Earth's History, Major Eras, and Origins of Life- LECTURE 2

Earth formed about $4.57 \times 10^{9}$ years ago.
Four major divisions of Earth history: Precambrian (super eon), Paleozoic, Mesozoic, Cenozoic.
Precambrian is the longest interval; ends around $5.43 \times 10^{8}$ years ago, after which the Paleozoic begins.
Precambrian is subdivided into three eons: Hadean, Archaean, Proterozoic.
The last three divisions on the slide (Paleozoic, Mesozoic, Cenozoic) are eras.

Duration: from Earth's formation at $4.6 \times 10^{9}$ years ago to around $5.43 \times 10^{8}$ years ago.
Divided into three eons: Hadean, Archaean, Proterozoic.
Precambrian biology is dominated by unicellular life for most of its duration; multicellular life appears only toward the end.
The concept of time-scale hierarchy (eons > eras) helps illustrate the immense span of the Precambrian.

No oceans, no protective atmosphere; highly reducing environment with little to no oxygen.
Oceans form later in the Hadean from rain in a gradually cooling atmosphere (by about $7.5 \times 10^{8}$ years after formation).
Early life not present or very simple; conditions harsh.

Early life appears; unicellular organisms dominate.
Fossils of cyanobacterial mats (stromatolites) are common by late Archaean.
Both Bacteria and Archaea representatives present in early days of life.
By the end of the Archaean, the first hints of multicellular life begin to emerge (though clearly identified later).

Paleozoic Era: $5.43 \times 10^{8}$ to $2.50 \times 10^{8}$ years ago (≈ $541 \times 10^{6}$ to $252 \times 10^{6}$ years ago).
Early Paleozoic life: trilobites and diverse marine organisms; early reef-builders include archaeocyathids (sponge/coral-like organisms).
Cambrian explosion reflects rapid diversification, but evidence suggests pre-Cambrian animals existed; the apparent rapidity is nuanced.
Late Paleozoic: plants colonize land (around 420 Ma); major vertebrate lineages evolve; large amphibians dominate land ecosystems.
End of Paleozoic: Permian extinction (the Great Dying) dramatically reduces biodiversity, opening ecological space for new groups.

Timeframe: $2.50 \times 10^{8}$ to $6.5 \times 10^{7}$ years ago (≈ 250–65 Ma).
Known as the age of reptiles; dinosaurs rise and diversify.
First mammals appear around $2 \times 10^{8}$ years ago (≈ 200 Ma).
Late Cretaceous mass extinction (KT boundary) around $6.6 \times 10^{7}$ years ago, linked to asteroid impact and volcanic activity; dust blocked sunlight, disrupting photosynthesis.

Timeframe: $5.5 \times 10^{7}$ years ago to present (66 Ma to today).
Age of mammals; major mammalian diversification.
Origin of the first humans occurred around $2 \times 10^{6}$ years ago.
Emphasis on how shifting land masses and plate tectonics shaped life distributions.

Foundational question: what is life? Two core features – organization and replication capability.
Evidence suggests life arose around $3.8 \times 10^{9}$ years ago.
Earth’s early environment: reducing atmosphere with little to no free oxygen, enabling chemical synthesis of complex molecules.
Step 1: Abiotic synthesis of organic molecules (building blocks like amino acids, nucleotides, sugars, lipids, ATP) in a reducing environment.
Step 2: Formation of polymers and concentration mechanisms to overcome the proximity problem (surfaces like clays can help assemble monomers into polymers).
Step 3: Emergence of compartmentalized structures (lipid bilayers forming liposomes) to create protobionts that protect and concentrate biochemical reactions.
Step 4: Origin of hereditary material; RNA world hypothesis suggests RNA was the first genetic/functional molecule, capable of catalysis and self-replication.
RNA properties supporting RNA world:
- RNA can form abiotically more readily than DNA;
- Can self-replicate in simple systems (especially with zinc);
- Can store hereditary information;
- Can catalyze reactions; small genomes (~250 nucleotides) could suffice for primitive life.
Overall idea: four steps lead from simple chemistry to organized, replicating systems; exact moment and location of life’s origin remain uncertain.

Early experiments showed that simple atmospheric mixtures plus energy input can produce amino acids, nucleotides, sugars, lipids, and ATP within about a week.
Original Miller–Urey setup used a reducing atmosphere (NH3, CH4); later work suggests early atmosphere may have been CO₂/N₂ with some H₂, altering exact products but supporting plausible pathways to building blocks.
Modern prebiotic chemistry explores diverse atmospheres and catalysts (e.g., clays) that help bring monomers together into polymers.

Proximity problem: how did monomers come together to form polymers without dispersing in the ocean?
Clay surfaces and mineral interfaces likely helped concentrate reactants and promote polymerization.
Formation of lipid bilayers creates liposomes that mimic cell compartments, concentrating materials and enabling more complex chemistry.

Earth history is vast: Precambrian dominates ~4.0 billion years; life begins in the Precambrian; major diversification occurs in the Phanerozoic (Paleozoic onward).
Major transitions: emergence of photosynthetic life (stromatolites), multicellularity, colonization of land, rise and extinction of dominant groups (dinosaurs, mammals).
Life likely arose through a sequence of steps from chemistry to organized, replicating systems with RNA as a key early genetic material.
The KT boundary marks a major turnover in life, reshaping ecosystems that lead to modern mammals.