Chapter 15

1. Chapter 15 Animal Evolution

2. Animal Origins • Animals are multicellular heterotrophs that ingest their food. • No cell walls • Most reproduce sexually, some reproduce asexually, and some can do both • Are motile during part or all of their lives • Have cells that specialize as the animal deveops from embryo to adult

3. Animal Origins • Probably evolved from heterotrophic, colonial protists that could reproduce • Scientists believe that some cells in the colonies underwent mutations that produced cells that could do some tasks and not do other tasks at all • These interdependent cells and the division of labor that came with them put colonies with these cells at a selective advantage • New, specialized cell types were the result and led to the evolution of animals

4. Animal Origins • Placozoans: early animals, very few genes, simplest body plan of any animal, 2mm across and has only 4 cell types • The first animal on Earth probably evolved in the ocean about 1 billion years ago but most animals came into being about 500 million years ago • This is about the time that O2 concentrations in the water increased dramatically and it allowed larger, more active animals to evolve • Also, at about the same, continents were breaking up, cutting off gene flow and leading to speciation

5. Major Animal Characteristics • All animals are descended from a common multicellular ancestor • The earliest animals were only aggregations of cells (like placozoans and sponges) • Most animals have tissues (one or more types of cells that are organized in a specific pattern and that carry out a particular task) • Early animal embryos had two tissue layers: ectoderm and endoderm. • Later animal embryos had a middle tissue layer called mesoderm • This embryo with three tissue layers allowed animals to increase in complexity • Most animal groups have organs derived from the mesoderm.

6. Embryonic Tissue Layers

7. Major Animal Characteristics • The simplest animals have no symmetry (asymmetrical); however later animals have radial symmetry (body parts arranged around a central axis with no front or back end) and the most recent animals have bilateral symmetry (two halves that are mirror images of one another with a distinctive head end and a concentration of nerve tissue) • How would bilateral symmetry be advantageous?

8. Symmetry Jellyfish and Sea Stars Cnidaria Sponges Most Animals

9. Major Animal Characteristics • Bilateral animals have a tubular gut with a mouth at one end and an anus at the other • This is called a complete digestive system and allows parts of the tube to become specialized for taking in food, digesting food, absorbing nutrients, and compacting wastes, allowing all of these to be going on at the same in the gut. • Two lineages of bilateral animals differ in the embryonic development of the tubular gut. • Protostomes are bilateral animals whose first opening that appears in the gut of the embryo becomes the mouth • In deuterostomes, the first opening in the embryo becomes the anus and second opening becomes the mouth.

10. Major Animal Characteristics • Most animals have a fluid-filled body cavity that surrounds the gut (rather than a solid body cavity). • When the tissue that lines this cavity is derived from the mesoderm, the body cavity is called a coelom. • The advantage of this fluid-filled body cavity is three-fold: • Allows materials to diffuse through the fluid to body cells • Muscles can redistribute the fluid to alter body shape and aid in movement • Gives organs more room to grow larger and move more freely

11. Major Animal Characteristics • A closed circulatory system with a heart distributes substances throughout the body much more quickly than diffusion through the fluid-filled body cavity alone. • Most bilateral animals have some degree of segmentation, which is the division of a body into interconnecting units that are repeated one after the other along the main body axis. • Early human embryos are segmented. • This segmentation allowed evolutionary innovations in body form since some segments could specialize in certain functions without endangering the life of the animal. • Most animals are invertebrates (do not have a backbone).

12. Invertebrates: Sponges(Porifera) • Aquatic • No symmetry, tissues, or organs • Adults don’t move about • Larval stage is a free-living, ciliated life stage that moves about • Filter-feeders • Hermaphroditic (produces both eggs and sperm)

13. Invertebrates: Cnidarians • Radial symmetry • Have nerve cells so that they can detect and respond to stimuli; however, they have no central information processing region that functions like a brain • Gastrovascular cavity with one opening that takes in and digests food, expels wastes, and also exchanges gases • Have specialized stinging cells called nematocysts • Two Cnidarian body plans: • Medusae: bell or umbrella shape (jellyfish) • Polyp: tubular with one end attached to a surface (sea anemone)

14. Invertebrates: Flatworms (Platyhelminthes) • Are protostomes • Bilateral symmetry • No body cavity • Hermaphroditic • Flukes and tapeworms are flatworms that spend part of their life cycle as parasites • Branching gut with a single opening • Pair of nerve cords that run the length of the body • Cluster of nerve cells in the head that serve as a simple brain • Head also contains chemical receptors and light detecting eyespots

15. Invertebrates: Segmented Worms (Annelids) • Segmented body • Coelom • Complete digestive system • Closed circulatory system • Nerve cord extends the the length of the body and connects to a simple brain • Multiple hears that pump blood through vessels • Hermaphroditic • Earthworms and leeches are Annelids

16. Invertebrates: Mollusks • Small coelom • Soft, unsegmented body • Have a mantle that forms a mantle cavity and secretes a hard, calcium-rich shell • Very diverse group • Gastropods: snails and slugs “belly foot”, only mollusks that have lung and can breathe air, use a radula (tongue-like organ) for eating • Bivalves: clams, oysters, etc., hinged, two-part shell, are filter feeders • Cephalopods: squids, octopi, etc., are predators, eat with a radula and biting mouthparts, move by jet propulsion, closed circulatory system, have eyes with lenses that focus light, are some of the fastest, biggest, and smartest invertebrates

17. Invertebrates: Roundworms (Nematodes) • More closely related to arthropod than to other worms • Unsegmented • Bilateral symmetry • Complete gut • Reproductive organs • Most are free-living decomposers • Some are parasites (pinworms, dog heartworms)

18. Invertebrates: Arthropods • There are more than a million arthropod species • Six evolutionary adaptations that have made these animals so successful: • Hard exoskeleton composed of chitin for support, protection, and aids in movement • Jointed appendages allow more freedom of movement • Specialized segments (head, thorax, abdomen) and appendages (claws, wings) • Respiratory structures such as gills or air-conducting tubes • Specialized sensory structures including one or more pairs of compound eyes (that are highly sensitive to movement) and one or two pairs of antennae (that can detect touch, odor, and vibrations) • Specialized developmental stages: metamorphosis(body plan is dramatically remodeled as larvae develop into adults)- this prevents juveniles and adults from competing for the same resources

19. Arthropod Classes • Arachnids: spiders, scorpions-live on land, four pair of walking legs, pair of touch sensitive palps, no antennae • Crustaceans: shrimps, crabs, lobsters-marine arthropods with two pairs of antennae • Insects: most diverse arthropods, three part body (head, thorax, abdomen), only winged invertebrates

20. Invertebrates: Echinoderms • The only invertebrates that are deuterostomes • Include, sea stars, sea urchins, and sea cucumbers • Spiny skin with interlocking plates made of calcium carbonate embedded in their skin form an endoskeleton • Radial symmetry but larvae have bilateral symmetry • No brain but do have a nervous system • Eyespots detect light and movement • Water vascular system for movement • Sexes are separate

21. Chordates • The majority of deuterostomes are chordates. • Chordate embryos have: • A notochord (a rod of stiff but flexible connective tissue that extends the length of the body and provides support) • A dorsal, hollow nerve cord that runs parallel to the notochord • Gill slits that open across the wall of the pharnyx • A muscular tail that extends beyond the anus • Some, all, or none of these traits persist in the adult, depending upon the chordate group.

22. Chordates • There are three groups of chordates: • Tunicates: invertebrates, attach to an undersea surface and filter food • Lancelets: invertebrates, fish-shaped, no brain or paired sensory organs but does have eyespots, are probably the closest invertebrate relatives of vertebrates • Vertebrates: have several major innovations • Vertebral column: protects spinal cord • Jaws: opened up new feeding opportunities • Swim bladder/lungs: allows fish to adjust buoyancy/exchange gases • Four limbs: more mobility • Amniote (waterproof) egg: most successful land tetrapods

23. Chordates

24. Vertebrates • All vertebrates have: • A brain • Closed circulatory system with one heart • Urinary system with a pair of kidneys • Complete digestive system

25. Vertebrates: Fishes • Jawless Lampreys: are ancient fishes, no jaws or fins but do have a backbone • Jawed Fishes: have paired fins and scales • Cartilaginous fishes: skeleton made of cartilage, including sharks • Bony fishes: embryonic skeleton made of cartilage transforms to bone in adult, protective gill cover, swim bladder • Ray-finned fishes: flexible fins (salmon, sardines. tuna, etc.) • Lobe-finned fishes: (coelacanths and lungfishes) fins are fleshy body extension with bony elements inside of them, lungfishes have lung-like sacs and many of them will drown if left underwater

26. Vertebrates: Transition to Land • All land vertebrates evolved from lobe-finned fishes. • Bones inside of lobe-finned fish’s pelvic and pectoral fins are homologous with amphibian limb bones. • Division of heart into three chambers also preceded movement to land allowed blood to flow to body and to LUNGS • Eyelids and changes in inner ear (to detect airborne sounds)also evolved

27. Land Vertebrates: Amphibians • Tetrapods that spend time on land but require water to breed • Larvae have gills but adults lose gills and develop lungs

28. Land Vertebrates: The Amniotes • Branched off from amphibian ancestors • Have amniotic eggs with four membranes that enable embryos to develop away from water • Also covered in a keratin rich covering that makes the egg waterproof to prevent dessication • Well-developed kidneys to conserve water • Internal fertilization is typical

29. Land Vertebrate Amniotes: Reptiles • Are ectotherms (adjust their internal body temperature by their behavior) • Dry, thick skin that is waterproof • Includes lizards, snakes, turtles, crocodilians (such as crocodiles, alligators, caimans) • Crocodilians have a four-chambered heart like birds and mammals. • They are the closest living relatives of birds and, like birds, lay eggs, then protect and care for their young • All teeth are similarly shaped

30. Land Vertebrate Amniotes: Birds • Only modern amniotes with feathers • Feathers are modified scales that help adapt bird for flight • Bones are hollow to make them lightweight (another adaptation for flight) • Most efficient respiratory system of any vertebrate • Four-chambered heart • Endotherms (produce their own heat through metabolic processes)

31. Land Vertebrate Amniotes: Mammals • Only amniotes in which females nourish their young with milk secreted from mammary glands • Only animals that have hair or fur • Endothermic • Most have more than one type of tooth • Three groups of mammals: • Monotremes: egg-laying mammals (platypus) • Marsupials: pouched mammals (koala, kangaroo) • Placental mammals: organ called placenta provides nourishment to developing offspring (humans)

32. Mammals Koala Platypus Kangaroo Placental mammals