Origins and Evolutionary Foundations

Lynn Margulis’ Endosymbiont Theory of Eukaryote Evolution

Eukaryotic cells originated through symbiotic relationships between different prokaryotic organisms, where aerobic bacteria and cyanobacteria were engulfed by a host cell and became mitochondria and chloroplasts.

Mitochondria Origin

Arose from aerobic bacteria engulfed by an ancestral host cell

Chloroplast Origin

Arose from cyanobacteria engulfed by an ancestral eukaryote

Symbiosis in Endosymbiont Theory

Instead of being digested, engulfed bacteria lived inside the host cell and became permanent organelles.

DNA Evidence for Endosymbiont Theory

Mitochondria and chloroplasts have their own circular DNA, similar to bacterial genomes.

Binary Fission Evidence for Endosymbiont Theory

Mitochondria and chloroplasts replicate independently of the host cell, using bacterial-like division.

Double Membrane Evidence for Endosymbiont Theory

Both organelles possess two membranes — one from the engulfed bacterium and one from the host.

Genetic Similarity Evidence for Endosymbiont Theory

Mitochondrial DNA resembles α-proteobacteria; chloroplast DNA resembles cyanobacteria.

Prokaryote Definition

Organism lacking a nucleus or membrane-bound organelles; includes Bacteria and Archaea.

Eukaryote Definition

Organism with a true nucleus and membrane-bound organelles such as mitochondria, ER, and Golgi.

Microscopy Evidence for Eukaryotes

Microscopy revealed nuclei, organelles, and larger cell size distinguishing eukaryotes from prokaryotes.

Molecular Evidence for Eukaryotes

Differences in ribosomes (70S vs 80S), cell-wall chemistry, and DNA packaging separate eukaryotes and prokaryotes.

Difference Between Bacteria and Archaea? How was this discovered?

Revealed by Carl Woese’s 16S rRNA sequencing showing two fundamentally different prokaryotic lineages.

• Bacteria: Cell wall composed of peptidoglycan, as well as fatty-acid membranes linked by ester bonds.

• Archaea: No peptidoglycan; membranes with isoprenoid chains linked via ether bonds; transcription/translation machinery resembles eukaryotes.

Carl Woese’s Contribution (1970s)

Used 16S rRNA sequencing to classify life into Bacteria, Archaea, and Eukarya.

Lokiarcheota Definition

Group of archaea discovered in deep-sea sediments possessing many genes once thought exclusive to eukaryotes.

Discovery of Lokiarcheota

Identified in 2015 via metagenomic sequencing of marine sediments — DNA extracted directly from environment.

Significance of Lokiarcheota

Suggests eukaryotes evolved from an archaeal ancestor within this group; possible “missing link” between prokaryotes and eukaryotes.

Eukaryotic Signature Proteins (ESPs)

Genes in Lokiarchaeota involved in cytoskeleton formation, membrane trafficking, and information processing that resemble eukaryotic genes.

LUCA Acronym

Last Universal Common Ancestor — the hypothetical most recent common ancestor of all modern life.

Evidence for LUCA

Shared traits among all life:

• Universal genetic code

• DNA→RNA→protein system

• ATP use, ribosomes

• Core metabolic pathways like glycolysis and TCA intermediates.

Nature of LUCA

Likely a simple single-celled organism with a lipid membrane, ribosomes, and metabolic flexibility; not the first life, but ancestor of all living organisms.

Two-Domain Hypothesis

Proposes only Bacteria and Archaea as domains, with eukaryotes branching from within Archaea (specifically Asgard group such as Lokiarcheota).

Implication of Two-Domain Model

Reclassifies “Eukarya” as a lineage within Archaea that acquired mitochondria, reducing life’s domains from three to two.

Dear King Philip Came Over For Good Soup

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

(Taxonomic hierarchy from broadest to most specific)