AP Biology 7.12 Origin of Life on Earth Study Notes - New Syllabus Effective 2025
AP Biology 7.12 Origin of Life on Earth Study Notes – New syllabus
AP Biology 7.12 Origin of Life on Earth Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the scientific evidence that supports models of the origin of life on Earth.
Key Concepts:
- Origin of Life on Earth
7.12.A – Scientific Evidence for the Origin of Life on Earth
🌋 Step-by-Step Model of Life’s Origin:
1. Earth’s Early Conditions 🌫️
- Earth formed ~4.6 billion years ago.
- The early atmosphere likely had no oxygen but was full of gases like methane (CH₄), ammonia (NH₃), water vapor (H₂O), and hydrogen (H₂).
- These were reducing conditions good for forming complex molecules.
2. Abiotic Synthesis of Organic Molecules 🧪
- Simple molecules (like amino acids and nucleotides) could form without life.
- Miller-Urey experiment (1953) showed that lightning + gases = amino acids.
- Proved that organic molecules can form spontaneously in early Earth conditions.
3. Formation of Polymers 🧬
- Organic monomers could link into polymers like proteins and RNA.
- Likely occurred on hot surfaces (like volcanic rock or clay) that acted as catalysts.
4. Self-Replicating Molecules 🔁
- RNA world hypothesis: RNA was likely the first genetic material.
- RNA can store information and catalyze reactions (ribozymes).
- Eventually, DNA took over as the more stable information molecule.
5. Formation of Protocells 🫧
- Protocells = early, simple cell-like structures.
- Lipid membranes can spontaneously form vesicles in water.
- These protocells could maintain internal conditions and divide an early step toward life.
🧬 Summary:
| Step | Process |
|---|---|
| 1 | Early Earth had no oxygen but lots of energy and simple gases |
| 2 | Organic molecules (amino acids, nucleotides) formed abiotically |
| 3 | Polymers formed from monomers |
| 4 | RNA evolved to replicate and pass on information |
| 5 | Protocells enclosed molecules in membranes |
🔍 Evidence Comes From:
- Lab experiments (e.g., Miller-Urey)
- Fossils of early microbes
- Geochemical data from ancient rocks
- Comparisons of DNA & RNA across species
- Modern extremophiles (organisms in harsh environments) show how early life could’ve survived
7.12.A.1 – Scientific Evidence for the Origin of Life on Earth
🧠 Key Idea:
The origin of life isn’t just a guess it’s backed by solid scientific and geological evidence from Earth’s history.
🌍 Geological Evidence Supports Life’s Origin
Scientists use fossils, ancient rocks, and isotopes to understand when and how life began.
- Stromatolites, which are layered structures formed by ancient bacteria, are among the oldest fossils we’ve found.
- Chemical signatures of life (like carbon isotope ratios) in ancient rocks show life was already active billions of years ago.
📅 Timeline of Early Earth & Life
| Event | Approximate Time |
|---|---|
| 🌍 Earth formed | ~4.6 billion years ago (bya) |
| 🌋 Conditions too hostile for life | Until ~3.9 bya |
| 🧫 Earliest fossil evidence of life | ~3.5 bya |
➡️ So, life most likely began sometime between 3.9 and 3.5 bya.
🔬 What This Tells Us:
- Life didn’t appear instantly early Earth needed to cool down and stabilize first.
- Once conditions were right, simple life forms emerged probably starting with self-replicating molecules like RNA.
- These findings match up with scientific models that explain how life may have developed naturally from non-living matter.
7.12.A.2 – The RNA World Hypothesis
🧠 Key Idea:
The RNA world hypothesis suggests that RNA not DNA was likely the first genetic material on early Earth. This model helps explain how life might have started before complex cells evolved.
🧬 Why RNA?
RNA is unique because it can:
- Store genetic information (like DNA)
- Catalyze reactions (like enzymes these RNA molecules are called ribozymes)
This means RNA could have acted both as the message and the machine in early life.
📌 Three Core Assumptions:
RNA replication ensured genetic continuity- Early life needed a way to copy genetic info, and RNA could do that without DNA or proteins.
Base-pairing allowed RNA to replicate itself – Just like in DNA, complementary base pairing (A-U and G-C in RNA) allowed one strand to guide the formation of another.
Proteins weren’t involved as catalysts – In this early world, RNA had to do the job alone – there were no enzymes or DNA polymerases yet.
🧬 Bonus Insight:
Ribozymes, which are RNA molecules that act like enzymes, still exist today – giving evidence that RNA-based catalysis is real and ancient.