The Tree of Life, Microbial Lifestyles, and Early Earth Evolution

Comprehensive vocabulary flashcards reviewing the classification of life, metabolic pathways, early Earth atmospheric history, endosymbiosis, and host-microbiome interactions.

Early Earth Geological and Evolutionary Timeline

The chronological progression from the origin of Earth ($4.5\,\text{bya}$) through the Hadean, Archaean, and Proterozoic eons, highlighting the emergence of prokaryotes ($3.5\,\text{bya}$), the rise of atmospheric oxygen ($2.6\text{--}2.5\,\text{bya}$), and the appearance of single-celled eukaryotes.

Prokaryotes

Small unicellular organisms that lack a membrane-enclosed nucleus and specialized membrane-bound organelles to compartmentalize cellular processes.

Eukaryotes

Organisms characterized by larger cells with membrane-bound organelles and genetic material primarily enclosed within a membrane-bound nucleus.

Polymerase Chain Reaction (PCR)

A molecular technique enabling the fast reproduction of copies of specific DNA segments, allowing DNA sequencing from environmental samples.

Plasmids

Small, additional circular DNA molecules frequently carried by prokaryotes alongside their main circular chromosome.

Binary Fission

The asexual reproductive mechanism utilized by prokaryotes (as well as mitochondria and chloroplasts) to divide.

Photoautotroph

An organism that captures energy from sunlight and acquires carbon from carbon dioxide ($CO_2$) to produce organic molecules.

Photoheterotroph

An organism that harnesses energy from sunlight but must obtain its carbon from organic molecules rather than $CO_2$.

Chemoautotroph

An organism that obtains energy from inorganic chemical reactions and utilizes carbon dioxide ($CO_2$) as its carbon source.

Chemoheterotroph

An organism that derives both its required metabolic energy and carbon directly from organic molecules.

Methanogens

Chemoautotrophic Archaea that produce methane via the reaction $CO_2 + 4H_2 \rightarrow CH_4 + 2H_2O$, historically warming Earth's early Archaean atmosphere.

Stromatolites

Dome-shaped structures formed by microbial mats trapping sediment over time; extant ones are produced by cyanobacteria, and fossil forms date back $3.5\,\text{billion years}$.

Oxygenic Photosynthesis

A metabolic process utilized by cyanobacteria and chloroplasts that converts carbon dioxide and water into sugars while releasing oxygen gas ($CO_2 + H_2O \rightarrow \text{"CH}_2\text{O"} + O_2$).

Oxygen Revolution

The massive environmental transformation starting approximately $2.6\text{--}2.7\,\text{bya}$ where photosynthetic oxygen oxidized marine iron, escaped into the air by $2.45\,\text{bya}$, cooled Earth by destroying methane, and generated a stratospheric ozone ($O_3$) layer.

Endosymbiont Theory

The evolutionary model stating that eukaryotic organelles (mitochondria and chloroplasts) originated as free-living bacteria that took up residence inside ancestral host cells.

Microbiome

The complete assemblage of microorganisms—including bacteria, archaea, fungi, algae, and single-celled eukaryotes—residing within a well-defined habitat.

Vertical Transmission

The passage and acquisition of microbiome members directly from mothers to offspring during the first months of life.

Horizontal Transmission

The uptake and colonization of microbiota from external environmental sources, including consumed food.

Holobiont

A discrete ecological unit consisting of a multicellular host along with the diverse community of symbiotic microbes living on or within it.

Genome Reduction

The evolutionary downsizing of a symbiotic microbe's genome resulting from specialized dependency on a host to supply essential nutrients.