Origin of Earth and Cellular Life – Key Points

Surface conditions on early Earth: extremely hot, high UV ➔ hostile to life
Subsurface (hydrothermal-vent) hypothesis:
- Stable environment, continuous energy from $\text{H}2,\;\text{H}2\text{S}$
- Facilitated synthesis of organic molecules & compartment formation
Prebiotic chemistry ➔ self-replicating RNA ("RNA world")
- RNA binds small molecules & shows catalytic self-replication
- Gradual takeover of catalysis by proteins; DNA later adopted as stable genetic store

Lived $\approx 3.8\,\text{–}\,3.7\,\text{Ga}$ in an anoxic Earth
Metabolism: anaerobic, heat-stable, chemolithoautotrophic
- Carbon source: $\text{CO}_2$ (auto-)
- e⁻ donor: $\text{H}_2$ (litho-)
- e⁻ acceptor: $\text{S}^0$
High horizontal gene transfer; divergence into Bacteria & Archaea once DNA replication, transcription, translation fully established

Early chemolithoautotrophy produced large pools of reduced organic carbon
Transition to chemoorganoheterotrophy followed as organic substrates accumulated

First phototrophs: anoxygenic, using $\text{H}_2\text{S}$ ➔ $\text{S}^0$
Evolution of Cyanobacteria & oxygenic photosynthesis $\approx 2.6\,\text{Ga}$
- e⁻ donor $\text{H}2\text{O}$ ➔ $\text{O}2$ production
Stromatolites (layered fossils) $\approx 3.5\,\text{Ga}$ confirm ancient phototrophy

Initial $\text{O}_2$ reacted with dissolved $\text{Fe}^{2+}$ ➔ insoluble $\text{Fe}^{3+}$ precipitates (banded iron)
After oxidation of all $\text{Fe}^{2+}$ , atmospheric $\text{O}_2$ began accumulating (Great Oxidation Event, $\approx 2.4\,\text{Ga}$ , reaching $10^{-6}$ present level)
Shift from anoxic ➔ oxic environment
- Anaerobes confined to anoxic niches
- Aerobic respiration evolved, yielding higher energy and faster growth

$\text{O}2$ + UV ➔ $\text{O}3$ (ozone)
Ozone absorbs UV (< $300\,\text{nm}$ ), reducing DNA damage
Enabled colonization of surface environments and diversification into complex multicellular life