The Evolution of Animals
Animals
Highly diverse group totaling over 1,000,000 species
Animals are eukaryotic, multicellular, heterotrophic organisms that obtain
nutrients by eating and, are able to digest food within their bodies
We classify them based on evolutionary innovations that evolved as animals
evolved
The hypothetical ancestor of all animals was a colonial flagellated protist
The oldest animal fossils date to 550-575 million years ago
Animals must have evolved earlier than that because these fossils already
show a diversity of size and shape
Early Animals and the Cambrian Explosion
Animal diversification appears to have accelerated rapidly from 525 to 535
million years ago, during the Cambrian period
Because so many animal body plans and new phyla appear in the fossils from
such an evolutionarily short time span, biologists call this episode the
Cambrian explosion
Most of the body plans were “rejected”
Why the “explosion”?
The Cambrian explosion may have been ignited by increasingly complex
predator-prey relationships and/or an increase in atmospheric oxygen
The genetic framework for complex bodies, a set of “master control”
genes, was already in place, so these were small “tweaks”
Evolutionary Relationships
Historically, biologists have categorized animals by “body plan,” general
features of body structure
More recently, a wealth of genetic data has allowed evolutionary biologists to
modify and refine groups
Sponges (Porifera) do not have bodies composed of tissues; all other phyla do
Porifera and Cnidaria do not exhibit bilateral symmetry; all other phyla do
Cnidaria exhibits radial symmetry
Porifera exhibits no symmetry
Porifera, Cnidaria, and Platyhelminthes do not possess a central body cavity known
as a coelom; all other phyla do
Porifera
Sponges
Most ancient animal lineage
Collection of cells with basic functions
Lack true tissues
Sponges live by drawing water into themselves through a series of tiny
pores on their exterior and then filtering food and extracting oxygen
from the water
Cnidaria
Jellyfishes, sea anemones, hydras, and corals use stinging tentacles to capture
prey
Many cnidarians have two stages of life
The immature polyp stage of life is sessile, remaining fixed to a solid
surface
The adult, medusa stage of life is motile, capable of swimming freely in
the water
Have true tissues
Nervous tissue, muscle-like tissue, digestive tissue
Lack true organs
Mollusca
All molluscs have a similar body plan with three main parts:
a muscular foot usually used for movement,
a visceral mass containing most of the internal organs, and
a mantle, a fold of tissue that secretes the shell if present
Many molluscs feed by extending a file-like organ called a radula to scrape up
food
There are three important classes within the phylum Mollusca
Gastropods (snails, slugs)
Bivalves (oysters, clams, mussels)
Cephalopods (octopus, squid, nautilus)
Platyhelminthes
Flatworms include mostly small creatures, dwelling either in aquatic or moist
terrestrial environments
Have bilateral symmetry and organs, but have no coelom or system of blood
circulation
Nervous and reproductive systems
Annelida
Clear body segmentation
Most annelids are marine, some are freshwater
Annelids exhibit two characteristics shared by all other bilateral animals
except flatworms:
a complete digestive tract with two openings: a mouth and an anus, and
a body cavity
Nematoda
The mostly microscopic roundworms of phylum Nematoda exist in enormous
numbers in all kinds of habitats on Earth
A number of roundworms are agricultural pests and some are human parasites
They are cylindrical in shape and tapered at both ends
Arthropoda
Includes arachnids, crustaceans, millipedes and centipedes, and insects
All have the following characteristics
Paired, jointed appendages
Specialized segments
Exoskeleton, requiring molting for growth
Insect Lives
Many insects undergo metamorphosis in their development
Young resemble adults but are smaller and have different body
proportions
The insect goes through a series of molts, each time looking more like an
adult, until it reaches full size
Other insects have distinctive larval stages specialized for eating and
growing that look entirely different from the adult stage, which is
specialized for dispersal and reproduction
Metamorphosis from the larva to the adult occurs during a pupal stage
Echinoderms
All members of phylum Echinodermata (sea stars, sea urchins, sea cucumbers,
sand dollars) are marine and all inhabit the ocean floor
Echinoderms
usually have radial symmetry as adults
have an endoskeleton (interior skeleton) constructed from hard plates
just beneath the skin
have a water vascular system, a network of water-filled canals that
circulate water throughout the echinoderm’s body, facilitating gas
exchange and waste disposal
Chordata
Includes lancelets, tunicates and sea squirts, all vertebrates
Only the vertebrates have a vertebral column
All chordates possess the following at some point in their lives
Notochord
A hollow dorsal nerve cord
Post-anal tail
Series of pharyngeal slits
Two groups of chordates, tunicates and lancelets, are invertebrates
All other chordates are vertebrates, which retain the basic chordate
characteristics but have additional features
All vertebrates have unique endoskeletons composed of a skull and a
backbone made of a series of bones called vertebrae
Hagfish lack jaws and scavenge dead or dying animals
Lampreys use their jawless mouths as suckers to attach to the sides of larger
fish, extracting nutrients
Cartilaginous fish (sharks and rays) have a flexible skeleton made of cartilage
Bony fish include ray-finned fish (the majority) and lobe-finned fish (only a
few representative species)
Amphibians are tied to water because their eggs, lacking shells, dry out quickly
in the air
They typically undergo metamorphosis from an aquatic larva to a
terrestrial adult
Amphibians were the first vertebrates to colonize land and descended
from lobe-finned fishes that had lungs, fins with muscles, and skeletal
supports strong enough to enable some movement on land
Reptiles (including birds) and mammals are amniotes, producing fluid-filled
amniotic eggs, with waterproof shells
Reptiles include snakes, lizards, turtles, crocodiles, alligators, birds and a
number of extinct groups, including most of the dinosaurs
The first mammals arose about 200 million years ago and were probably
small, nocturnal insect-eaters
Most mammals are terrestrial, but there are the aquatic mammals, and nearly
1,000 species are bats
Only mammals have mammary glands (which produce milk, a nutrient-rich
substance to feed the young) and hair, which insulates the body
There are three major groups of mammals:
monotremes, egg-laying mammals
marsupials, pouched mammals with a placenta
eutherians, also called placental mammals