End-of-topic test: Origins of Life on Earth

Unit 7 · Topic 7.12 end-of-topic test

Suggested time: about 44 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.

Answer every question. For each multiple-choice question, pick one option. When you have answered every question, press Submit the test; the feedback then gives the reasoning for each. 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. Then open the scoring guide and mark your own work against it. Every case that opens with ‘Suppose’ is imagined for the question.

Question 1

Geologists read the fossils, minerals and chemical traces in Earth’s oldest rock layers.

Which of the following can that evidence tell biologists?

A. The way the first cell formed B. The molecule that was the first genetic material C. The time by which single-celled life existed D. Whether the first cells held DNA

Question 2

Five layers of land rock, with the iron minerals in layers 1 to 4 marked; layer 1 is the bottom layer.

The column shows five layers of undisturbed land rock. Layer 1 is the bottom layer. Only layers 1 to 4 hold iron minerals. The dots mark unrusted dark iron minerals. The solid band marks a red layer of rusted iron.

Which layer is the youngest layer that formed before free oxygen had built up in the air and sea?

A. Layer 1 B. Layer 3 C. Layer 4 D. Layer 5

Question 3

Suppose a geologist reports that a mineral grain is about 4,400 million years old. A billion is a thousand million.

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

A. 0.44 billion years B. 4.4 billion years C. 44 billion years D. 440 billion years

Question 4

Two fossil finds placed on the timeline by their ages, in billions of years ago.

The timeline marks two fossil finds by their ages, in billions of years ago. A billion is a thousand million.

Which of the following is the interval between the two finds’ ages, in million years?

A. 1.5 million years B. 150 million years C. 1,500 million years D. 15,000 million years

Question 5

Suppose geologists report four new findings.

Which finding would widen the window in which biologists place the origin of life?

A. New dating puts the oldest stromatolites at about 3.3 billion years old B. A stromatolite about 3.85 billion years old is found C. A second stromatolite about 3.5 billion years old is found D. New dating puts Earth’s formation at about 4.5 billion years ago

Question 6

Suppose researchers hypothesize that sparks make amino acids only when they jump through the gas mixture. They build two sealed apparatuses, identical in every part, and spark both for nine days. In the gas-sparked apparatus the electrodes sit in the upper flask, so the sparks jump through the gas. In the water-sparked apparatus the electrodes sit under the water in the lower flask, so the sparks jump through the water.

If the hypothesis is correct, which of the following is the result the researchers expect?

A. Amino acids form in the water-sparked apparatus and few or none in the gas-sparked apparatus B. Amino acids form in both apparatuses in about equal amounts C. Amino acids form in the gas-sparked apparatus and few or none in the water-sparked apparatus D. Amino acids form in neither apparatus

Question 7

Suppose a researcher wants to find what adding carbon dioxide to the gas mixture does to the amount of amino acid formed. She builds a sealed apparatus whose upper flask holds methane, ammonia, hydrogen, water vapor and carbon dioxide, and sparks it for six days.

Which of the following is the control to compare it with?

A. An identical apparatus, sparked for six days, whose mixture holds carbon dioxide only B. An identical apparatus, never sparked, whose mixture holds the same gases and carbon dioxide C. An identical apparatus, sparked for twelve days, whose mixture holds the same gases and carbon dioxide D. An identical apparatus, sparked for six days, whose mixture holds the same gases with the carbon dioxide left out

Question 8

Suppose amino acids are found in the water of a sparked apparatus after eight days.

Which feature of the design rules out the possibility that living cells from outside made them?

A. The apparatus was sealed B. The sparks jumped through the gas C. The water was heated D. The mixture held no oxygen

Question 9

An origin-of-life apparatus holds methane, ammonia, hydrogen and water vapor in its upper flask. Oxygen is left out of the mixture.

Which of the following is the reason oxygen is left out?

A. Living cells in the flask would use the oxygen up B. Oxygen would put the sparks out C. Oxygen is heavier than the other gases and would sink into the water D. The oldest land rocks show that the early air and sea held almost no oxygen

Question 10

Biologists who offer the RNA world hypothesis say that RNA could have been the earliest genetic material.

Which property of RNA supports that idea?

A. An RNA molecule is copied more accurately than a DNA molecule B. An RNA molecule stores more information in each unit than a DNA molecule C. A folded RNA molecule speeds up a reaction as well as carrying a copyable sequence D. RNA is the genetic material in every cell today

Question 11

Suppose molecule U is an RNA. Folded, molecule U speeds up the splitting of a fat into fatty acids. Straightened out, molecule U leaves the same reaction at its slow rate.

Which of the following explains why the straight molecule leaves the reaction slow?

A. The fold makes the pocket that the fat fits into B. Straightening the molecule changes the order of its bases C. A straight RNA cannot pair with any other molecule D. Straightening the molecule breaks it into nucleotides

Question 12

A student writes: “Each nucleotide joined the growing copy opposite the one base on the old strand that it fits.”

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 13

Suppose four molecules are candidates for the earliest genetic material, and the only molecules present are the four candidates and free nucleotides.

Under the RNA world hypothesis, which candidate could pass its own sequence on by itself, with only free nucleotides to build from?

A. An RNA that, folded, speeds up the joining of nucleotides into a partner strand B. A DNA molecule that stays straight and speeds up no reaction C. A protein that, folded, speeds up the joining of nucleotides into a strand D. An RNA that stays straight and speeds up no reaction

Question 14

In every cell today, the part of a ribosome that joins one amino acid to the next is made of RNA rather than protein.

Which of the following does this fact provide for the RNA world hypothesis?

A. Evidence that the earliest cells held no protein at all B. Proof of the hypothesis C. Evidence against the hypothesis D. Evidence consistent with the hypothesis

Question 15

A student writes: “The first RNAs were copied between 3.9 and 3.5 billion years ago.”

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 16

Suppose a molecule in a cell speeds up a reaction and is exactly as it was afterwards. Tests show that the molecule is a chain of nucleotides.

Which term names such a molecule?

A. A ribosome B. A ribozyme C. A nucleotide D. A protein enzyme

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 · 4 points

Suppose a researcher builds five sealed apparatuses, identical in every part. The upper flask of each holds methane, ammonia, hydrogen and water vapor, with oxygen left out, and a flame heats the water in each for the same 20 days. She sparks four of them for 2, 5, 10 and 20 of those days; the fifth stays unsparked, so only the days of sparking differ. At the end of the 20 days she finds how much amino acid the water in each holds. The results are in the table.

Five sealed apparatuses, each heated for 20 days: the days each was sparked, and the amino acid in its water afterwards.

(a) Based on the data in the table, describe the effect of the number of days of sparking on the amount of amino acid in the water. (1 point)

A full-credit answer: The amount of amino acid in the water rises with the number of days of sparking.
It rises from 1.6 mg after 2 days of sparking to 16.4 mg after 20 days.

Check the box for each point your answer earns

the amount of amino acid in the water rises as the number of days of sparking rises. Accept with or without values from the table.

(b) State the null hypothesis for the experiment. (1 point)

A full-credit answer: The number of days of sparking 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 (the number of days of sparking) 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.

(c) Explain why the researcher includes the apparatus that stays unsparked rather than sparking all five. (1 point)

A full-credit answer: The researcher includes the apparatus that stays unsparked because it has the sparks left out and everything else the same.
Its water held no amino acid after 20 days of heating.
So heating the gases and the water for 20 days, on its own, made no amino acid.
Comparing the sparked apparatuses with it rules out the heat, and the time, as what made the amino acids.
So the amino acids can be credited to the sparks.

Check the box for each point your answer earns

the unsparked apparatus shows what forms with the heat, the gases and 20 days but no sparks (its water held no amino acid), so the comparison rules out the heat (accept: the time; the gas mixture on its own) as what made the amino acids and credits them to the sparks. Calling it the control, with no comparison stated, earns no point.

(d) A student says: “The sparked apparatuses held amino acids, so this experiment shows that the first living cells formed in a sparked sea.” Evaluate the student’s statement. (1 point)

A full-credit answer: The statement is not supported.
The sparked apparatuses held amino acids, which are building blocks of proteins.
No apparatus held a living cell.
So the experiment shows that life’s building blocks can form without living cells.
The experiment does not show that the first cells formed this way.

Check the box for each point your answer earns

not supported, because no living cell formed — the result shows that life’s building blocks can form without living cells, not that life did begin this way. A verdict alone earns no point.

Free-response score: 0 of 4

Free response 2 · Analyze Data · 4 points

Suppose geologists find a stromatolite in a rock layer about 1.4 billion years old and add it to the timeline of Earth’s early history. The timeline marks the new find and three dates already known.

Three dates from Earth’s early history and a new find, in billions of years ago.

(a) Using the timeline, describe what the oldest stromatolites show about when life existed on Earth. (1 point)

A full-credit answer: The oldest stromatolites on the timeline are about 3.5 billion years old.
A stromatolite is a layered rock built by a mat of living single-celled organisms.
So living cells existed when those rocks formed, 3.5 billion years ago.

Check the box for each point your answer earns

the oldest stromatolites (3.5 billion years old) were built by mats of living cells, so cells were alive by then. Naming the date alone earns no point.

(b) Explain why this find leaves the window for the origin of life unchanged. (1 point)

A full-credit answer: The latest possible date for the origin of life rests on the oldest fossil of living cells, the 3.5-billion-year-old stromatolites.
The new stromatolite shows that single-celled life existed by 1.4 billion years ago.
Life by 1.4 billion years ago was already shown by the older stromatolites.
So neither date of the window moves.

Check the box for each point your answer earns

the latest possible date rests on the oldest fossil (3.5 billion years); a younger fossil shows life at a time when life was already known to exist, so it moves neither date of the window.

(c) New dating then places the date the surface became habitable at about 3,650 million years ago. A billion is a thousand million. Predict how the window for the origin of life changes. (1 point)

A full-credit answer: The window narrows.
The window runs from 3.65 to 3.5 billion years ago instead of from 3.9 to 3.5 billion years ago.

Check the box for each point your answer earns

the window narrows (accept: its earliest possible date moves later, to 3.65 billion years ago, so the window is 3.65 to 3.5 billion years ago).

(d) Justify your prediction in part (c). (1 point)

A full-credit answer: The prediction in part (c) holds because no cell could have formed before the surface was habitable.
So the earliest possible date for the origin of life moves later, from 3.9 to 3.65 billion years ago.
The oldest stromatolites still set the latest possible date at 3.5 billion years ago.
So the stretch between the two dates shrinks: the window narrows.

Check the box for each point your answer earns

reasoning consistent with the student’s own prediction in part (c): the habitable date is the earliest possible date for the origin of life (no cell before a habitable surface), so moving it later moves the window’s earlier edge later while the oldest stromatolites still set the later edge.

Free-response score: 0 of 4

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Multiple choice checked: 0 of 16 correct.

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