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. Data for every question: half-life of carbon-14 = 5,700 years; fraction remaining after n half-lives = (1/2)ⁿ. A calculator is assumed. All species, sites and data sets are fictional unless a real example is named.
Question 1 (multiple choice · foundation)
Which observation is an example of a vestigial structure that provides evidence of common ancestry?
- (A) Bats and moths both have wings that they use for flight.
- (B) Whales have small pelvic bones embedded in muscle that are not attached to the backbone and play no part in walking.
- (C) Bacteria and humans use the same codons for the same amino acids.
- (D) A fossil fish is found in a rock layer below a fossil amphibian.
Answer and explanation
Answer: (B). The whale pelvis is a reduced remnant of the hip bones that supported the hind legs of land-living ancestors. It makes sense only if whales descended from four-legged animals.
- (A) describes analogous structures: bat and moth wings are built differently and evolved separately, so they are not evidence of a recent common ancestor.
- (C) is biochemical evidence of common ancestry, but it is not a vestigial structure.
- (D) is geological evidence about the order in which organisms appeared, not a vestigial structure.
Question 2 (multiple choice · core)
A piece of wood from an ancient boat contains 6.25% of the carbon-14 found in living wood. How old is the wood?
- (A) 11,400 years
- (B) 17,100 years
- (C) 22,800 years
- (D) 91,200 years
Answer and explanation
Answer: (C). 100% → 50% → 25% → 12.5% → 6.25% is 4 half-lives. Check: (1/2)⁴ = 1/16 = 0.0625. Age = 4 × 5,700 = 22,800 years.
- (A) is 2 half-lives, the age at 25% remaining.
- (B) is 3 half-lives, the age at 12.5% remaining.
- (D) multiplies 5,700 by 16 (from 1/16) instead of by the number of halvings, 4.
Question 3 (multiple choice · core)
Why can carbon-14 not be used to date a fossil bone thought to be 70 million years old?
- (A) After so many half-lives, far too little carbon-14 would remain to measure.
- (B) Bones do not contain carbon, so they never take in carbon-14.
- (C) Carbon-14 decays faster as a sample gets older, so its half-life is unreliable.
- (D) Carbon-14 is only found in rocks, not in the remains of organisms.
Answer and explanation
Answer: (A). 70 million years is about 12,000 half-lives (70,000,000 ÷ 5,700 ≈ 12,300). Even after about 50,000 years (under 9 half-lives) the amount left is too small to measure reliably. Such fossils are dated using isotopes with much longer half-lives in volcanic layers around them.
- (B) is false: bone contains carbon compounds, and living animals take in carbon-14 through their food.
- (C) is false: decay happens at a constant rate, which is what makes half-lives useful.
- (D) reverses the facts: carbon-14 enters living things from the atmosphere through photosynthesis and food.
Question 4 (data analysis · core)
Scientists compared the amino acid sequence of the same 120-amino-acid protein in five fictional mammal species. The table shows the number of differences between species A and each of the others.
| Species compared with A | B | C | D | E |
|---|---|---|---|---|
| Amino acid differences (out of 120) | 1 | 9 | 17 | 30 |
(a) Identify the species most closely related to A and the species least closely related to A, using the data. (b) Calculate the percentage identity between A and E. (c) Explain why the number of differences indicates how recently two species shared a common ancestor. (d) When the DNA of the gene for this protein is compared, A and B differ at 6 nucleotides. Explain how the DNA can show more differences than the protein.
Worked solution and suggested Marlbridge rubric
(a) Most closely related: B, with only 1 difference (99.2% identical). Least closely related: E, with 30 differences. The number of differences rises from B to C (9) to D (17) to E (30).
(b) Matching positions = 120 − 30 = 90. Percentage identity = 90 ÷ 120 × 100 = 75%.
(c) After two lineages split, each builds up its own mutations, and some of these change amino acids. The longer ago the split, the more time there has been for differences to accumulate. So fewer differences means a more recent common ancestor.
(d) The genetic code is redundant: most amino acids have more than one codon. A change in DNA, often in the third base of a codon, can be silent, giving the same amino acid. So some DNA differences do not appear in the protein. (Changes outside the coding sequence, such as in introns, are also acceptable.)
| Point | What earns it |
|---|---|
| 1 | B most related and E least, each supported by a value |
| 1 | 75% |
| 1 | Differences accumulate over time after lineages split, so fewer differences means a more recent ancestor |
| 1 | Silent mutations / redundant code (or non-coding DNA) explain extra DNA differences |
Question 5 (constructed response · core)
A cliff at a fictional site shows these layers, listed from the top down. The layers are undisturbed.
| Layer (top to bottom) | Description |
|---|---|
| 1 | siltstone with fossil leaves |
| 2 | volcanic ash, dated at 0.9 million years |
| 3 | sandstone with fossil T, a horse-like tooth |
| 4 | conglomerate with fossil U, a large tortoise shell |
| 5 | volcanic ash, dated at 1.6 million years |
| 6 | shale with fossil V, a fish skeleton |
(a) Put fossils T, U and V in order from oldest to youngest, and state the principle you used. (b) Give the possible age range of fossil U. (c) A student suggests checking the age of fossil T with carbon-14. Explain why this would not work. (d) At a second site 40 km away, a shell of the same tortoise species as fossil U is found in a layer just above an ash bed dated at 1.5 million years. Explain how this geographical data narrows the age of fossil U, and state one assumption.
Model answer and suggested Marlbridge rubric
(a) V (oldest), U, T (youngest). In undisturbed sedimentary rock, lower layers formed first, so they are older (the principle of superposition).
(b) U is between the two ash layers, so it is between 0.9 and 1.6 million years old.
(c) T is at least 0.9 million years old. That is about 160 half-lives of carbon-14 (900,000 ÷ 5,700 ≈ 158), so essentially no carbon-14 would remain. Carbon-14 only works for remains up to about 50,000 years old.
(d) The species was alive at the second site just after 1.5 million years ago. If the species existed for only a short period, fossil U is likely to be close to that age, so near the older end of its 0.9–1.6 million-year range. Assumption (any one): the tortoise species lived for only a short time, so its fossils mark a narrow time band; the layers at the second site are also undisturbed; the shell has not been moved from an older layer.
| Point | What earns it |
|---|---|
| 1 | Correct order with superposition stated |
| 1 | 0.9–1.6 million years |
| 1 | Too many half-lives / beyond about 50,000 years, so too little carbon-14 left |
| 1 | Explains how the matching fossil at the second site narrows the range |
| 1 | One valid assumption |
Question 6 (constructed response · core)
A fictional salamander lives only in dark caves. It has small eyes covered by skin, which cannot form images. Closely related salamanders living in nearby surface streams have fully working eyes.
(a) Explain why the cave salamander’s eyes are described as vestigial. (b) Explain how these eyes provide evidence that cave salamanders and surface salamanders share a common ancestor. (c) A cave cricket in the same caves is also pale and has reduced eyes. A student says this shows the cricket and the salamander are closely related. Evaluate the claim. (d) Suggest one type of molecular evidence that could test which surface species is the cave salamander’s closest relative, and state the result that would support it.
Model answer and suggested Marlbridge rubric
(a) The eyes are reduced versions of a structure that is fully developed and working in related species, and they have lost their main function (forming images).
(b) Building eyes in total darkness would make no sense if the salamander had arisen separately. The eyes are best explained as inherited from an ancestor that lived in light and used them. Their position and basic structure match the eyes of surface salamanders, which is what common ancestry predicts.
(c) The claim is not supported. Pale bodies and reduced eyes in a cricket and a salamander are analogous features: similar conditions in the cave favoured similar traits in two very distantly related groups. Crickets are insects, with compound eyes that are built quite differently from salamander eyes. Similarity of this kind is not evidence of a recent common ancestor.
(d) Compare the DNA sequence of one or more genes (or the amino acid sequence of a protein) between the cave salamander and each surface species. The surface species with the fewest differences is the closest relative.
| Point | What earns it |
|---|---|
| 1 | Vestigial defined: reduced, lost main function, compared with related species |
| 1 | Links the remnant eyes to inheritance from a sighted ancestor |
| 1 | Identifies the cricket and salamander traits as analogous (separate origin, similar environment) |
| 1 | Molecular comparison named with “fewest differences = closest relative” |
Question 7 (data analysis · stretch)
A chain of fictional volcanic islands lies off a mainland coast. Each island has its own lizard population, descended from a mainland lizard species. Scientists dated each island’s oldest volcanic rock and measured the percentage DNA difference between each island’s lizards and the mainland species.
| Island | K | L | M | N |
|---|---|---|---|---|
| Age of oldest rock / million years | 4.0 | 2.5 | 1.2 | 0.4 |
| DNA difference from mainland species / % | 6.2 | 4.1 | 2.0 | 0.7 |
(a) Describe the relationship between island age and DNA difference, using values from the data. (b) Calculate the DNA difference per million years for island K and for island N. (c) Explain how the geological data and the molecular data together support the claim that the island lizards evolved from mainland ancestors after the islands formed. (d) The age of the oldest rock is the maximum time the lizards could have lived on each island. Explain how the true rate of DNA change would compare with your answer in (b) if lizards reached island K long after it formed.
Worked solution and suggested Marlbridge rubric
(a) The older the island, the greater the DNA difference. The difference rises from 0.7% on the youngest island, N (0.4 million years), to 6.2% on the oldest, K (4.0 million years). The points lie close to a straight line through the origin, so the difference increases roughly in proportion to island age.
(b) K: 6.2 ÷ 4.0 = 1.55% per million years. N: 0.7 ÷ 0.4 = 1.75% per million years. (L and M give 1.64 and 1.67, so all four are similar.)
(c) The geological (physical) data give the islands’ ages independently of any biology. The molecular data show that lizards on older islands have diverged further from the mainland species, at a fairly steady rate. Two independent methods agree: each population has been changing for about as long as its island has existed. This fits colonisation from the mainland followed by gradual genetic change.
(d) If the lizards arrived later, the real time available for change was shorter than the rock age. The same 6.2% difference built up in less time, so the true rate would be higher than 1.55% per million years. For example, arrival 2.0 million years ago would give 6.2 ÷ 2.0 = 3.1% per million years. The calculated values are therefore minimum rates.
| Point | What earns it |
|---|---|
| 1 | Positive relationship described with at least two quoted pairs of values |
| 1 | 1.55 and 1.75 (% per million years) |
| 1 | Geological ages and DNA differences are independent lines of evidence |
| 1 | The two agree: longer time on island → more divergence, consistent with descent from mainland ancestors |
| 1 | Later arrival means less time, so the true rate is higher; calculated values are minimums |
How did you do?
- Q2, Q3 or Q5(c) wrong: re-read “Radioactive decay of isotopes” and Worked example 1 in the study guide, and use Figure 2.
- Q5 wrong: re-read “The age of the rock layers” and “Geographical data”, with Figure 1.
- Q1 or Q6 wrong: re-read “Morphological evidence”, especially the contrast between homologous, vestigial and analogous structures.
- Q4 or Q7 wrong: rework Worked example 2 and “Describing data well”.
Then tick off the topic checklist.