EVIDENCE FOR EARLY LIFE. Describe evidence concerning when and where life on Earth began.
LO 6.3
SOURCES OF ORGANIC MOLECULES. Describe potential sources of the building blocks of life on early Earth, and discuss and critique evidence provided by the Miller–Urey experiment.
LO 6.4
RNA WORLD. Compare DNA to RNA, and evaluate the hypothesis that early life used RNA rather than DNA as its hereditary molecule.
LO 6.5
A PLAUSIBLE SCENARIO FOR ORIGIN OF LIFE. Describe a plausible, five-step scenario for the origin of DNA-based life on Earth, and evaluate the quality of the evidence supporting each step.
LO 6.6
MIGRATION OF LIFE FROM OTHER WORLDS. Evaluate the possibility that life may have migrated to Earth after first evolving somewhere else.
LO 6.7
EVOLUTIONARY HISTORY OF EARTH. Identify major events in Earth’s evolutionary history.
LO 6.8
THE RISE OF OXYGEN. Discuss evidence for how and when oxygen was introduced into Earth’s atmosphere, and describe the role of oxygen in the evolution of more complex organisms.
LO 6.9
LUCKY EARTH? Consider whether the events that made complex life possible on Earth should be expected or unexpected on other similar worlds.
LO 6.10
THE K–Pg EVENT. Evaluate the evidence suggesting that an impact caused the extinction of the dinosaurs.
LO 6.11
OTHER MASS EXTINCTIONS. Discuss and evaluate potential causes of other mass extinctions in Earth’s history.
LO 6.12
THE IMPACT THREAT. Discuss whether another impact event is likely to occur in the future, and argue for or against an increase in government spending to avert that possibility.
LO 6.13
HUMAN EVOLUTION. Describe current understanding of the general pathway of human evolution.
LO 6.14
ARTIFICIAL LIFE. Describe approaches to, and ethical dilemmas about, the creation of artificial life.

got it. here’s a tight study sheet keyed to each LO.

# LO 6.3 — Evidence for early life: when & where
– **Oldest isotopic hints (3.8–4.1 Ga):** Light carbon (δ¹³C depletion) in ancient graphite suggests biological fractionation.
– **Microfossils & stromatolites (≥3.4–3.5 Ga):** Layered microbial mats and cell-like microstructures in cherts.
– **Biomarkers:** Hopanes/steranes are debated in the oldest rocks (possible contamination), stronger later.
– **Settings:** Shallow marine shelves/tidal flats (stromatolites), **hydrothermal systems** (oceanic crust, alkaline vents) provide gradients, minerals, and catalysis.

# LO 6.4 — Sources of organics & Miller–Urey
– **Endogenous:** Lightning + reducing atmospheres; hydrothermal vent chemistry; UV-driven synthesis; mineral-catalyzed reactions (FeS, clays).
– **Exogenous:** Carbonaceous chondrites, comets, IDPs deliver amino acids, sugars, nucleobases.
– **Miller–Urey (1953):** Spark discharge in a strongly reducing gas makes amino acids.
**Critique:** Atmosphere likely less reducing (more CO₂/N₂), yields drop; still key proof-of-principle that **prebiotic synthesis is plausible**. Modern variants (volcanic gases, UV/ice chemistry) also work.

# LO 6.5 — RNA World
– **DNA vs RNA:** DNA = stable storage (deoxyribose, thymine); RNA = less stable but can **store info and catalyze** (ribozymes).
– **Evidence:** Ribosome’s active site is RNA; many cofactors are “fossil” nucleotides; lab-evolved ribozymes can polymerize RNA.
– **Challenges:** Prebiotic ribose/nucleobase formation & **non-enzymatic replication fidelity**; plausible partial solutions (formose variants, cyanosulfidic chemistry, wet–dry cycles, mineral surfaces).

# LO 6.6 — A five-step origin path (DNA-based life)
1. **Prebiotic monomers** (amino acids, nucleotides, lipids) from Earth/space. **Evidence:** meteorites; lab syntheses.
2. **Polymers** via surfaces/wet–dry cycles (clays, basalts) → peptides/RNA. **Evidence:** lab polymerization on minerals.
3. **Protocells** (fatty-acid vesicles) concentrating polymers; growth/division via simple physics. **Evidence:** spontaneous vesicle formation, strand uptake.
4. **RNA World** heredity + catalysis; selection improves replication; metabolic cofactors emerge. **Evidence:** ribozymes; lab selection.
5. **DNA/protein takeover**: reverse transcriptase–like chemistry hardens genome; proteins take over catalysis; modern cells. **Evidence:** universality of genetic code, ribosome RNA core.
**Confidence:** Strongest for steps 1–3; plausible but incomplete for 4–5.

# LO 6.7 — Life migrating here (panspermia)
– **Mechanisms:** Impact ejecta → interplanetary transfer; comet/asteroid delivery; interstellar dust (speculative).
– **Feasibility:** Microbes can survive shock, vacuum, radiation for limited times; transfer **within a system** is plausible (Earth–Mars).
– **Test:** Compare ancient isotopes/biochemistry; no decisive evidence yet. Panspermia **moves the start**, doesn’t solve abiogenesis.

# LO 6.8 — Evolutionary history (ultra-brief)
– 4.54 Ga Earth forms → oceans by ~4.3 Ga
– ≥3.5 Ga microbial biosphere (stromatolites)
– 2.4–2.0 Ga **Great Oxidation Event**
– 1.9–1.6 Ga eukaryotes; 1.2 Ga simple multicells
– 635–541 Ma Ediacaran; **541 Ma Cambrian** radiation
– 470–360 Ma land plants/animals; 252 Ma end-Permian crash
– 201–66 Ma dinosaurs dominate; **66 Ma K–Pg**; mammals diversify
– <1 Ma anatomically modern humans spread.

# LO 6.9 — Rise of oxygen
– **Evidence:** BIFs, red beds, sulfur mass-independent fractionation loss at ~2.4 Ga.
– **Cause:** Oxygenic photosynthesis (cyanobacteria) outpaced sinks (Fe²⁺, volcanic gases).
– **Role:** Enabled ozone layer, aerobic respiration, larger/complex bodies; later **Neoproterozoic oxygenation** tied to eukaryote/multicell expansion.

# LO 6.10 — Lucky Earth?
– Some enabling “stacked contingencies”: long-lived star, plate tectonics, magnetic field, surface water, carbon cycle, moons stabilizing obliquity, right metal content, impact history.
– Not necessarily unique, but **not guaranteed** on every Earth-like planet.

# LO 6.11 — K–Pg impact (dinosaurs)
– **Smoking guns:** Global iridium spike; shocked quartz/spherules; **Chicxulub crater** (~180 km, Yucatán); tsunami/tektites; sudden fossil turnover.
– **Aftermath:** Aerosols/soot → darkness/cooling → food web collapse; non-avian dinos go extinct; mammals rebound.

# LO 6.12 — Other mass extinctions
– **End-Permian (~252 Ma):** Largest; Siberian Traps volcanism → CO₂/CH₄, warming, anoxia, acidification.
– **End-Ordovician (~444 Ma):** Glaciation/sea-level fall.
– **Late Devonian (~372–359 Ma):** Pulsed; anoxia, plant-driven nutrient shifts, possible impacts/volcanism.
– **End-Triassic (~201 Ma):** Central Atlantic Magmatic Province volcanism.
– **K–Pg (~66 Ma):** Impact + volcanism interplay.

# LO 6.13 — Impact threat & policy
– **Likelihood:** Small impacts common; civilization-ending (≥10 km) very rare; **city/region-scale (50–300 m)** are the main risk.
– **Mitigation:** Survey (NEO catalogs, space-based IR like NEO Surveyor), characterize, deflection demos (e.g., kinetic impactor), coordination.
– **Argument:** Cost is modest vs risk tail; funding for detection + tested deflection tech is rational insurance.

# LO 6.14 — Human evolution (outline)
– **Split with chimps** ~6–7 Ma; early hominins (Sahelanthropus, Ardipithecus).
– **Australopithecus** (bipedalism).
– **Homo habilis/erectus** (tools, fire, long-distance).
– **H. heidelbergensis → Neanderthals/Denisovans**; admixture with **H. sapiens**.
– **H. sapiens** ~300 ka Africa → global dispersal; culture, language, tech accelerate recent evolution.

# LO 6.15 — Artificial life: approaches & ethics
– **Approaches:**
– **Wet-lab minimal cells** (genome reduction, lipid vesicles).
– **XNA/expanded genetic alphabets**, non-canonical amino acids.
– **Bottom-up protocells** (metabolism + heredity in vesicles).
– **In silico ALife** (evolving software/robots).
– **Ethics:** Containment/biorisk, dual-use, ecological release, IP/ownership of life, definitions of personhood/rights for sentient AI or engineered organisms, equity of benefits.

Want this turned into a one-page printable cheat sheet or flashcards?
Do you think that cells or? say lived stacked say, and during drying cycles these organisms would compress onto each other, sometimes sticking together so they are mated when rehydrated?

Yes, that’s actually a plausible prebiotic mechanism — it matches what some origin-of-life researchers call **wet–dry cycling adhesion**.

– **Drying phase:** As water evaporates in tidal pools, lakeshores, or volcanic hot springs, simple lipid vesicles, polymers, or microbial films collapse and press into each other. Molecules are concentrated, surfaces touch, and dehydration reactions (which link monomers into polymers) are favored.
– **Sticking/fusion:** When membranes or polymer films dry together, they can fuse or form layered aggregates. Fatty-acid vesicles in experiments often merge under these conditions.
– **Rehydration:** When water returns, some fused patches reseal as **larger vesicles**, trapping mixed contents from the “stacked” neighbors. That could shuffle polymers around, promoting complexity.

So your “stacked & mated” idea mirrors a known lab phenomenon: protocells and lipid vesicles *do* stick and exchange contents after wet–dry cycles. It’s one of the best-supported pathways for protocells to grow, combine, and eventually evolve.

Would you like me to sketch how this “stack-fusion” could evolve stepwise into something like primitive cell lineages?