Chapter 32 An Overview of Animal Diversity 1 Chapter 32 An Overview of Animal Diversity Introduction Biologists have identified 1 3 million extant living species of animals Concept 32 1 Animals are multicellular heterotrophic eukaryotes with tissues that develop from embryonic layers Nutritional Mode Plants are autotrophic eukaryotes capable of generating organic molecules through photosynthesis Fungi are heterotrophs that grown on or near their food and that feed by absorption Animals ingest their food by eating other living organisms or by eating nonliving organic material and then use enzymes to digest it within their bodies Cell Structure and Specialization Animals are multicellular eukaryotes Animals lack the structural support of cell walls Instead external proteins provide structural support to animal cells Most abundant protein is collagen which is only found in animals Animals have two types of specialized cells muscle cells and nerve cells responsible for body movement and conducting nerve impulses Cells are organized into tissues groups of cells that have a common structure function or both Reproduction and Development Most animals reproduce sexually and the diploid stage usually dominates the life cycle Sperm and egg are produced by meiotic division Diploid zygote is formed when a flagellated sperm fertilizes a larger non motile egg Zygote undergoes cleavage a succession of mitotic cell divisions without cell growth between the divisions Cleavage leads to the formation of a multicellular stage called a blastula which in many animals take s the form of a hollow ball Following the blastula stage is the process of gastrulation during which the layers of embryonic tissues that will develop into adult body parts are produced The resulting developmental stage is called a gastrula The life cycle of most animals include at least one larval stage A larva is a sexually immature form of an animal that is morphologically distinct from the adult usually easts different food and may even have a different habitat than the adult Animal larvae eventually undergo metamorphosis a developmental transformation that turns the animal into a juvenile that resembles an adult All animals have developmental genes that regulate the expression of other genes and many of these regulatory genes contain sets of DNA sequences called homeoboxes Homeoboxes contain families of genes known as Hox genes that play important roles in the development of animal embryos Sponges lack Hox genes Hox genes regulate the formation of the anterior posterior front to back axis as well as other aspects of development Concept 32 1 Summary Animals are multicellular heterotrophic eukaryotes with tissues that develop from embryonic layers Animals are heterotrophs that ingest their food Animals are multicellular eukaryotes Their cells are supported and connected to one another by collagen and other structural proteins located outside the cell membrane Nervous tissue and muscle tissue are key animal features In most animals gastrulation follows the formation of the blastula and leads to the formation of embryonic tissue layers All animals have Hox genes that regulate the development of body form Although Hox genes have been highly conserved over the course of evolution they can produce a wide diversity of animal morphology Concept 32 2 The history of animals span more than half a billion years Some estimates suggest that the ancestors of animals diverged from the ancestors of fungi about a billion years ago A combination of morphological and molecular evidence indicates that choanoflagellates are among the closest living relatives of animals The ancestor of living animals may have been a suspension feeder Neoproterozic Era 1 Billion 542 Million Years Ago Some are sponges while others may be related to living cnidarians The first fossils are members of an early group of soft bodied multicellular eukaryotes known as the Ediacaran biota Neoproterozoic rocks have also yielded what may be microscopic signs of early animals The fossil record as it is known today shows that the late Neoproterozoic era was a time of increasing animal diversity Paleozoic Era 542 251 Million Years Ago A wave of animal diversification occurred 525 525 million years ago during the Cambrian period of the Paleozoic era a phenomenon referred to as the Cambrian explosion Paleontologists have found the oldest fossils of about half of all extant animal phyla including the first arthropods chordates and echinoderms Chapter 32 An Overview of Animal Diversity 2 The increase in the diversity of animal phyla during the Cambrian was accompanied by a decline in the diversity of Ediacaran life forms Some evidence suggests that during the Cambrian period predators acquired novel adaptations such as forms of locomotion that helped them catch prey Another hypothesis focuses on an increase in atmospheric oxygen that preceded the Cambrian explosion More oxygen would have enabled animals with higher metabolic rates and larger body sizes to thrive while potentially harming other species Another hypothesis suggests that the Hox genes and other genetic changes affecting the regulation of developmental genes facilitated the evolution of new body forms The Cambrian period was followed by the Ordovician Silurian and Devonian periods when animal diversity continued to increase although punctuated by episodes of mass extinction Vertebrates made the transition to land around 365 million years ago and diversified into numerous terrestrial groups Vertebrates fishes emerged as the top predators of the marine food web Arthropods began to adapt to terrestrial habitats Two of these survive today the amphibians frogs and salamanders and the amniotes reptiles including birds and mammals Mesozoic Era 251 65 5 Million Years Ago The animal phyla that evolved during the Paleozoic began to spread to new habitats Coral reefs formed in the ocean providing other marine animals with new habitats On land descent with modification in some tetrapods led to the origin of wings and other flight equipment in pterosaurs and birds Large and small dinosaurs emerged both as predators and herbivores Flowering plants angiosperms and insects both underwent dramatic diversification Cenozoic Era 65 5 Million Years Ago to the Present Mass extinction of both terrestrial and marine animals ushered in a new era Large nonflying dinosaurs and marine reptiles disappeared Fossil records indicate the rise of large