Major Events in the History of Life

Vocabulary flashcards covering the major evolutionary timeline events of Earth, prokaryotic and eukaryotic diversification, endosymbiosis, multicellularity, and modern eukaryotic taxonomy.

Origin of the Earth

The formation of planet Earth, occurring approximately $4.6\text{ billion}$ years ago.

First prokaryotes

The earliest single-celled organisms without membrane-bound nuclei, dating to approximately $\sim 3.5\text{--}3.7\text{ BYA}$ and possibly as old as $4.3\text{ BYA}$.

Stromatolites

Colonial bacteria that grow in mats and incorporate sediment into distinct layers; they represent the oldest known fossils on Earth and can still be found in warm, shallow bays of Australia and the Bahamas.

Early prokaryotic diets

Metabolic strategies used by ancestral prokaryotes, which included oxidizing chemicals such as $\text{H}_2\text{S}$ to synthesize organic sugars or absorbing pre-made organic molecules, generating ATP via anaerobic respiration.

Bacteria and Archaea divergence

The early evolutionary split of prokaryotes into two modern domains that are physically very similar under the microscope but molecularly distinct.

Oxygen Revolution (Buildup of atmospheric $\text{O}_2$)

The accumulation of oxygen in Earth's atmosphere occurring around $\sim 2.2\text{--}2.7\text{ BYA}$, triggered by photosynthetic prokaryotes through the reaction $6\,\text{CO}_2 + 6\,\text{H}_2\text{O} \rightarrow \text{sugar} + 6\,\text{O}_2$.

Aerobic respiration

A highly efficient cellular respiration pathway that utilizes $\text{O}_2$ to yield substantially more ATP per molecule of sugar than anaerobic respiration; selected for as oxygen accumulated in the biosphere.

First eukaryotes

Organisms with isolated, membrane-bound nuclei and internal organelles that evolved from within an Archaea lineage approximately $\sim 2.1\text{ BYA}$ (and possibly as old as $2.7\text{ BYA}$).

In-folding of the plasma membrane

The evolutionary mechanism by which the eukaryotic nucleus, endoplasmic reticulum, and other endomembrane system components originally developed from internal folds of the ancestral prokaryotic cell membrane.

Eukaryotic cytoskeleton

An internal structural network that evolved in early eukaryotes, facilitating cellular motility and the engulfment of food particles via phagocytosis.

Endosymbiotic theory

The theory stating that specific eukaryotic organelles originated from once free-living prokaryotes that were engulfed via phagocytosis by an ancestral eukaryote, survived digestion, and formed a permanent mutualistic association.

Endosymbiotic origin of mitochondria

The event in which an ancestral anaerobic eukaryote engulfed an aerobic bacterium that continued living within the host, ultimately becoming an oxygen-processing, ATP-generating organelle.

Mitochondrial double membrane evidence

Structural evidence supporting endosymbiosis: the inner membrane retains bacterial proteins and represents the ancestral bacterium's membrane, while the outer membrane contains eukaryotic proteins from the phagosomal vesicle that engulfed it.

Primary endosymbiosis

The evolutionary engulfment of a photosynthetic cyanobacterium by a heterotrophic eukaryote, resulting in two-membrane plastids found in red algae and green algae.

Secondary endosymbiosis

The evolutionary process wherein a heterotrophic eukaryote engulfs a photosynthetic red or green alga, producing plastids surrounded by three or four membranes in lineages such as stramenopiles, alveolates, and euglenids.

Coloniality

An organization in which multiple genetically identical single cells live in close association but do not differentiate into specialized cell types or tissues.

Multicellularity

An organizational state in which multiple genetically identical cells live together and differentiate to divide labor and perform specialized roles; evolved independently multiple times beginning around $\sim 1.5\text{ BYA}$.

First animals (Metazoans)

Multicellular, heterotrophic eukaryotes lacking cell walls that first appeared in the fossil record approximately $\sim 700\text{ million}$ years ago.

First plants and colonization of land

The macroevolutionary event occurring around $\sim 500\text{ MYA}$ where photosynthetic autotrophs with cellulose walls transitioned from aquatic environments to terrestrial habitats.

Kingdom Protista (Archaic definition)

An obsolete taxonomic category formerly used to classify any eukaryotic organism that is not classified as an animal, plant, or fungus; it represents a polyphyletic/paraphyletic convenience grouping rather than a valid clade.

Four eukaryotic supergroups

The modern phylogenetic framework that organizes all Eukarya into Excavata, SAR, Archaeplastida, and Unikonta.

Archaeplastida

The eukaryotic supergroup containing red algae, green algae (chlorophytes and charophytes), and land plants, all of which descended from an ancestor that acquired plastids via primary endosymbiosis.

Unikonta

The eukaryotic supergroup encompassing Amoebozoans (tubulinids and slime molds) and Opisthokonts (nucleariids, fungi, choanoflagellates, and animals).

SAR Supergroup

A major eukaryotic clade comprised of Stramenopiles (diatoms, brown algae), Alveolates (dinoflagellates, apicomplexans, ciliates), and Rhizarians (forams, cercozoans).

Excavata

A major eukaryotic supergroup consisting of flagellated unicellular organisms including diplomonads, parabasalids, and euglenozoans.