Lecture 2: Time Scales and the Origin of Life

Vocabulary practice flashcards covering geologic time scales, Earth history eras, and origin of life concepts based on the lecture material.

Precambrian Supereon

The geologic time period spanning from $4.6\text{ BYA}$ to $543\text{ MYA}$, during which Earth formed and early single-celled and simple multicellular life arose.

Paleozoic Era

The geologic era spanning from $543\text{ MYA}$ to $250\text{ MYA}$, characterized by the origin of land plants, early arthropods, tetrapods, and ending with the Permian extinction.

Mesozoic Era

The geologic era spanning from $250\text{ MYA}$ to $65\text{ MYA}$, known as the Age of Reptiles, during which dinosaurs, conifers, and early mammals appeared.

Cenozoic Era

The geologic era spanning from $65\text{ MYA}$ to the present day, known as the Age of Mammals, during which mammals and flowering plants rapidly radiated.

Protoplanetary Disk

A rotating circumstellar disk of dense gas and dust surrounding a newly formed star, where perturbations in outer rings lead to the formation of protoplanets.

Birth of the Sun

The astronomical event triggered when pressure within the central mass of a protoplanetary disk becomes sufficiently high to initiate nuclear fusion.

Formation of Earth

The process driven by gravity outside the reach of the sun, causing interstellar matter to condense around protoplanets approximately $4.6\text{ BYA}$.

Hadean Eon

The earliest eon of Earth's history ($4.6\text{ BYA}$ to between $4.2\text{ BYA}$ and $3.8\text{ BYA}$), characterized by a high-energy environment, molten rock, and a reducing atmosphere without free oxygen.

Archean Eon

The geologic eon beginning between $4.2\text{ BYA}$ and $3.8\text{ BYA}$ marked by the origin of the first living organisms on Earth.

Stromatolites

Layered sedimentary structures produced by mats of cyanobacteria, forming some of the earliest fossil evidence of life on Earth.

Trace Fossil

A type of fossil that preserves evidence of the activity or movement of an organism rather than the physical body itself.

Archaeocyatha

A group of sponge-like marine organisms that lived during the Cambrian period.

Cambrian Explosion

A rapid diversification event early in the Paleozoic era during which major animal body plans and complex marine life appeared.

Permian Extinction

A mass extinction event at the end of the Paleozoic era ($250\text{ MYA}$) in which $96\%$ of Earth's species died out and atmospheric oxygen levels dropped rapidly.

Morganucodon watsoni

An early mammal species that appeared during the Mesozoic era around $205\text{ MYA}$.

K-T Extinction

A mass extinction event caused by an impact at the boundary between the Cretaceous and Tertiary periods ($65\text{ MYA}$), wiping out non-avian dinosaurs and about $65\%$ of all species.

Panspermia

The hypothesis proposing that life on Earth originated from extra-terrestrial sources, such as organic compounds or microbes carried by comets or meteorites.

Reducing Atmosphere

An atmospheric condition on early Earth lacking free oxygen ($O_2$), composed primarily of methane ($CH_4$), ammonia ($NH_3$), hydrogen ($H_2$), and water vapor ($H_2O$).

Abiotic Synthesis

The non-biological generation of organic chemical building blocks—such as amino acids, nucleotides, sugars, lipids, and ATP—from simple inorganic compounds under early Earth conditions.

Proximity Problem

The chemical challenge in prebiotic origin-of-life reaction mixtures where organic monomers were too dilute to react and form polymers, hypothesized to be solved by mineral surfaces like clay rich in $Fe^+$ and $Zn^+$.

Liposomes

Microscopic vesicles composed of a lipid bilayer surrounding an aqueous cavity, which serve as models for protobionts by enclosing reactants and helping resolve the proximity problem.

RNA World Hypothesis

The hypothesis that early life forms relied entirely on RNA to both store hereditary information and catalyze chemical reactions prior to the evolution of DNA and proteins.

Viroids

Infectious agents consisting solely of short, circular single-stranded RNA lacking a protein coat, considered potential modern clues to the ancient RNA world.