Life On Planet Earth
Chapter 17a: Evolution of Invertebrate Animals
The Origins of Animal Diversity
Animal life began in Precambrian seas (~600 million years ago) with the evolution of multicellular creatures that consumed other organisms.
What Is an Animal?
Animals are:
Eukaryotic (cells contain nuclei).
Multicellular.
Heterotrophic, obtaining nutrients by ingestion.
Able to digest food internally.
Unique Features of Animal Cells
Lack cell walls, unlike plants and fungi.
Most have muscle cells and nerve cells for movement and control.
Animal Reproduction & Development
Diploid (two sets of chromosomes).
Reproduce sexually via meiosis to produce gametes (sperm & egg).
Undergo a series of developmental stages during growth.
Early Animals and the Cambrian Explosion
Animals likely evolved from a colonial flagellated protist.
The oldest known animal fossils date back 550–575 million years.
Molecular data suggest an even earlier origin for animals.
The Cambrian Explosion (525–535 million years ago)
Marked a rapid increase in animal diversification.
Many new animal body plans and phyla appeared in a short evolutionary timeframe.
Possible causes:
Increasingly complex predator-prey relationships.
Rise in atmospheric oxygen levels
Causes of the Cambrian Explosion (C.E.)
Possible factors that triggered the C.E.:
Complex predator-prey relationships drove evolutionary adaptations.
Increased atmospheric oxygen supported larger, more active animals.
The role of Hox genes:
Hox genes (master control genes) regulate body plans in animals.
However, these genes existed before the C.E., meaning their evolution did not directly cause the explosion of diversity.
Animal Phylogeny and Body Plans
Classification Criteria:
Body Plan – General features of body structure.
Genetic Data – DNA analysis to determine evolutionary relationships.
Key Evolutionary Branch Points:
1st Split: Sponges vs. Other Animals
Sponges lack true tissues, setting them apart from more complex animals.
Tissues = Groups of similar cells performing a specific function.
Organs = Functional groupings of multiple tissue types.
2nd Split: Body Symmetry: External body structures
Radial Symmetry: Identical around a central axis (e.g., jellyfish).
Bilateral Symmetry: Can be divided into equal halves only one way (e.g., humans, insects).
Bilateral Symmetry is linked to cephalization, the development of a head region.
Major Invertebrate Phyla
Invertebrates:
Animals without backbones.
Make up 95% of all animal species.
Ancestral to vertebrates.
Sponges (Phylum Porifera):
Likely represent multiple phyla (similar to how “protists” is a convenience grouping).
Key Features:
Stationary animals (sessile).
Lack true tissues.
Likely evolved early from colonial protists.
~8,700 described species today.
Sponges (Phylum Porifera) - Structure and Function
Body Structure:
The sponge's body resembles a sac perforated with holes.
Choanocytes (Collar Cells):
Set up water currents to draw water through the sponge's walls.
Collect small food particles carried by the water.
Amoebocytes:
Mobile cells that move via pseudopodia.
Function similarly to white blood cells in vertebrates (e.g., macrophages).
Located in the sponge's wall, these cells:
Play a role in defending the organism against pathogens.
Digest and distribute food, dispose of wastes.
Can transform into other cell types, acting as stem cells.
Cnidarians (Phylum Cnidaria) - Structure and Characteristics
Key Features:
Body tissues (unlike sponges)
Radial symmetry
Tentacles with stinging cells
Approximately 10,300 described species today.
Basic Body Plan:
A sac with a gastrovascular cavity (a central digestive compartment with only one opening).
Body Plan Variations:
Polyp: Stationary form.
Medusa: Floating form.
Cnidarians (Phylum Cnidaria) - Feeding Mechanism
Carnivores:
Use tentacles, armed with cnidocytes (stinging cells), for:
Defense
Capturing prey
Cnidocyte Mechanism:
The trigger is activated when prey comes into contact with the tentacle.
Coiled thread within the capsule is discharged to entangle and capture the prey.
Molluscs (Phylum Mollusca) - Characteristics
General Features:
Soft-bodied animals, usually protected by a hard shell.
Relatively speciose, with approximately 117,400 described species today.
Feeding Mechanism (except bivalves):
Use a radula, a file-like organ, to scrape food from hard substrates.
Mollusc Body Parts:
Muscular Foot:
Used for movement.
Visceral Mass:
Contains most of the internal organs.
Mantle:
A fold of tissue that secretes the shell (if present).
Mollusc Digestive System:
Complete Digestive Tract:
Has two openings (mouth and anus).
One-way movement of food leads to increased digestive efficiency.
Major Groups of Molluscs
Gastropods:
Include snails (protected by a single, spiraled shell).
Some slugs and sea slugs lack shells entirely.
Bivalves:
Include clams, oysters, mussels, and scallops.
Have a shell divided into two halves, hinged together.
Have no head (lack cephalization) but are highly successful evolutionarily.
Cephalopods:
Include squids and octopuses.
Typically lack an external shell.
Built for speed and agility.
Flatworms- simplest bilateral animals
Free living in marine, freshwater, or damp terrestrial environments
Gastrovascular cavity:
Highly branched.
Provides extensive surface area for nutrient absorption.
Annelids
Phylum: Annelida
Species: ~17,200 described species
Key feature: Body segmentation (body divided into repeated segments)
Main Groups:
Earthworms:
Eat their way through soil.
Polychaetes:
Marine worms.
Have segmental appendages for movement and gas exchange.
Leeches:
Mostly free-living carnivores, some are bloodsuckers.
Roundworms
Phylum: Nematoda (~24,800 described species)
Characteristics:
Cylindrical shape, tapered at both ends
Most numerous and widespread of all animals
Importance:
Act as decomposers
Can be dangerous parasites in plants, humans, and other animals
Arthropods
Phylum: Arthropoda (>1.2 million described species, mostly insects)
Characteristics:
Named for their jointed appendages
Extremely diverse and successful, found in nearly all habitats
Four Main Groups of Arthropods
Arachnids
Crustaceans
Millipedes & Centipedes
Insects
General Characteristics of Arthropods
Segmented Body:
Specialized body segments
Appendages adapted for different functions, enabling efficient division of labor
Exoskeleton:
Provides protection
Serves as an attachment point for muscles
Limits body growth → must be periodically shed (molting), making the animal temporarily vulnerable to predation
Body Regions of Arthropods
Cephalothorax (head + thorax combined)
Abdomen
Antennae (sensory organs)
Swimming Appendages (in some arthropods)
Arachnids:
Habitat: Usually live on land
Characteristics:
Four pairs of walking legs
Specialized pedipalps (feeding appendages)
Examples: Spiders, scorpions, ticks, mites
Crustaceans
Habitat: Nearly all aquatic
Characteristics:
Multiple pairs of specialized appendages
Examples: Crabs, lobsters, crayfish, shrimp, barnacles
Millipedes and Centipedes
Millipedes:
Eat decaying plant matter
Have two pairs of short legs per body segment
Centipedes:
Terrestrial carnivores with poison claws
Have one pair of short legs per body segment
Insect Anatomy
Body Structure: Three-part body
Head
Thorax
Abdomen
Head Features:
Pair of sensory antennae
Pair of eyes
Mouthparts adapted for various types of eating
Key Adaptation: Flight contributes to their success
Insect Diversity
Abundance: Insects outnumber all other forms of life combined.
Habitats:
Found in almost every terrestrial habitat
Present in fresh water and air
Largely absent from saltwater habitats
Metamorphosis in Insects
Many insects undergo metamorphosis during development.
Types of Development:
Incomplete Metamorphosis: Young insects (nymphs) resemble smaller forms of the adult.
Complete Metamorphosis: Insects change from a larval form to a significantly different adult form.
Echinoderms (Phylum Echinodermata)
Species Count: ~7,500 described species.
Key Characteristics:
Lack body segments
Symmetry:
Adults: Pentamerous radial symmetry.
Larvae: Bilateral symmetry (indicating evolution from bilaterally symmetrical ancestors).
Endoskeleton: Internal skeletal structure.
Water Vascular System: Facilitates movement and gas exchange.
Morphological Diversity: Echinoderms exhibit a wide range of body forms.
17b
Vertebrate Evolution and Diversity
Unique Endoskeleton:
Cranium (Skull)
Backbone made of vertebrae
Classification: Vertebrates are a subgroup within Phylum Chordata
Characteristics of Chordates (Phylum Chordata) (~68,000 species)
Four Key Features (Present in Embryo & Sometimes Adult):
Dorsal, hollow nerve cord
Notochord
Pharyngeal slits
Post-anal tail
Body Segmentation:
Backbone in vertebrates
Segmental muscles in all chordates
Chordate Groups
Lancelets – Bladelike animals without a cranium
Tunicates (Sea Squirts) – Lack a cranium
Vertebrates
Fishes
First vertebrates:
Evolved during the early Cambrian period (~542 MYA)
Lacked jaws
Represented today by lampreys (Agnatha = jawless vertebrates)
Hagfish (Agnatha)
Lack jaws
Have a cranium
Lost vertebral column
All other vertebrates belong to Gnathostomata (= jawed vertebrates)
Two major groups of living fishes:
Cartilaginous fishes (e.g., sharks and rays)
Flexible skeleton made of cartilage
Bony fishes
Skeleton reinforced by hard calcium salts
Bony fishes include:
Ray-finned fishes
Lobe-finned fishes
Key features:
Both cartilaginous and bony fishes have a lateral line system, which detects minor vibrations in the water.
Lift and buoyancy:
Cartilaginous fish must swim continuously to maintain lift off the bottom.
Bony fish typically have swim bladders (gas-filled sacs, the antecedent to our lungs), which help them maintain buoyancy.
Amphibians:
Characteristics of amphibians:
Exhibit a mixture of aquatic and terrestrial adaptations.
Typically require water for reproduction.
Undergo metamorphosis from an aquatic larva to a terrestrial adult.
Historical transition:
The transition from aquatic to terrestrial vertebrate life occurred ~375 million years ago.
Characteristics of amphibians:
Exhibit a mixture of aquatic and terrestrial adaptations.
Typically, they require water for reproduction.
Undergo metamorphosis from an aquatic larva to a terrestrial adult.
Historical transition:
The transition from aquatic to terrestrial vertebrate life occurred ~375 million years ago.
Amphibians as the first vertebrates to colonize land:
Descended from fishes that had:
Lungs
Fins with muscles
Skeletal supports strong enough to enable some movement on land
Tetrapods:
Terrestrial vertebrates are collectively called tetrapods, which means “four feet.”
Tetrapods include:
Amphibians
Reptiles
Mammals
Reptiles
Reptiles (including birds) and mammals are amniotes, meaning they produce amniotic eggs, which:
Are fluid-filled
Have “waterproof” shells
Enclose the developing embryo
Reptiles include:
Snakes
Lizards
Turtles
Crocodiles
Alligators
Birds (plus other dinosaurs!)
Reptile adaptations to living on land:
Amniotic egg
Scaled, “waterproof” skin
Nonbird Reptiles
Non-bird reptiles are ectotherms, sometimes mistakenly called “cold-blooded.” This means that:
They obtain body heat from the environment.
Energy efficiency:
A non-bird reptile can survive on less than 10% of the calories required by a bird or mammal of equivalent size.
The advantage of being an ectotherm is particularly significant in food-limited environments.
Reptile Evolution and the Mesozoic Era
Reptiles diversified extensively during the Mesozoic Era (250–66 million years ago).
Dinosaurs evolved ~230 million years ago (mya) and were:
The most diverse reptile group.
The largest animals ever to live on land.
The Age of Reptiles started to decline about 70 mya as the global climate became cooler and more variable.
Birds: Evolution and Adaptations
Origin:
Genetic and fossil evidence suggests that birds evolved from a lineage of small, two-legged dinosaurs called theropods during the great reptilian radiation of the Mesozoic Era.
A current hypothesis suggests that their relatively small body size helped them survive the mass extinction event (~66 mya) that killed all other dinosaurs.
Adaptations for Flight:
Lighter body structure:
Honeycombed bones (light but strong).
One ovary instead of two.
Beak instead of teeth.
Nine air sacs that enhance respiratory efficiency.
Endothermy:
Unlike other reptiles, birds are endotherms, maintaining a warm and steady body temperature.
Wing Structure and Aerodynamics:
Bird wings function as airfoils, utilizing the same aerodynamic principles as airplane wings.
Flight power comes from breast muscles anchored to a keel-like breastbone.
Feathers (made of the same protein as reptile scales) likely first evolved for:
Insulation (retaining body heat).
Courtship displays.
Only later were they adapted for flight.
Mammals
Origin and Early Evolution:
First mammals arose ~220 mya.
Likely small, nocturnal insect-eaters.
Diversity and Adaptation:
Most mammals are terrestrial.
Some groups, such as dolphins, porpoises, and whales, are fully aquatic.
Whale evolution:
Transitioned from land to water ~55 mya.
This transition took approximately 20 million years.
Unique characteristics of mammals:
Hair
Mammary glands that produce milk to nourish young
Three middle ear bones
Three major groups of mammals:
Monotremes – Egg-laying mammals
Marsupials – Pouched mammals with a placenta
The placenta consists of embryonic and maternal tissues.
It joins the embryo to the mother within the uterus.
The embryo receives oxygen and nutrients from maternal blood flowing near the embryonic blood system.
Eutherians (Placental Mammals)
Their placentas provide a more intimate and longer-lasting connection between the mother and the developing young compared to marsupial placentas.
Human Ancestry and Primate Evolution
Humans belong to the primate group, which also includes:
Lorises
Pottos
Lemurs
Tarsiers
Monkeys
Non-human apes
Primates:
The mammalian group that includes Homo sapiens and our closest kin.
Evolved from insect-eating mammals in the late Cretaceous (~65 million years ago).
Early primates were small, arboreal (tree-dwelling) mammals.
Natural selection shaped their adaptations for tree life.
Arboreal Adaptations of Primates
Limber shoulder joints → Allow swinging from branch to branch.
Eyes positioned close together → Overlapping fields of vision enhance depth perception.
Excellent eye-hand coordination.
Extensive parental care → Primates are among the most attentive parents in the mammal group.
Classification of Primates
Three main groups of primates:
Lemurs, lorises, and bush babies
Found in Madagascar, southern Asia, and Africa.
Tarsiers
Small, nocturnal tree-dwellers found only in Southeast Asia.
Anthropoids
Includes monkeys and apes.
Many anthropoids (excluding most New World monkeys) have a fully opposable thumb (can touch the tips of all four fingers with the thumb).
Apes: Our Closest Anthropoid Relatives
Non-human apes include:
Gibbons
Orangutans
Gorillas
Chimpanzees
Apes live only in tropical regions of the Old World.
Behavioral and physical traits:
Gibbons and orangutans → Primarily arboreal.
Gorillas and chimpanzees → Highly social.
Apes have larger brains (relative to body size) than monkeys and exhibit more adaptable behavior.
Apes include humans.
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