How can anyone study something that happened billions of years ago?
No one watched life begin. So scientists build models of what might have happened and test them against evidence that survives today. Two kinds of evidence matter in this topic:
- Geological evidence: the ages of rocks and what they show about early Earth’s conditions, plus fossils preserved in those rocks. This tells you when life could have started.
- Molecular evidence: what the molecules of life can do, today and in the laboratory. This suggests how the first genetic system might have worked.
Geological evidence: setting the time window
Three dates frame the story. Learn them as the course states them.
| Date (approximate) | What happened | What it tells you |
|---|---|---|
| 4.6 bya | Earth formed | nothing could live on Earth before this |
| 4.6 to 3.9 bya | the environment was too hostile for life | life could not have started and survived before about 3.9 bya |
| 3.5 bya | earliest fossil evidence of life | life already existed by this time |
(bya = billion years ago.)
Why hostile? Young Earth was hot, and large asteroid and comet impacts were frequent. Big impacts released enough energy to melt rock and boil away surface water, so any early life could have been wiped out.
How are rocks dated? Radioactive isotopes in minerals decay at known, steady rates. Measuring the ratio of a parent isotope to its decay product gives the age of the rock (radiometric dating).
What do the oldest fossils look like? Not bones or shells. The oldest widely accepted evidence comes from stromatolites: layered mounds of rock built up by mats of microorganisms trapping sediment. Stromatolites about 3.5 billion years old are found in Western Australia. Living stromatolites still form today in a few salty, shallow seas, which helps scientists recognise the fossil ones.
Reasoning with the dates. The two kinds of evidence work from opposite ends:
- The end of the hostile period gives the earliest plausible date: life could not have started and lasted before then.
- The oldest fossils give a minimum age: life must have existed by then, and it may well be older. Fossils of the very first cells, if any formed, are unlikely to survive billions of years of heat and pressure in rock.
Taken together, they give a plausible range of dates, roughly 3.9 to 3.5 bya. Claims of even older signs of life exist, but they are disputed, because non-living processes can make similar layered shapes and chemical traces. That is why the course treats about 3.5 bya as the earliest fossil evidence.
Background (beyond the required content): from simple molecules to building blocks
Before any genetic system, the building blocks of life had to form without living cells. In an experiment published in 1953, Stanley Miller, working in Harold Urey’s laboratory, sealed water, methane, ammonia and hydrogen in glass apparatus and passed electric sparks (a model of lightning) through the gases. Within a week, amino acids such as glycine and alanine had formed. Today most scientists think the early atmosphere was different, with more carbon dioxide and nitrogen, but later experiments show that organic building blocks can still form under a range of early-Earth conditions. The required content for this topic starts at the next step: how a genetic system could begin.
The RNA world hypothesis
Modern cells have a “chicken and egg” problem. DNA stores genetic information, but copying DNA needs protein enzymes. Proteins catalyse reactions, but making a protein needs the information in DNA (through RNA). So which came first?
The RNA world hypothesis suggests that the first molecule to carry genes from one generation to the next was RNA, not DNA. RNA can do both jobs:
- It stores information. The sequence of bases (A, U, G, C) along an RNA strand is a code, just like DNA’s.
- It can be copied by base pairing. A pairs with U and G pairs with C, so one strand acts as a template for a complementary strand.
- It can catalyse reactions. A single RNA strand can fold back on itself, forming base-paired regions and loops. Some folded RNAs are catalysts, called ribozymes.
The three assumptions
The hypothesis rests on three assumptions. Learn them in your own words:
- RNA replication once carried genetic continuity. At some point, genetic information passed from one generation to the next because RNA molecules were copied.
- Base pairing was needed for replication. Copies were accurate because each base on the template paired with its complementary base.
- Genetically encoded proteins were not the catalysts. The reactions, including copying, were catalysed by RNA (or by simple minerals or metal ions), not by proteins made from genes.
Evidence that fits the hypothesis
- Ribozymes exist in cells today. Thomas Cech and Sidney Altman shared the 1989 Nobel Prize in Chemistry for showing that RNA can be a catalyst.
- The ribosome is a ribozyme. The site where peptide bonds form is made of ribosomal RNA, not protein. Protein synthesis itself is catalysed by RNA, as you would expect if RNA came first.
- Laboratory ribozymes can copy RNA. RNA molecules evolved in the laboratory can copy RNA templates by base pairing, although fully copying a molecule as long and complex as the ribozyme itself remains a major challenge.
None of this proves that an RNA world existed. It shows that the idea is chemically possible and fits features of modern cells.
Why DNA and proteins later took over
If RNA came first, DNA and proteins must have taken over most of its jobs later. DNA is more stable (its sugar lacks the reactive –OH group that makes RNA easier to break), and its double strand allows damage to be repaired from the other strand. Proteins, built from 20 kinds of amino acid, can fold into a far wider range of shapes than RNA, so they make faster, more specific catalysts.
Worked example 1: putting the dates in proportion
Question. Imagine Earth’s whole history, from 4.6 bya to now, squeezed into one 24-hour day starting at midnight. (a) At what times do conditions become suitable for life (3.9 bya) and do the earliest fossils appear (3.5 bya)? (b) How long is the plausible window? (c) For what percentage of Earth’s history has there been fossil evidence of life?
- Scale: 24 hours represent 4.6 billion years, so 1 billion years = 24 ÷ 4.6 = 5.217 hours (about 313 minutes).
- 3.9 bya is 4.6 − 3.9 = 0.7 billion years after Earth formed: 0.7 × 5.217 = 3.652 hours, about 03:39.
- 3.5 bya is 4.6 − 3.5 = 1.1 billion years after Earth formed: 1.1 × 5.217 = 5.739 hours, about 05:44.
- Window: 3.9 − 3.5 = 0.4 billion years (400 million years), which is 05:44 − 03:39 ≈ 2 hours 5 minutes on the clock (0.4 × 313 ≈ 125 minutes).
- Fossil evidence of life covers 3.5 ÷ 4.6 × 100 = 76% of Earth’s history.
Check. 0.7 + 0.4 + 3.5 = 4.6, so the three parts add up to the whole.
Interpretation. Life appeared early: by breakfast time on the 24-hour clock, with the rest of the day still ahead. The window is short on this scale, but 400 million years is still a very long time for chemistry to produce a self-copying system. Remember that 3.5 bya is a minimum: life may have begun earlier in the window.
Worked example 2: a null hypothesis for an RNA world experiment
Question. Researchers have a fictional laboratory-made RNA, called R7. They want to test whether R7 can copy a short RNA template without any protein enzyme. Each tube contains the template, a short starting strand that base-pairs with it, and free nucleotides. They measure the percentage of templates fully copied after 24 hours.
(a) State the null hypothesis. (b) Predict the results of the controls if R7 is an RNA catalyst that works by base pairing. (c) Interpret the fictional results below.
| Tube | Contents | Templates copied / % |
|---|---|---|
| 1 | no R7 | 0.4 |
| 2 | R7 | 18.6 |
| 3 | R7 treated first with RNase (an enzyme that destroys RNA) | 0.5 |
| 4 | R7 treated first with protease (an enzyme that destroys protein) | 18.1 |
| 5 | R7, but a template whose bases cannot pair with the starting strand | 0.6 |
(a) Null hypothesis. R7 has no effect on the percentage of templates copied. (A null hypothesis predicts no difference; the experiment tries to reject it.)
(b) Predictions.
- Tube 2 should copy far more templates than tube 1, if R7 is a catalyst.
- Tube 3 should fall to the tube 1 level, because destroying RNA destroys the catalyst.
- Tube 4 should stay at the tube 2 level, because no protein is involved (assumption 3).
- Tube 5 should fall to the tube 1 level, because copying depends on base pairing (assumption 2).
(c) Interpretation. The results match every prediction. Tube 2 copies about 46 times as many templates as tube 1 (18.6 ÷ 0.4 = 46.5), so the null hypothesis is rejected. RNase removes the effect (0.5%) but protease does not (18.1%, about 97% of tube 2), so the catalyst is RNA, not a contaminating protein. Without base pairing (tube 5), almost nothing is copied. The data support two assumptions of the RNA world hypothesis. They do not show that such an RNA arose on early Earth: R7 was made in a modern laboratory, with purified ingredients.
Common misconceptions
- “The fossils show exactly when life began.” Fossils give a minimum age. Life may be older than its oldest surviving fossils.
- “Earth was suitable for life as soon as it formed.” For several hundred million years it was too hostile: hot and frequently hit by large impacts.
- “The first genetic material was DNA.” The RNA world hypothesis proposes RNA, because RNA can both store information and catalyse reactions; DNA needs protein enzymes to be copied.
- “All biological catalysts are proteins.” Ribozymes are RNA catalysts, and the ribosome’s core is one.
- “A hypothesis supported by evidence is proven.” The RNA world is a model supported by several lines of evidence, not a proven fact.
- “The null hypothesis is the result you expect.” It states no effect or no difference, whatever you predict.
- “The Miller–Urey experiment created life.” It made some organic building blocks, such as amino acids, not living cells.
Where this leads
Unit 8 moves from the history of life to how organisms interact with their environment today: read Responses to the Environment next. To review how genetic diversity affects survival, go back to Variations in Populations. Test yourself with the practice questions, then use the revision notes and the checklist.
Quick questions
Do I need to know the Miller–Urey experiment?
It is not part of the required content for this topic, but it is a useful example of how scientists test ideas about early Earth chemistry. This guide includes it as labelled background.
Has the RNA world hypothesis been proven?
No. It is a well-supported model, not a proven fact. Several observations fit it, and laboratory experiments show that parts of it are possible, but no one has shown how the first self-copying RNA arose.