Practice questions · Topic 7.12

Unit 7 · Practice for the Topic 7.12 end-of-topic test

You’ve gone through everything in this topic. The summary video below recaps it all, so you’re ready for the questions.

Watch first: Origins of life on Earth, summed up

Video coming soon

What the oldest rocks say; three dates and the window between two of them; predicting the flask; the RNA world hypothesis and its three assumptions; the molecule that does two jobs.

These are practice questions in the shape of the topic test. Work through them before you take the test; every question tells you what it wanted.

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback gives the reasoning. For the free-response questions, write one short sentence for each step of your reasoning, each on its own line, and make every link clear (so, because, therefore). That is what the exam’s ‘paragraph form’ means for you: linked sentences, not bullet points. The first free-response question walks you through one case one part at a time, and you can open a hint for each part; the second is at the level of the test. When you finish a question, open the scoring guide and mark your own work against it. Every case that opens with ‘Suppose’ is imagined for the question.

Question 1

Suppose a land rock in a quarry face holds a stromatolite.

Which of the following does the stromatolite show about the time when the rock formed?

A. Living cells were there B. The first cell formed then C. Animals lived there D. The rock formed from cooled lava

Question 2

Suppose a land rock about 0.9 billion years old holds red layers of rusted iron.

Which of the following rusted the iron?

A. Free oxygen reaching the iron B. The heat of the rock’s burial C. The weight of the rock layers above D. Living cells growing on the rock

Question 3

Suppose a rock layer is about 600 million years old. A billion is a thousand million.

Which of the following is the layer’s age in billion years?

A. 0.06 billion years B. 0.6 billion years C. 6 billion years D. 60 billion years

Question 4

Biologists place the origin of life in a window between two dates.

Which two facts set the two dates?

A. The date the surface became habitable and the date free oxygen built up B. The date Earth formed and the age of the oldest stromatolites C. The date Earth formed and the date the surface became habitable D. The date the surface became habitable and the age of the oldest stromatolites

Question 5

Suppose a researcher sparks two sealed apparatuses for ten days. One gas mixture holds twice as much methane as the other, and everything else is the same. She then finds how much amino acid the water in each holds.

Which of the following is the null hypothesis for this experiment?

A. More methane in the mixture gives more amino acid B. The amount of methane in the mixture has no effect on the amount of amino acid formed C. The sparks in the apparatus have no effect on the amount of amino acid formed D. Less methane in the mixture gives more amino acid

Question 6

Suppose a sealed flask of methane, ammonia, hydrogen and water vapor, with oxygen left out, is warmed and sparked for a month. Afterwards its water holds amino acids and sugars, and the researchers find the water free of living cells.

Which of the following does this result show?

A. Life’s building blocks can form without living cells B. The first living cell formed in a flask like this one C. Life on Earth began in a sparked sea D. The early sea held living cells

Question 7

Suppose molecule W is a chain of nucleotides. Straight, molecule W carries a fixed sequence of bases. Folded, molecule W speeds up the joining of two amino acids and is exactly as it was afterwards.

Which of the following does molecule W show?

A. A protein can carry a copyable sequence of amino acids B. A DNA molecule can fold up and speed up a reaction C. One molecule can work as an enzyme while it carries a copyable sequence D. An RNA can be copied only when a protein enzyme is present

Question 8

A student writes: “The first RNAs were copied inside warm, shallow pools of water.”

Which assumption of the RNA world hypothesis is this statement, if it is one?

A. (i) RNA copying carried the sequence on B. (ii) Base pairing did the copying C. (iii) The catalysts were RNAs D. Not one of the three assumptions

Question 9

A student says: “DNA could have been the first genetic material, on its own, before any protein existed.”

Which of the following is the problem with the student’s idea?

A. DNA holds no information until a protein reads it B. DNA cannot carry a copyable sequence C. DNA is found only in cells with a nucleus D. Copying DNA needs protein enzymes, which did not yet exist

Question 10

Biologists give a name to evidence read from rocks: their layers, and the fossils and chemical traces in them.

Which term names that kind of evidence?

A. Biochemical evidence B. Geological evidence C. Behavioral evidence D. Experimental evidence

How to tackle the free-response questions. Read the verb first: describe asks what you see or know; explain asks why or how, so name the mechanism; predict asks what will happen and why; justify asks for the evidence that supports a claim. Each point is earned by one idea, stated in a sentence that names the thing and the mechanism. Extra words earn nothing; a wrong extra can lose the point. If there is a figure or table, use what it shows. When you finish, check the box for each point your answer earns and compare your sentences with the full-credit answer.

Free response 1 · Scientific Investigation · 5 points

Suppose researchers hypothesize that heating the water is needed for amino acids to form in the apparatus: the heat turns water into vapor, the vapor carries water up into the sparked gas mixture, and the molecules that form there are carried back down into the water. They build two sealed apparatuses, identical in every part, whose upper flasks hold methane, ammonia, hydrogen and water vapor. They heat the water in one apparatus; the water in the other stays cold. They spark both for twelve days. Then they find how much amino acid the water in each apparatus holds.

(a) Identify what is changed in this experiment. (1 point)

Hint: Which one condition differs between the two apparatuses?

A full-credit answer: What is changed is whether the water is heated: heated in one apparatus, cold in the other.

Check the box for each point your answer earns

whether the water is heated (accept: the heating of the water; the temperature of the water).

(b) Identify what is measured. (1 point)

Hint: What do the researchers find out at the end of the twelve days?

A full-credit answer: What is measured is the amount of amino acid in the water of each apparatus.

Check the box for each point your answer earns

the amount of amino acid in the water.

(c) State the null hypothesis for this experiment. (1 point)

Hint: Which two things does a null hypothesis always name, and what does it claim about them?

A full-credit answer: The null hypothesis is that heating the water has no effect on the amount of amino acid formed in the water.

Check the box for each point your answer earns

a sentence naming the thing changed (heating the water) and the thing measured (the amount of amino acid in the water) and claiming no effect of one on the other. A sentence that predicts a difference, or that names only one of the two, earns nothing.

(d) Predict the result if the researchers’ hypothesis is correct. (1 point)

Hint: The hypothesis says heat is needed. What does that mean for each apparatus?

A full-credit answer: If the researchers’ hypothesis is correct, amino acids form in the heated apparatus and few or none form in the cold apparatus.

Check the box for each point your answer earns

amino acids form in the heated apparatus and none (accept: far less) in the cold apparatus.

(e) Explain why the researchers include the cold apparatus rather than testing the heated apparatus alone. (1 point)

Hint: What could the heated apparatus alone leave unsettled about where its amino acids came from?

A full-credit answer: The researchers include the cold apparatus because it has the tested factor, the heating, left out and everything else the same.
The cold apparatus shows how much amino acid forms without heating.
So any extra amino acid in the heated apparatus can be credited to the heating.

Check the box for each point your answer earns

the cold apparatus is the control — it shows what forms without the heating, so a difference between the two apparatuses can be credited to the heating (accept: it rules out the sparks or the gases alone as the cause).

Free-response score: 0 of 5

Free response 2 · Analyze Data · 4 points

Suppose geologists drill a borehole through undisturbed rock layers and date each layer, as the column shows. Layer 1 is the bottom layer. Layer 1 holds a stromatolite and a red layer of rusted iron. Layer 2 holds a red layer of rusted iron and no fossil. Layer 3 holds a stromatolite. Layer 4 holds fossil shells of sea animals.

A borehole through four dated rock layers, with what each layer holds marked; layer 1 is the bottom layer.

(a) Describe what the red layers of rusted iron in layers 1 and 2 show about the air and sea when those layers formed. (1 point)

A full-credit answer: Iron rusts only when oxygen reaches it.
So free oxygen had built up in the air and sea by the time layers 1 and 2 formed.

Check the box for each point your answer earns

free oxygen had built up in (accept: had reached) the air and sea when those layers formed, because iron rusts only when oxygen reaches it. Stating that free oxygen had built up, with no link to the rusted iron, earns no point.

(b) A student says: “Layer 1 holds the oldest fossil in the borehole, so the first cell formed 2.35 billion years ago.” Evaluate the student’s statement. (1 point)

A full-credit answer: The statement is not supported.
A stromatolite is the fossil of a mat of cells that was already alive, so layer 1 shows that single-celled life existed by 2.35 billion years ago.
The oldest stromatolites on Earth are about 3.5 billion years old, so life existed long before layer 1 formed.
The first cell formed some time before 3.5 billion years ago.

Check the box for each point your answer earns

not supported, because a stromatolite shows that cells were already alive, so its date is a time by which life existed (accept: older stromatolites, about 3.5 billion years old, exist elsewhere). A verdict alone earns no point.

(c) Suppose the geologists drill deeper and reach a land-rock layer about 2.75 billion years old. Free oxygen began to build up in the air and sea about 2.4 billion years ago. Predict what they find when they examine the layer’s iron minerals. (1 point)

A full-credit answer: Its iron minerals are unrusted dark minerals, because 2.75 billion years ago is before free oxygen began to build up, about 2.4 billion years ago.

Check the box for each point your answer earns

unrusted (accept: dark iron minerals; no red layer of rusted iron). Accept with or without the reason.

(d) Explain what these layers leave open about the first cell. (1 point)

A full-credit answer: The stromatolite in layer 1 is the fossil of a mat of cells that was already alive, so it shows only that the first cell had formed by 2.35 billion years ago.
No layer records the first cell forming.
So the layers leave open how the first cell formed, and exactly when.

Check the box for each point your answer earns

how the first cell formed (accept: exactly when it formed — the same class), because a stromatolite is the trace of a mat of cells already alive and no layer records the first cell forming. A bare ‘how it formed’ with no link to what the layers record earns no point.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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