Animal Evolution – The Invertebrates

Comparing Key Groups

Evolutionary trends toward • Greater size • Compartmentalization (division of labor among

cells, tissues, and organs)• Integration of specialized activities that keep the

organism alive

Body Symmetry: Radial and Bilateral

Most animals are bilateral

Body Organization

Cephalization • Sensory cells concentrated at the head

Segmentation• Repetition of body units, front-to-back

Body Cavities

Most bilateral animals have a coelom and a complete gut

Key Concepts: INTRODUCING THE ANIMALS

Animals • Multicelled heterotrophs (ingest other organisms) • Grow and develop through a series of stages• Actively move about during all or part of life cycle

Cells of most animals form tissues and extracellular matrixes

Sponges (Phylum Porifera)

Sponges• No symmetry, tissues, or organs • Flattened cells line the body wall (many pores;

spikes of silica and/or proteins)• Filter feeders (flagellated collar cells absorb food;

amoeboid cells digest and distribute it)• Zygote develops into free-living larva

Sponges

Cnidarians (Phylum Cnidaria)

Jellyfishes, corals, and sea anemones• Radial, tentacled carnivores• Gastrovascular cavity (respiration and digestion)• True epithelial tissues with a jellylike matrix in

between• Simple nervous system

Cnidarian Body Plans

Unique Cnidarian Weapons

Nematocysts• Used to capture prey and for defense

Cnidarian Predators

Cnidarian Life Cycles

Flatworms (Phylum Platyhelminthes)

Free-living turbellarians (planarians), parasitic tapeworms and flukes• Simplest animals with organ systems• Paired nerve cords

Parasite Life Cycle: Blood Fluke

Parasite Life Cycle: Tapeworm

Annelids

Segmented worms (earthworms, polychaetes) and leeches• Closed circulatory system• Digestive and excretory systems• Nervous system, ganglia in each segment • Muscles and fluid in chambers act as a

hydrostatic skeleton

secretory organ

headend

Polychaete

Mollusks (Phylum Mollusca)

Including gastropods (snails), bivalves (scallops), chitons, nudibranchs, cephalopods • 100,000 named species

Aquatic Snail Body Plan

Cephalopods

The fastest (squids), largest (giant squids), and smartest (octopuses) invertebrates

Have a mantle • Sheetlike part of the body mass, draped back on

itself

Cephalopods

Cuttlefish Body Plan

Roundworms (Phylum Nematoda)

More than 22,000 kinds of roundworms• Free-living decomposers or parasites• Some agricultural pests and human parasites• Cylindrical body with bilateral features• A complete gut• Organ systems in a false coelom

Parasitic Roundworms

Key Concepts: BILATERAL INVERTEBRATES

Most animals show bilateral symmetry

Bilateral animals have tissues, organs, and organ systems

All adult tissues arise from two or three simple layers that form in early embryos

Simple Arthropods

Trilobite, millipede, centipede

Arthropod Characteristics

Key arthropod adaptations• Hardened exoskeleton • Jointed appendages • Specialized and fused segments (wings)• Efficient respiratory and sensory structures (eyes,

antennae)

Chelicerates

Horseshoe crabs and arachnids (spiders, scorpions, ticks, and mites)• Predators, parasites, or scavengers

Crustaceans

Mostly marine crustaceans (crabs, lobsters, barnacles, krill, and copepods)

Insect Diversity

Unwelcome Arthropods

Spiders, ticks, and mosquitoes

Echinoderms

Sea stars, sea urchins, sea cucumbers, etc.• Exoskeleton with spines, spicules, or plates of

calcium carbonate • Water-vascular system with tube feet• Adults are radial, but bilateral traits appear in

larval stages

Echinoderms: “Spiny-Skinned”

Fig. 23.34, p.381

Animal Evolution – The Vertebrates

Chordate Heritage

Four features define chordates • A notochord • A dorsal hollow nerve cord • A pharynx with gill slits• A tail extending past the anus

All features form in embryos• May or may not persist in adults

Invertebrate Chordates

Tunicates and lancelets (marine filter-feeders)

Lancelet Body Plan

Craniates

Chordates with a braincase of cartilage or bone • Hagfish (jawless fish): Simplest modern craniate

Vertebrate Evolution

Key innovations laid the foundation for adaptive radiations of vertebrates• Vertebral column of cartilaginous or bony

segments • Jaws evolved in predatory fishes• Gills evolved in water, then lungs for dry land• Paired fins were a starting point for other limbs

Gill-Supporting Structures

Key Concepts: TRENDS AMONG VERTEBRATES

In some vertebrate lineages, a backbone replaced the notochord as the partner of muscles used in motion

Jaws evolved, sparking the evolution of novel sensory organs and brain expansions

On land, lungs replaced gills, and more efficient blood circulation enhanced gas exchange

Fleshy fins with skeletal supports evolved into limbs, now typical of vertebrates on land

Jawed Fishes and Tetrapods

Jawed fishes• Cartilaginous fishes (sharks and rays)• Bony fishes

Body plans adapted to life in water• Streamlined shape reduces drag • Swim bladder (in bony fishes) adjusts buoyancy

Cartilaginous Fishes

Bony Fishes

The most diverse vertebrates• Lungfishes • Lobe-finned fishes (coelacanth)• Ray-finned fishes

Early Lineages

Coelacanth, lungfish, and Devonian tetrapod

Amphibians

Frogs, toads, and salamanders• Carnivorous vertebrates• Adapted to life on land (lungs, 3-chambered

heart)• Nearly all return to the water to reproduce

Amphibian Evolution

Amniotes

First vertebrates able to complete their life cycle on dry land• Water-conserving skin and kidneys• Amniote eggs (four membranes)• Active life-styles

Dinosaur Extinctions

K-T asteroid impact hypothesis• A huge asteroid impact caused extinction of last

dinosaurs; spared earliest birds and mammals

Modern Reptiles

Major Groups• Turtles (shell attached to skeleton)• Lizards (the most diverse reptiles)• Snakes (limbless)• Crocodilians (closest relatives of birds)

Fig. 24.17, p.397

Fig. 24.17, p.397

Fig. 24.17, p.397

Reptile Characteristics

General characteristics• Live on land or in water• Cold-blooded• Have a cloaca (opening for wastes and

reproduction) • Eggs are fertilized in the body, usually laid on

land

Fig. 24.16, p.396

cloaca

olfactory lobe(sense of smell)

hindbrain, midbrain,forebrain

spinalcord

vertebral column gonad

kidney (control of water,solute levels in internalenvironment)

unmatched rowsof teeth on upperand lower jaws

snout

stomachesophagus lung heart liver intestine

Birds

Birds are the only modern animals with feathers

Bird Eggs

Birds are warm-blooded amniotes

Adaptations for Flight and Migration

Feathers, lightweight bones, and highly efficient respiratory and circulatory systems

Mammals

Animals with hair, females that nourish young with milk from mammary glands, a single lower jawbone and four kinds of teeth

Modern Mammals

Three major lineages• Egg-laying mammals (monotremes)• Pouched mammals (marsupials)• Placental mammals, the most diverse and

widespread mammals

Three Major Lineages

Placental Mammals

Primates

Primate Evolution

Key trends• Better daytime vision• Upright walking (bipedalism)• More refined hand movements• Smaller teeth• Bigger brains• Social complexity (extended parental care;

culture evolved in some lineages)

The Foramen Magnum

Four-legged walkers versus upright walkers

Emergence of Early Humans

Hominoids and hominids originated in Africa

Australopiths: Upright Walking

Early Humans

Humans (Homo) arose 2 million years ago• H. habilis was an early toolmaking species • H. erectus dispersed into Europe and Asia

Emergence of Modern Humans

Extinct Neandertals and modern humans are close relatives with distinct gene pools

Modern H. sapiens evolved 195,000 years ago

Dispersal of Homo sapiens

Based on fossils and studies of genetic markers