Biology 1204: Eukaryotic Origins

Practice vocabulary flashcards covering the evolutionary origins of eukaryotes, organelle endosymbiosis, phylogenetic models, and the evolution of multicellularity.

Eukaryote

An organism characterized by cells containing a membrane-bound nucleus; eukaryotic fossils first appeared approximately $1.8\text{ billion}$ years ago (with an estimated range of $1.5 - 2.1\text{ billion}$ years ago), typically ranging from $10\,\mu\text{m}$ to $100\,\mu\text{m}$ in size.

Endomembrane System

An interconnected complex of internal membranes in eukaryotic cells that includes the smooth and rough endoplasmic reticulum, Golgi apparatus, vesicles, nuclear envelope, and the interacting plasma membrane.

Infolding (Invagination) Hypothesis

The hypothesis proposing that the eukaryotic endomembrane system and nuclear envelope evolved through the inward folding and pinching off of the ancestral prokaryotic plasma membrane.

Endosymbiosis Hypothesis

The evolutionary theory proposing that specific eukaryotic organelles, namely mitochondria and chloroplasts, originated when a larger host cell engulfed smaller prokaryotic cells that lived symbiotically rather than being digested.

Asgard Archaea

A group of anaerobic archaea first discovered in 2010 at the Loki's Castle hydrothermal vent system off the coast of Norway that encodes proteins previously thought unique to eukaryotes, representing the ancestral lineage of the host cell in primary endosymbiosis.

Sterile Inflammation

An inflammatory immune response that occurs when damaged host cells release mitochondria into extracellular space, causing the immune system to mistake mitochondrial DNA ($\text{mtDNA}$) for invading bacterial DNA due to their close evolutionary homology.

Primary Endosymbiosis

The process by which an ancestral eukaryotic or archaeal cell engulfed a living prokaryote (an aerobic proteobacterium for mitochondria, or a photosynthetic cyanobacterium for chloroplasts), resulting in organelles bounded by two membranes.

Archaeplastida

The eukaryotic supergroup (including red algae, green algae, and land plants) whose common ancestor first acquired a photosynthetic plastid through primary endosymbiosis of a cyanobacterium.

Secondary Endosymbiosis

The evolutionary event in which a heterotrophic eukaryotic cell engulfed a photosynthetic eukaryote (a single-celled red or green alga), producing complex plastids surrounded by more than two membranes (typically three or four).

Mixotrophy

A nutritional strategy in which an organism can obtain energy and carbon through both autotrophy (photosynthesis) and heterotrophy (consuming other organisms or organic matter), common in secondary endosymbionts like Euglena.

Horizontal Gene Transfer (HGT)

The non-vertical transmission of genetic material between distinct organisms or across species boundaries, such as bacterial conjugation or the lateral transfer of endosymbiont genes into the host cell's nuclear genome.

Ring of Life Model

A phylogenetic model of early cellular evolution proposing that the three domains of life (Bacteria, Archaea, and Eukarya) developed from a single, laterally connected pool of primitive prokaryotes rather than from a single linear ancestor.

Web of Life Model

A phylogenetic model proposed by W. Ford Doolittle that features multiple trunks and interconnected horizontal branches to account for pervasive horizontal gene transfer throughout early evolutionary history.

Two-Domain Model

An evolutionary hypothesis based on phylogenomic data asserting that Eukarya is not an independent domain, but rather a nested lineage that evolved directly from within the Archaea.

Colonial Flagellate Hypothesis

A hypothesis stating that multicellularity originated when single-celled flagellated protists formed colonial aggregates, developed into hollow spheres, established specialized reproductive cells, and underwent tissue-forming invaginations.