges) – early branch  Parazoa; structural simplicity; no true tissue differentiation  Separated from Eumetazoa – all have true tissue differentiation. The next phyla are all Eumetazoa: 2. Radiata – radial symmetry in body structure (no left/right side) Example: jellyfish Separated from Bilateria – two-sided symmetry  left/right, dorsal/ventral, anterior/posterior; cephalization = sensory organs concentrated on anterior end

Body symmetry

No symmetry Radial symmetry Bilateral symmetry

3. Acoelomates – no body cavity Example: flatworms Separated from Pseudocoelomates (e.g., nematodes) and Coelomates (e.g., humans) – have body cavities

4. Protostomia – body cavity forms from cell masses, blastopore becomes mouth Examples: snails, worms, insects Separated from Deuterostomia – body cavity develops from digestive tube, blastopore becomes anus Examples: starfish, chordates (humans)

A comparison of early development in protostomes and deuterostomes

Two views of animal phylogeny: Which is right? Both are useful?

III. Origins of animal diversity A. Most animal phyla originated in a relatively brief span of geologic time. 1. Modern phyla developed in about 40 million years total. 2. During the Cambrian Explosion (543 to 524 million years ago), nearly all major body plans appeared. Really 3 explosions, 1 each within lophotrochozoa, ecdysozoa, & deuterostomia?

26.8 The Cambrian radiation of animals

B. What caused the Cambrian explosion? 1. Development of predators and evolution toward prey escaping/predator hunting. Increased need for speed and better sensory equipment. 2. Oxygen levels reached present levels that allow for rapid metabolism exhibited by animals. 3. Hox genes evolved at that time and allowed for differential development.