These are original Marlbridge practice questions, not past exam questions. The mark points are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. Use the course dates: Earth formed about 4.6 bya; it was too hostile for life until about 3.9 bya; the earliest fossil evidence of life is about 3.5 bya. The research reports and laboratory data in Questions 5 and 6 are fictional.
Question 1 (multiple choice · foundation)
Which statement correctly combines the geological evidence about the origin of life?
- (A) Life began 4.6 bya, when Earth formed, and the oldest fossils date from 3.9 bya.
- (B) Life most plausibly began between about 3.9 and 3.5 bya: conditions were too hostile before 3.9 bya, and fossils show life existed by 3.5 bya.
- (C) Life began exactly 3.5 bya, because that is the age of the oldest fossils.
- (D) Life most plausibly began between 4.6 and 3.9 bya, because the hostile conditions provided the energy for chemical reactions.
Answer and explanation
Answer: (B). The end of the hostile period gives the earliest plausible date, and the oldest fossils give a minimum age. Together they bracket a window of about 400 million years.
- (A) puts life’s origin at Earth’s formation, when conditions were far too hostile, and misdates the oldest fossils.
- (C) treats the fossil date as the origin. Fossils show life existed by then; it may have begun earlier.
- (D) places the origin in the period when the course evidence says life could not have survived.
Question 2 (multiple choice · core)
Which observation provides the strongest support for the RNA world hypothesis?
- (A) The site in the ribosome where peptide bonds form is made of RNA, not protein.
- (B) DNA is chemically more stable than RNA.
- (C) Nearly all reactions in modern cells are catalysed by protein enzymes.
- (D) The oldest fossils are about 3.5 billion years old.
Answer and explanation
Answer: (A). If RNA was once both the genetic material and the catalyst, you would expect traces of RNA catalysis in the oldest, most essential cell machinery. Protein synthesis itself is catalysed by RNA, which fits.
- (B) explains why DNA later took over storage, but it says nothing about RNA coming first.
- (C) describes modern cells; if anything, it is the observation the hypothesis must explain, not evidence for it.
- (D) is geological evidence about when life arose, not about which molecule was the first genetic material.
Question 3 (multiple choice · core)
A student writes four statements about the RNA world. Which one contradicts the hypothesis?
- (A) Genetic information was passed on when RNA molecules were copied.
- (B) Each new RNA strand was built by complementary base pairing with a template strand.
- (C) RNA copying was catalysed by protein enzymes encoded by early genes.
- (D) Some RNA molecules folded into shapes that catalysed reactions.
Answer and explanation
Answer: (C). One assumption of the hypothesis is that genetically encoded proteins were not the catalysts. Catalysis came from RNA itself (or from simple non-protein factors such as minerals).
- (A) matches the assumption that RNA replication carried genetic continuity.
- (B) matches the assumption that replication needed base pairing.
- (D) describes ribozymes, the reason RNA could act as a catalyst.
Question 4 (calculation · core)
A student makes a model of Earth’s history on a paper strip 4.6 m long. One end is Earth’s formation and the other end is the present day.
(a) Calculate how many millions of years each centimetre represents. (b) Calculate how far from the “Earth forms” end the student should mark 3.9 bya and 3.5 bya, and the length of the strip between the two marks. (c) The student writes: “The strip shows that life took 1.1 billion years to appear.” Evaluate this statement.
Worked solution and suggested Marlbridge rubric
(a) 4.6 m = 460 cm; 4.6 billion years = 4600 million years. 4600 ÷ 460 = 10 million years per cm.
(b) 3.9 bya is 4.6 − 3.9 = 0.7 billion years = 700 million years after formation: 700 ÷ 10 = 70 cm. 3.5 bya is 1.1 billion years = 1100 million years after formation: 110 cm. Between the marks: 110 − 70 = 40 cm (400 million years).
(c) 4.6 − 3.5 = 1.1 billion years is the time from Earth’s formation to the oldest fossil evidence, not necessarily to the origin of life. Fossils give a minimum age; life could have begun anywhere in the window from about 3.9 bya, and earlier fossils may simply not have survived. Also, for the first 0.7 billion years conditions were too hostile, so life did not have 1.1 billion years of suitable conditions to “take”. A better statement: “Life had appeared by 1.1 billion years after Earth formed, and possibly within a few hundred million years of conditions becoming suitable.”
| Point | What earns it |
|---|---|
| 1 | 10 million years per cm with consistent units |
| 1 | 70 cm and 110 cm (and 40 cm between) |
| 1 | Recognises 3.5 bya as a minimum age, not the origin |
| 1 | Uses the 3.9 bya limit to refine the statement |
Question 5 (constructed response · core)
A research team reports layered mounds in rocks dated to 3.8 bya and claims they are stromatolites.
(a) Explain what a stromatolite is and why fossil stromatolites count as evidence of life. (b) If the claim were accepted, describe how the plausible window for the origin of life would change, with numbers. (c) Other scientists are cautious. Explain why, and suggest two kinds of further evidence that would make the claim more convincing.
Model answer and suggested Marlbridge rubric
(a) A stromatolite is a layered mound of rock built up by mats of microorganisms that trap and bind sediment, layer on layer. Living mats still build them today, so similar fossil layers are taken as signs that such communities existed when the rock formed.
(b) The minimum age of life would move from 3.5 to 3.8 bya. The window would shrink from 3.9 − 3.5 = 0.4 billion years (400 million years) to 3.9 − 3.8 = 0.1 billion years (100 million years), a quarter of its old length. Life would have arisen soon after conditions became suitable.
(c) Layered shapes can also form without life, for example when rock is folded or chemicals settle out in layers, and very old rocks have been heated and squeezed, which can create or distort such patterns. Further evidence (any two): fine internal layering that matches living microbial mats; microscopic structures of consistent, cell-like size and shape; chemical traces in the rock that only living processes are known to leave; the same features at several independent sites; confirmation of the rock’s age by a second dating method.
| Point | What earns it |
|---|---|
| 1 | Stromatolite described as layers built by microbial mats |
| 1 | New window of about 100 million years (3.9 to 3.8 bya), with the old 400 million for comparison |
| 1 | Non-living processes can form similar layers, or old rocks are altered |
| 1 | Two suitable kinds of further evidence |
Question 6 (experimental design · stretch)
Researchers test a laboratory-made RNA, called L3, which may join two short RNA pieces into one strand when both pieces are base-paired side by side on a template. They measure the percentage of pieces joined after 2 hours.
| Tube | Contents | Pieces joined / % |
|---|---|---|
| 1 | pieces + template, no L3 | 0.2 |
| 2 | pieces + template + L3 | 24.0 |
| 3 | as tube 2, but L3 first treated with RNase (destroys RNA) | 0.3 |
| 4 | as tube 2, but L3 first treated with protease (destroys protein) | 23.5 |
| 5 | pieces + L3, no template | 0.4 |
| 6 | as tube 2, but the template has one mismatched base where the two pieces meet | 6.1 |
(a) State a null hypothesis for the comparison of tubes 1 and 2. (b) Calculate how many times more joining occurs in tube 2 than in tube 1. (c) Explain what tubes 3 and 4 together show. (d) Explain how tubes 5 and 6 relate to one of the assumptions of the RNA world hypothesis. (e) Give one limitation in using this experiment as evidence that an RNA world existed.
Model answer and suggested Marlbridge rubric
(a) “L3 has no effect on the percentage of RNA pieces joined.”
(b) 24.0 ÷ 0.2 = 120 times.
(c) Destroying RNA removes the activity (0.3%, close to tube 1), but destroying protein does not (23.5%, about 98% of tube 2). So the catalyst is the RNA L3, not a protein contaminant. This fits the assumption that encoded proteins were not needed as catalysts.
(d) Without a template the pieces are not held side by side, and joining stays near background (0.4%). With one mismatch at the junction, pairing is weakened and joining falls to 6.1%, about a quarter of tube 2. Accurate joining depends on base pairing with the template, the assumption that base pairing is needed for replication.
(e) Any one: L3 was made and selected in a modern laboratory, so it may not resemble any RNA on early Earth; purified pieces and controlled conditions differ from early-Earth conditions; joining two pieces is only one step, not full self-copying of an RNA; only one RNA and one set of conditions was tested.
| Point | What earns it |
|---|---|
| 1 | Null hypothesis states no effect / no difference |
| 1 | 120 times |
| 1 | Tubes 3 and 4 interpreted together: catalyst is RNA, not protein |
| 1 | Tubes 5 and 6 linked to base pairing and to the correct assumption |
| 1 | A valid limitation |
Question 7 (argument · stretch)
Modern cells face a “chicken and egg” problem: DNA is copied by protein enzymes, but proteins are made using information in DNA.
(a) Explain how the RNA world hypothesis solves this problem. (b) Give one piece of evidence from modern cells that supports your explanation. (c) If RNA came first, explain why DNA and proteins could later have replaced it in most roles.
Model answer and suggested Marlbridge rubric
(a) RNA can do both jobs. Its base sequence stores information, and it can be copied by complementary base pairing; and some RNA strands fold into shapes that act as catalysts (ribozymes). So one kind of molecule could carry genes and catalyse its own copying, with no need for DNA or protein at the start.
(b) Any one: the catalytic core of the ribosome, where peptide bonds form, is RNA; ribozymes that cut and join RNA occur in living cells today.
(c) DNA is more stable: its sugar lacks the reactive –OH group that makes RNA break more easily, and its double strand allows damaged bases to be repaired using the other strand, so it is a safer long-term store. Proteins are made from 20 kinds of amino acid, so they can fold into many more shapes and make faster, more specific catalysts. Cells using DNA and proteins would therefore out-reproduce RNA-only cells, and natural selection would favour them.
| Point | What earns it |
|---|---|
| 1 | RNA stores information and is copied by base pairing |
| 1 | RNA can act as a catalyst, so proteins are not needed at first |
| 1 | A valid piece of supporting evidence |
| 1 | A reason DNA is better for storage and a reason proteins are better catalysts |
How did you do?
- Q1, Q4 or Q5 wrong: re-read “Geological evidence: setting the time window” in the study guide, look again at Figure 1, and rework Worked example 1.
- Q2, Q3 or Q7 wrong: re-read “The RNA world hypothesis”, its three assumptions and Figure 2.
- Q6 incomplete: rework Worked example 2; check that your null hypothesis says “no effect”, and match each control to the assumption it tests.
Then tick off the topic checklist.