Early Earth Life Origins and Biological Classification

Posted on Dec 11, 2025 in Biology

Part 5: Early Earth, Origin of Life, and Evolution

Characteristics of Early Earth & Prebiotic Simulation

Early Earth lacked oxygen and an ozone layer, resulting in high UV radiation, noxious gases, frequent electrical storms, and abundant water.

The prebiotic simulation (1950s) recreated these early Earth conditions in the lab, leading to the formation of amino acids—the basic building blocks of life. This supports the chemical evolution hypothesis.

Hypotheses for the Origin of Life

Chemical Evolution (Prebiotic Simulation): Life arose from non-living molecules under early Earth conditions.
Panspermia: Simple cells may have arrived on Earth via meteorites and subsequently evolved.

General Order of Organisms & Evolutionary Advantages

Prokaryotes: The first life forms; simple and single-celled, living in oceans.
Photosynthesis: Some prokaryotes evolved the ability to photosynthesize, utilizing abundant sunlight and low competition.
Oxygen: Photosynthesis increased atmospheric oxygen, enabling new evolutionary pathways.
Eukaryotes: Evolved from prokaryotes as oxygen levels rose; these cells are more complex and possess organelles.
Multicellularity: Evolved because larger single cells struggled with nutrient exchange and waste removal; multicellular organisms allowed for specialization.
Plants on Land: The first organisms to colonize land, made possible by photosynthesis.
Animals on Land (Lobe-finned Fish): The first animals to move onto land, possessing adaptations for oxygen storage and movement between pools.
Amphibians: Evolved from lobe-finned fish; required water for reproduction and had moist skin.
Reptiles: Adapted to drier climates with waterproof eggs and scaly skin, allowing them to live farther from water.
Birds & Mammals: Developed insulation (feathers for birds, hair for mammals) to survive in diverse environments.

Primates & Hominins

Primates: Characterized by opposable thumbs, nails (instead of claws), binocular vision, depth perception, and larger brains and bodies relative to length.

Hominins: Human-like primates; the oldest fossils originate in Africa. This group includes humans and their close relatives.

Australopithecines to Genus Homo

Australopithecines: Early hominins exhibiting both ape-like and human-like traits.
Genus Homo: Diverged from Australopithecines; characterized by larger brains and bodies, shorter arms, longer legs, and tool use.

Homo erectus & Neanderthals

Homo erectus: The first hominin to migrate out of Africa; it is not a direct ancestor of modern humans and became extinct.
Neanderthals: Also migrated out of Africa; they are extinct and not direct ancestors of modern humans.

Homo sapiens & Genetic Diversity

Homo sapiens: Appeared approximately 150,000 years ago.

Genetic diversity: Is highest in Africa due to the founder effect; diversity decreases in populations that migrated away from Africa.

Part 6: Taxonomy & Phylogenetics

Taxonomy: Genus & Species

Scientific names use genus and species (e.g., Homo sapiens). The genus is capitalized, and the species is lowercase; both must be italicized or underlined.

Taxonomic Classifications

Hierarchy (DKPCOFGS): Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

If two species share the same order, they automatically share all higher categories (family, class, phylum, kingdom, domain).

Three Domains & Four Eukaryotic Groups

Domains: Bacteria, Archaea, Eukarya.

Eukarya includes four major groups: animals, plants, fungi, and protists.

Phylogenetic Trees & Evolutionary Relationships

Phylogenetic trees illustrate evolutionary relationships.

Nodes: Represent speciation events (the last common ancestors).
Clades: Groups that include a common ancestor and all of its descendants.

The left/right positioning of organisms on a tree does not affect relatedness; the shared node is the critical factor. Removing a descendant breaks the definition of a true clade; all descendants must be included.

Part 7: Prokaryotes (Bacteria & Archaea)

General Characteristics & Similarities

Prokaryotes are the smallest living things and are single-celled.

They exhibit three common shapes: spherical, rod, or spiral.

Reproduction occurs via binary fission (asexual cell division).

Conjugation: Genetic material is exchanged via a sex pilus (bridge) connecting the donor to the recipient; this process transfers plasmids (circular DNA).

Rapid evolution results from fast reproduction and genetic exchange, often leading to antibiotic resistance.

Differences Between Bacteria & Archaea

Molecular & Structural Differences

Bacteria possess peptidoglycan in their cell walls; archaea do not.
Bacteria can be classified as gram-positive (purple stain, thick peptidoglycan) or gram-negative (red/pink stain, extra fatty layer).
Archaea can produce methane; bacteria generally cannot.
Only some bacteria can photosynthesize; archaea cannot.
Some bacteria produce endospores (resistant to harsh conditions) or biofilms (slimy, protective colonies); archaea do not.
Only some bacteria are pathogenic; archaea are not pathogenic.

Anthrax

Acquisition: Inhalation, cutaneous (through skin), or gastrointestinal (ingestion).

Virulence: Plasmids PXO1 and PXO2 make anthrax virulent by producing anthrax toxin and a capsule.

Category A Pathogen: Considered an effective bioweapon because it can be aerosolized, inhaled, or spread as a powder, affecting large areas.

Key Terms for Prokaryotes

Heterotroph: Absorbs nutrients from the surroundings.
Autotroph: Self-feeding, usually via photosynthesis.
Pathogen: An organism or agent that causes harm, disease, illness, or death.

Prokaryotes & the Environment: Benefits and Risks

Four Positives:

Nutrition: Aid in digestion and nutrient absorption in animal guts.
Recycling: Decompose dead and decaying matter, recycling essential nutrients.
Nitrogen Fixation: Convert atmospheric nitrogen into forms usable by plants and animals.
Bioremediation: Break down environmental pollutants (e.g., oil spills).

One Negative:

Some bacteria are pathogenic, causing disease.

Viruses, Viroids, and Prions

Viruses

Non-living infectious agents; they are not cells.
Structure: Contain either DNA or RNA (never both), surrounded by a protein coat.
They are smaller than bacteria and require a host to reproduce.
They can infect all living organisms. (Example: SARS-CoV-2 contains only RNA.)

Viroids

Infectious agents composed solely of short RNA strands. They affect plants/crops only and lack protein or DNA.

Prions

Infectious misfolded proteins that contain no genetic material.
They are resistant to heat and difficult to destroy.
Prions cause brain diseases (e.g., mad cow disease, Kuru, Creutzfeldt-Jakob disease).
They reproduce by converting normal proteins into the misfolded, disease-causing form.

Distinguishing Characteristics

Bacteria: Living, possess DNA, have a cell wall (peptidoglycan), and reproduce independently.
Viruses: Non-living, possess DNA or RNA, have a protein coat, and require a host to reproduce.
Prions: Consist only of protein, lack genetic material, and cause disease by altering normal proteins.

Part 8: Protists

Characteristics & Definition

Protists are eukaryotic organisms that are not classified as animals, plants, or fungi.

They exhibit traits resembling animals, plants, and fungi, and are ubiquitous (found everywhere).

Five Major Areas of Diversity

Size/Structure: Single-celled (e.g., diatoms) versus multicellular (e.g., kelp).
Habitat: Terrestrial (in soil/damp areas) versus aquatic.
Motility: Motile (using flagella or pseudopods) versus stationary.
Reproduction: Sexual versus asexual reproduction (binary fission).
Nutrition: Heterotrophic (ingesting/absorbing nutrients) versus autotrophic (photosynthetic algae). Some are mixotrophic (doing both).

Pseudopods: Extensions of the cell membrane used by some protists to surround and engulf food.

Protists & the Environment: Benefits and Risks

Three Positives:

Photosynthesis: Major contributors to Earth’s photosynthesis and energy production.
Food Sources: Important components of aquatic and terrestrial food webs.
Commercial Uses: Used in products like agar, cosmetics, and toothpaste.

One Negative (Pathogens): Some cause diseases in plants and animals:

Late blight: Affects potatoes.
Downy mildew: Affects grapes.
Red tide: Caused by dinoflagellates, releasing neurotoxins harmful to aquatic and land animals.

Key Terms & Definitions Summary

Prebiotic simulation: Lab recreation of early Earth conditions to study the origin of life.
Prokaryote: Single-celled organism without a nucleus; includes Bacteria and Archaea.
Binary fission: Asexual reproduction in prokaryotes.
Conjugation: Transfer of plasmids (genetic material) between bacteria via a sex pilus.
Plasmid: Circular DNA molecule in bacteria, often carrying genes for antibiotic resistance or toxins.
Peptidoglycan: Structural molecule in bacterial cell walls.
Gram-positive/negative: Bacterial classification based on cell wall structure and staining.
Endospore: Resistant bacterial structure for surviving extreme conditions.
Biofilm: Protective, slimy colony formed by bacteria.
Clade: Group of species sharing a common ancestor.
Node: Point on a phylogenetic tree representing a speciation event.
Virus: Non-living infectious agent with DNA or RNA and a protein coat; requires a host to replicate.
Viroid: Infectious RNA molecule affecting plants.
Prion: Infectious misfolded protein causing brain disease.
Protist: Diverse group of eukaryotic organisms not classified as animal, plant, or fungi.
Pseudopod: Membrane extension used by some protists to engulf food.
Autotroph: Organism that produces its own food (e.g., via photosynthesis).
Heterotroph: Organism that obtains nutrients by consuming other organisms.
Mixotroph: Organism capable of both autotrophic and heterotrophic nutrition.
Bioremediation: Use of organisms (often bacteria) to break down environmental pollutants.
Virulent factors: Molecules produced by pathogens that contribute to their ability to cause disease (e.g., toxins, capsules).