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CHAPTER 32 INTRODUCTION TO ANIMAL EVOLUTION Introduction Animal life began in Precambrian seas with the evolution of multicellular forms that lived by eating other organisms. Early animals populated the seas, fresh waters, and eventually the land. A. What is an animal? 1. Structure, nutrition and life history define animals While there are exceptions to nearly every criterion for distinguishing an animal from other life forms, five criteria, when taken together, create a reasonable definition. (1) Animals are multicellular, heterotrophic eukaryotes. They must take in preformed organic molecules through ingestion , eating other organisms or organic material that is decomposing. (2) Animal cells lack cell walls that provide structural supports for plants and fungi. The multicellular bodies of animals are held together with the extracellular proteins, especially collagen. In addition, other structural proteins create several types of intercellular junctions, including tight junctions, desmosomes, and gap junctions, that hold tissues together. (3) Animals have two unique types of tissues: nervous tissue for impulse conduction and muscle tissue for movement. (4) Most animals reproduce sexually, with the diploid stage usually dominating the life cycle.
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In most species, a small flagellated sperm fertilizes a larger, nonmotile eggs. The zygote undergoes cleavage , a succession mitotic cell divisions, leading to the formation of a multicellular, hollow ball of cells called the blastula . During gastrulation , part of the embryo folds inward, forming the blind pouch characteristic of the gastrula. This produces two tissue layers: the endoderm as the inner layer and the ectoderm as the outer layer. Some animals develop directly through transient stages into adults, but others have distinct larval stages. The larva is a sexually immature stage that is morphologically distinct from the adult, usually eats different foods, and may live in a different habitat from the adult. Animal larvae eventually undergo metamorphosis , transforming the animal into an adult. (5) The transformation of a zygote to an animal of specific form depends on the controlled expression in the developing embryo of special regulatory genes called Hox genes. These genes regulate the expression of other genes. Many of these Hox genes contain common “modules” of DNA sequences, called homeoboxes. Only animals possess genes that are both homeobox-containing in structure and homeotic in function. All animals, from sponges to the most complex insects and vertebrates have Hox genes, with the number of Hox genes correlated with the complexity of the animal’s anatomy. 2. The animal kingdom probably evolved from a
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