Origin and History of Life, Phylogeny, Prokaryotes, Protists, Plants, Fungi, Animal Diversity, and Evolutionary Evidence

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Unit 25: Origin and History of Life

Evolution of Eukaryotic Cells

Eukaryotic cells evolved from prokaryotic ancestors through a process known as endosymbiosis. This process involved the engulfment of one cell by another, leading to the development of complex cellular structures.

Endosymbiosis: A heterotrophic host cell engulfed an aerobic bacterium (which became the mitochondrion) or a photosynthetic cyanobacterium (which became the plastid/chloroplast).
Evidence: Mitochondria and chloroplasts have their own DNA and double membranes, supporting their origin from free-living bacteria.
Example: The origin of mitochondria in all eukaryotes and chloroplasts in plants and algae.

Fossil Record Biases

The fossil record is an important but incomplete source of information about the history of life.

Biases: Organisms with hard parts (shells, bones) and those living in sediment-rich environments are more likely to be fossilized.
Example: Clams with hard shells are more commonly fossilized than soft-bodied sea slugs.

Stages in the Origin of Life

The origin of life is hypothesized to have occurred in four main stages:

Production of nucleotides and amino acids
Polymerization into larger molecules (proteins, nucleic acids)
Enclosure in membranes to form protocells
Origin of self-replicating molecules (inheritance)

Experimental Support: The first two stages are supported by laboratory synthesis experiments (e.g., Miller-Urey experiment).
Speculation: The fourth stage (self-replication) is the least understood.

Timeline of Life on Earth

Earth forms: ~4.6 billion years ago (BYA)
Prokaryotes appear: ~3.5–4 BYA
Single-celled eukaryotes: ~2 BYA
Multicellular eukaryotes: ~1.5 BYA
Animals: <1 BYA
Land plants: ~500 million years ago (MYA)

Environmental Changes and Evolution

Oxygen Revolution: Cyanobacteria increased atmospheric oxygen, drastically altering Earth's environment.
Mass Extinctions: Periodic events that eliminate many species, often followed by adaptive radiations.
Adaptive Radiation: Surviving groups diversify rapidly to fill ecological niches (e.g., mammals after dinosaur extinction).

Key Vocabulary

Protocell: An aggregate of molecules with a lipid bilayer boundary, maintaining a distinct internal environment.
Adaptive Radiation: Rapid diversification of a group into new ecological roles.

Unit 21 & 26: Phylogeny, Taxonomy, and Systematics

Phylogenetic Trees

Phylogenetic trees are hypotheses about evolutionary relationships among species.

Anagenesis: Evolution within a single lineage (one species evolves into another).
Cladogenesis: Branching evolution, where one species splits into two or more.

Group Types in Phylogeny

Group Type	Definition	Example
Monophyletic (Clade)	Includes a common ancestor and all its descendants	Birds
Paraphyletic	Includes ancestor and some, but not all, descendants	"Reptiles" (excluding birds)
Polyphyletic	Includes species with different ancestors	"Warm-blooded" animals

Principle of Parsimony

The simplest tree with the fewest evolutionary changes is preferred.

Molecular Clocks

Estimate divergence times using nucleotide differences in homologous genes.

Complications in Phylogenetic Analysis

Convergent Evolution: Leads to homoplasies (analogous traits not from a common ancestor, e.g., bird and bat wings).
Horizontal Gene Transfer: Movement of genes between lineages without reproduction, complicating tree construction.

Key Vocabulary

Taxon: A named group at any taxonomic level.
Sister Taxa: Lineages most closely related to each other.
Homology: Similarity due to shared ancestry (e.g., human arm and whale flipper).

Unit 27: Bacteria and Archaea (Prokaryotes)

Ecological Roles

Decomposers: Recycle nutrients by breaking down dead matter.
Producers: Cyanobacteria produce oxygen via photosynthesis.
Nitrogen Fixers: Convert atmospheric nitrogen to usable forms (e.g., Rhizobium in legumes).

Pathogenic Bacteria

Examples: Streptococcus mutans (tooth decay), Mycobacterium leprae (leprosy).

Horizontal Gene Transfer (HGT)

Mechanisms: Transformation (uptake of DNA), transduction (virus-mediated), conjugation (direct transfer between cells).
Importance: HGT is widespread and contributed to the evolution of eukaryotic organelles.

Shapes and Nutritional Modes

Shape	Description
Cocci	Spherical
Bacilli	Rod-shaped
Vibrios	Comma-shaped
Spirochaetes	Spiral-shaped

Photoautotrophs: Use light and CO2 for energy and carbon.
Chemoheterotrophs: Use organic compounds for both energy and carbon.

Key Vocabulary

Obligate Anaerobes: Organisms poisoned by oxygen.
Cyanobacteria: Only prokaryotes that generate oxygen via photosynthesis.

Unit 28: Protists

Unifying Characteristics

Eukaryotes that are not plants, animals, or fungi.
Most abundant in moist habitats; usually microscopic.

Classification by Ecological Role

Group	Role	Example
Algae	Producers	Green algae
Protozoa	Heterotrophs	Amoeba
Fungus-like	Decomposers	Slime molds

Endosymbiosis in Protist Evolution

Primary Endosymbiosis: Engulfment of a cyanobacterium led to red and green algae.
Secondary Endosymbiosis: Heterotrophic host engulfed a red or green alga, leading to further diversity.

Life Cycles

Gametic: Dominant diploid stage (e.g., diatoms).
Sporic: Alternation of generations (e.g., brown algae).
Zygotic: Dominant haploid stage (e.g., many green algae).

Key Vocabulary

Pseudopodia: "False feet" for movement (e.g., filose pseudopodia in Rhizaria).
Plastid: Organelle such as a chloroplast found in algae and plants.

Unit 29: Plants

Adaptations for Life on Land

Waxy Cuticle: Prevents water loss.
Stomata: Pores for gas exchange.
Vascular Tissue: Xylem (water transport) and phloem (food transport).
Lignin: Provides structural support for tall growth.

Alternation of Generations

Life cycle alternates between multicellular haploid gametophyte (produces gametes) and multicellular diploid sporophyte (produces spores).

Evolutionary Origin

Land plants evolved from green algae, specifically charophytes.

Key Vocabulary

Sporangia: Structures where spores are produced.
Rhizoids: Root-like structures in non-vascular plants.

Unit 31: Fungi

Major Characteristics

Multicellular heterotrophs using absorptive nutrition.
Cell walls made of chitin.
Bodies composed of mycelium (mass of filaments).

Body Structure

Hyphae: Individual filaments; growth occurs at the tip, driven by osmosis.

Ecological Roles

Decomposers: Break down organic matter.
Mutualisms: Mycorrhizae (with plant roots), lichens (with algae or cyanobacteria).

Key Vocabulary

Opisthokonta: Eukaryotic supergroup containing fungi and animals.
Karyogamy: Fusion of nuclei during sexual reproduction.

Unit 32: Animal Diversity

Major Characteristics

Multicellular, heterotrophic eukaryotes lacking cell walls.
Most have nerves, muscles, and the ability to move.

Cambrian Explosion

Rapid increase in animal diversity ~530 MYA.
Likely driven by new predator-prey relationships and evolution of Hox genes.

Developmental Patterns

Diploblastic: Two embryonic tissue layers (endoderm, ectoderm).
Triploblastic: Three layers (adds mesoderm).

Body Cavities

Coelom: Fluid-filled cavity that cushions organs and allows independent movement and growth.

Protostome vs. Deuterostome Development

Feature	Protostome	Deuterostome
Cleavage	Spiral	Radial
Blastopore Fate	Mouth	Anus

Unit 33 & 34: Invertebrates and Vertebrates

Major Invertebrate Phyla

Porifera: Sponges; lack true tissues.
Cnidaria: Jellyfish; possess cnidocytes (stinging cells).
Arthropoda: Jointed legs; undergo ecdysis (molting).
Echinodermata: Starfish; have a water vascular system.

Vertebrate Innovations

Vertebral Column: Backbone for support and protection.
Jaws: Present in gnathostomes (jawed vertebrates).
Lungs and Limbs: Adaptations for life on land (tetrapods).
Amniotic Egg: Allows reproduction away from water (amniotes).

Amniotes

Animals (reptiles, mammals) with amniotic eggs, internal fertilization, and water-resistant skin, enabling a fully terrestrial life cycle.

Mammal Groups

Group	Characteristic	Example
Monotremes	Egg-laying	Platypus
Marsupials	Pouch development	Kangaroo
Eutherians	Placental mammals	Human

Evidence of Evolution & Population Ecology

Natural Selection and Evidence for Evolution

Natural Selection: Variation in inherited traits leads to differential reproductive success.
Evidence: Fossil record (e.g., transitional forms like Tiktaalik), biogeography (similar species on separated continents), and homologies (shared structures).

Population Growth Models

Exponential Growth: Population grows without limits under ideal conditions.
Logistic Growth: Growth slows as resources become limited and population approaches carrying capacity (K).

Equations:

Exponential growth:
Logistic growth:

Life History Strategies

r-selected species: Many offspring, little parental care, thrive in unstable environments.
K-selected species: Few, competitive offspring, thrive in stable environments.

Key Vocabulary

Density-dependent: Factors (e.g., competition, disease) that limit population size based on density.
Net Reproductive Rate (R0): Per-generation population growth rate.