Chapter 32 An Introduction to Animal Diversity Overview Welcome to Your Kingdom Biologists have identified 1 3 million living species of animals Total number of animal species run far higher from 10 to 20 million to as many as 100 to 200 million Concept 32 1 There are exceptions to nearly every criterion for distinguishing an animal from other life forms However five criteria taken together comprise a reasonable definition Nutritional Mode Animals cant construct all of their own organic molecules so they ingest them either by eating other living organisms or by eating nonliving organic material Most animals use enzymes to digest their food only after they have ingested it Cell Structure and Cell Specialization Animals are multicellular Animals lack structural support of cell walls but instead animal bodies are held together by structural proteins the most abundant being collagen Animals have other unique types of intercellular junctions tight junctions desmosomes and gap junctions Consist of other structural proteins Among animal cells are two specialized forms not seen in other multicellular organisms muscle cells and nerve cells Responsible for movement and impulse conduction Reproduction and Development Most animals reproduce sexually with the diploid stage usually dominating the life cycle During the development of most animals cleavage leads to the formation of a multicellular stage called the In most species a small flagellated sperm fertilizes a larger nonmotile egg forming a diploid zygote The zygote undergoes cleavage a succession of mitotic cell divisions w o cell growth between division cycles blastula which in many animals takes the form of a hollow ball Following the blastula stage is the process of gastrulation during which layers of the embryonic tissues that will develop into adult body parts are produced The resulting development stage is called a gastrula Some animals develop directly through transient stages of maturation into adults but the life cycle of many animals also include at least one larval stage A larva is a sexually immature form of an animal 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 Animals share a unique homeobox containing family of genes known as Hox genes All eukaryotes have genes that regulate the expression of other genes Many of these regulatory genes contain common modules of DNA sequences called homeoboxes All animals share the unique family of Hox genes suggesting that this gene family arose in the eukaryotic lineage that gave rise to animals Hox genes play important roles in the development of animal embryos regulating the expression of dozens or hundreds of other genes Hox genes control cell division and differentiation producing different morphological features of animals Hox genes in sponges regulate the formation of channels the primary feature of sponge morphology In more complex animals the Hox gene family underwent further duplication In bilaterians Hox genes regulate patterning of the anterior posterior axis The same conserved genetic network governs the development of a large range of animals Concept 32 2 The history of animals may span more than a billion years Some calculations based on molecular clocks estimate that the ancestors of animals diverged from the ancestors of fungi as much as 1 5 billion years ago Similar studies suggest that the common ancestor of living animals lived 1 2 billion to 800 million years ago The common ancestor was probably a colonial flagellated protist and may have resembled modern choanoflagellates Neoproterozoic Era 1 billion 542 million years ago Although molecular data indicates a much earlier origin of animals the oldest generally accepted animal fossils are only 575 million years old These fossils are known as the Ediacara fauna named for the Ediacara Hills of Australia Ediacara fauna consist primarily of cnidarians but soft bodied mollusks were also present and numerous fossilized burrows and tracks indicate the presence of worms Paleozoic Era 542 251 million years ago Animals underwent considerable diversification between 542 525 million years ago during the Cambrian period of the Paleozoic Era There are several hypotheses regarding the cause of the Cambrian explosion 1 The new predator prey relationships that emerged in the Cambrian may have generated diversity through During this period known as the Cambrian explosion about half of extant animal phyla arose Fossils of Cambrian animals include the first animals with hard mineralized skeletons natural selection Predators acquired adaptations that helped them catch prey Prey acquired adaptations that helped them resist predation 2 A rise of atmospheric oxygen preceded the Cambrian explosion More oxygen may have provided opportunities for animals with higher metabolic rates and larger body sizes 3 The evolution of the Hox complex provided the developmental flexibility that resulted in variations in morphology These hypotheses are not mutually exclusive all may have played a role In the Silurian and Devonian periods animal diversity continued to increase punctuated by episodes of mass extinction Vertebrates fishes became the top predators of marine food webs By 460 million years ago arthropods began to adapt to terrestrial habitats Vertebrates moved to land about 360 million years ago and diversified into many lineages Two of these survive today amphibians and amniotes Mesozoic Era 251 65 5 million years ago Few new animal body plans emerged among animals during the Mesozoic era Animal phyla began to spread into new ecological niches In the oceans the first coral reefs formed On land birds pterosaurs dinosaurs and tiny nocturnal insect eating mammals arose Cenozoic Era 65 5 million years ago to the present Insects and flowering plants both underwent a dramatic diversification during the Cenozoic era This era began with mass extinctions of terrestrial and marine animals Among the groups of species that disappeared were large nonflying dinosaurs and the marine reptiles Large mammalian herbivores and carnivores diversified as mammals exploited vacated ecological niches Some primate species in Africa adapted to open woodlands and savannas as global climates cooled Our ancestors were among these grassland apes Concept 32 3 Animals can be characterized by body plans Zoologists