Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Geological evidence (rock ages, signs of early conditions and fossils) supports models of when life began.
- Earth formed about 4.6 bya. Heat and frequent large impacts kept it too hostile for life until about 3.9 bya. The earliest fossil evidence of life, such as stromatolites (layered mounds built by microbial mats), is about 3.5 bya.
- The hostile period sets the earliest plausible date; fossils set a minimum age. Together they give a plausible range.
- The RNA world hypothesis proposes that RNA was the earliest genetic material. RNA stores information in its base sequence, is copied by base pairing, and can fold into catalysts (ribozymes).
- Later, DNA (more stable) took over information storage and proteins (more varied shapes) took over most catalysis.
Key relationships
| Evidence or idea | What it shows |
|---|---|
| Earth’s age, 4.6 bya | oldest possible date: nothing could live on Earth earlier |
| Hostile conditions until about 3.9 bya | life could not start and survive before then |
| Earliest fossils, 3.5 bya | life existed by then (a minimum age) |
| Ribozymes, including the ribosome’s core | RNA can be a catalyst, so proteins are not essential for catalysis |
| Lab RNAs that copy templates | RNA-catalysed, base-pairing replication is chemically possible |
| Assumption of the RNA world | In one line |
|---|---|
| 1 | Genetic continuity came from copying RNA |
| 2 | Copying depended on complementary base pairing (A–U, G–C) |
| 3 | Catalysts were not proteins made from genes |
Assumptions behind the reasoning
- The dates are approximate, and the course values (4.6, 3.9, 3.5 bya) are the ones to use.
- Experiments in modern laboratories show what is possible, not what did happen.
- A null hypothesis always states no effect or no difference.
Mistakes to avoid
- Treating the oldest fossil date as the moment life began.
- Saying DNA was the first genetic material.
- Writing “proteins catalysed early RNA replication”: that contradicts assumption 3.
- Stating the expected result as the null hypothesis.
- Calling the RNA world hypothesis proven.
Quick self-check
- How long was the plausible window? (3.9 − 3.5 = 0.4 billion years = 400 million years.)
- Why can RNA, but not DNA alone, explain the first genetic system? (RNA can both carry information and act as a catalyst; DNA needs protein enzymes to be copied.)
- Null hypothesis for “Does RNA X speed up copying?” (RNA X has no effect on the rate of copying.)
Next: practice questions.