Chapter 15Animal Evolution

Animal OriginsAnimals are multicellular

heterotrophs that ingest their food.

No cell wallsMost 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

Animal OriginsProbably evolved from heterotrophic,

colonial protists that could reproduceScientists 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

Animal OriginsPlacozoans: 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

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.

Embryonic Tissue Layers

Major Animal CharacteristicsThe 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?

Symmetry

Sponges Jellyfish and Sea StarsCnidaria

Most Animals

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.

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

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).

Invertebrates: Sponges(Porifera)AquaticNo symmetry, tissues, or organsAdults don’t move aboutLarval stage is a free-living,

ciliated life stage that moves about

Filter-feedersHermaphroditic (produces both

eggs and sperm)

Invertebrates: CnidariansRadial symmetryHave 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)

Invertebrates: Flatworms (Platyhelminthes)

Are protostomesBilateral symmetryNo body cavityHermaphroditicFlukes and tapeworms are flatworms that

spend part of their life cycle as parasitesBranching gut with a single openingPair of nerve cords that run the length of the

bodyCluster of nerve cells in the head that serve as

a simple brainHead also contains chemical receptors and

light detecting eyespots

Invertebrates: Segmented Worms (Annelids)

Segmented bodyCoelomComplete digestive systemClosed circulatory systemNerve cord extends the the length of

the body and connects to a simple brain

Multiple hears that pump blood through vessels

HermaphroditicEarthworms and leeches are Annelids

Invertebrates: MollusksSmall coelomSoft, unsegmented bodyHave a mantle that forms a mantle cavity and

secretes a hard, calcium-rich shellVery 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

Invertebrates: Roundworms (Nematodes)More closely related to arthropod

than to other wormsUnsegmented Bilateral symmetryComplete gut Reproductive organsMost are free-living decomposersSome are parasites (pinworms,

dog heartworms)

Invertebrates: ArthropodsThere are more than a million arthropod speciesSix 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

Arthropod ClassesArachnids: 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

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 systemEyespots detect light and movementWater vascular system for movementSexes are separate

ChordatesThe 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 anusSome, all, or none of these traits persist in

the adult, depending upon the chordate group.

ChordatesThere are three groups of chordates:

1. Tunicates: invertebrates, attach to an undersea surface and filter food

2. Lancelets: invertebrates, fish-shaped, no brain or paired sensory organs but does have eyespots, are probably the closest invertebrate relatives of vertebrates

3. 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

Chordates

VertebratesAll vertebrates have:

◦A brain◦Closed circulatory system with one

heart◦Urinary system with a pair of kidneys◦Complete digestive system

Vertebrates: FishesJawless Lampreys: are ancient fishes, no

jaws or fins but do have a backboneJawed 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

Vertebrates: Transition to LandAll 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

Land Vertebrates: AmphibiansTetrapods that spend time on

land but require water to breedLarvae have gills but adults lose

gills and develop lungs

Land Vertebrates: The AmniotesBranched off from amphibian ancestorsHave 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

Land Vertebrate Amniotes: ReptilesAre ectotherms (adjust their internal body

temperature by their behavior)Dry, thick skin that is waterproofIncludes 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

Land Vertebrate Amniotes: BirdsOnly modern amniotes with feathersFeathers are modified scales that help

adapt bird for flightBones are hollow to make them

lightweight (another adaptation for flight)

Most efficient respiratory system of any vertebrate

Four-chambered heartEndotherms (produce their own heat

through metabolic processes)

Land Vertebrate Amniotes: Mammals

Only amniotes in which females nourish their young with milk secreted from mammary glands

Only animals that have hair or furEndothermicMost have more than one type of toothThree groups of mammals:

◦Monotremes: egg-laying mammals (platypus)◦Marsupials: pouched mammals (koala,

kangaroo)◦Placental mammals: organ called placenta

provides nourishment to developing offspring (humans)

Mammals

Platypus

Koala

Kangaroo

Placental mammals