Animal Evolution Notes
Animal Evolution
Origin of Animals
Animals appeared more than half a billion years ago.
Their ancestor might resemble choanoflagellates.
Opisthokonts include animals, fungi, and choanoflagellates, belonging to the Unikonta group.
Timeline of Animal Evolution
Late Proterozoic Era (1 Billion–542 Million Years Ago): First animal fossils.
Paleozoic Era (542–251 Million Years Ago): Early fossils of most major animal groups, vertebrates appear and move onto land.
Mesozoic Era (251–65.5 Million Years Ago): Dinosaurs roamed the land, first mammals, insects diversify.
Cenozoic Era (65.5 Million Years Ago to the Present): Mammals diversify and replace dinosaurs as dominant terrestrial animals.
Earliest Animals: Ediacaran Biota
Existed 635-538 million years ago.
Fossils resemble Cnidarians.
Most fossils are difficult to interpret.
Examples include Mawsonites spriggi and Spriggina floundersi.
Cambrian Explosion
The Cambrian explosion (535 to 525 million years ago) marks the earliest fossil appearance of many major groups of living animals.
Example: Hallucigenia fossil (530 mya).
Causes of the Cambrian Explosion
External Changes: Increase in atmospheric oxygen, the appearance of ozone shielding against UV, and an increase in calcium in seawater.
Internal Changes: Appearance of new structures for locomotion and defense allowed predators and prey to diversify, and evolution of Hox genes enables new body plans.
Hox Genes
A special set of genes present only in animals.
Organize embryonic development.
Determine head-to-tail body pattern.
Present in all Bilaterian animals in relatively unchanged form.
Hox genes appeared around 550 million years ago.
Paleozoic Era (550-250 mya)
Animals continue to diversify.
Several mass extinctions drastically change animal diversity.
Plants appear on land and produce a lot of oxygen.
Vertebrates transition from water to land.
The Paleozoic era ends in “the Great Dying” – End-Permian mass extinction event that destroyed 90% of all life.
Mesozoic Era (251-65.5 mya)
Dinosaurs dominate the earth.
First mammals emerge.
Corals reappear, forming new ecological niches in the sea.
Flowering plants appear, and insects diversify.
First mammals appear (in the shadow of dinosaurs).
Cenozoic Era (65.5 mya-present)
Dinosaurs go extinct, leaving space for mammals.
Mammals and birds diversify greatly.
Primates emerge!
The climate cools down, and mammals and birds diversify.
Evolution of Vertebrates: From Sea to Land
Tetrapods have specific adaptations for life on land:
Four limbs and feet with digits.
A neck, allowing separate movement of the head.
Fusion of the pelvic girdle to the backbone.
Lungs but not gills (with some exceptions).
Ears for detecting airborne sounds.
Origin of Tetrapods: Tiktaalik
Tiktaalik, nicknamed a “fishapod,” shows both fish and tetrapod characteristics:
Fins, gills, lungs, and scales.
Ribs to breathe air and support its body.
A neck and shoulders.
Fins with the bone pattern of a tetrapod limb.
Tiktaalik could most likely prop itself on its fins but not walk.
The first tetrapods appeared 365 million years ago.
Tetrapod Limb Homology
Features Eusthenopteron, Panderichthys, Tiktaalik, Acanthostega, Tulerpeton, Amphibians.
Key limb bones: Ulna, Radius, and Humerus.
Why Animals Evolve
All animals have the same needs:
Obtain energy (eat).
Obtain oxygen (breathe).
Maintain water and salt balance.
Reproduce.
Defend against predators and pathogens.
In diverse environments, the best ways to meet these needs will be different!
Form vs. Function
Form (studied by anatomy) evolves to match function (studied by physiology).
Unrelated species can evolve similar form when faced with a similar challenge.
E.g., Fusiform shape – tapered at both ends – helps fast movement in water.
Organ Systems in Mammals
Specialized cells are organized into tissues that perform a specific function.
Tissues are organized into organs.
Organs are organized into organ systems.
Tissues
Tissues are classified into four main categories:
Connective
Muscle
Epithelial
Nervous
Connective Tissue
Connective tissue originates from mesoderm and is one of the most versatile tissues.
Function: To bind and support other tissues, but also to provide storage space, defense, and transport.
Form: Sparsely packed cells scattered throughout an extracellular matrix made of fibers in a liquid, jelly-like, or solid foundation.
Examples: Blood, Cartilage, Adipose tissue, Bone, Fibrous connective tissue, Loose connective tissue.
Muscle Tissue
Muscle tissue also originates from mesoderm.
Muscle proteins are relatively similar across animals.
Function: To move!
Form: Long cells called muscle fibers, which contract in response to nerve signals. Some muscle cells form a syncytium.
Examples: Skeletal muscle, Smooth muscle, Cardiac muscle.
Epithelial Tissue
Epithelial tissue covers the outside of the body and lines the digestive system, the lungs, and some glands.
Epithelium lining the digestive system, the lungs, and the glands is derived from endoderm, while the skin is derived from ectoderm.
Function: To interact with the environment.
Form: Tightly joined cells that can be cuboidal (like cubes), columnar (like pillars), or squamous (like tiles).
Epithelial tissue can be simple (one layer of cells), stratified (multiple layers of cells), or pseudostratified (one layer with cells of varying height).
Epithelial cells have polarity – two sides of the cell differ.
Examples: Cuboidal epithelium, Simple columnar epithelium, Simple squamous epithelium, Pseudostratified columnar epithelium, Stratified squamous epithelium.
Nervous Tissue
Nervous tissue is derived from ectoderm and present in all animals except sponges.
Function: To sense stimuli (from the outside and from the inside) and to transmit signals between different body parts.
Form: Nerve cells (neurons) have long projections (to reach remote places) and are supported and nourished by glial cells.
Invertebrate and vertebrate nervous systems differ in chemistry, structure, and function of the nervous tissue!
Examples: Neurons, Glia.