7.12 Origins of Life on Earth

1. AAbout 4.6 bya
   A student who thinks Earth could support life as soon as it formed picks this. Earth formed about 4.6 bya, but its environment was too hostile for life until about 3.9 bya.
2. BAbout 3.5 bya
   A student who takes the age of the earliest fossils as the start of life picks this. The earliest fossils, about 3.5 bya, show that life already existed; the environment could support life from about 3.9 bya.
3. CAbout 3.9 bya Correct
   Earth formed about 4.6 bya, and its environment was too hostile for life until about 3.9 bya.
4. DAbout 2.4 bya
   A student who thinks all life needs oxygen picks the time when O₂ began to rise in the atmosphere. Life existed long before then: the earliest fossils are about 3.5 billion years old.

CED 7.12.A.1.ii · Read this in Fix

1. AIt can store information in its base sequence and can also catalyze reactions. Correct
   RNA's base sequence can carry genetic information and be copied by base pairing, and some RNA molecules (ribozymes) act as catalysts. A molecule with both abilities could have been copied and could have catalyzed reactions before DNA and encoded proteins existed.
2. BIt can carry information, but only after it is copied into DNA, the genetic material.
   A student who thinks only DNA can be genetic material picks this. RNA can store information in its own base sequence; the hypothesis proposes RNA as genetic material before DNA.
3. CIt can catalyze reactions, but the information it carries came from early proteins.
   A student who thinks proteins carried the first genetic information picks this. Proteins are not copied by base pairing; the hypothesis proposes RNA as the earliest carrier of information.
4. DIt arose because the first cells needed a molecule that could carry their information.
   A student who thinks molecules arise because organisms need them picks this. Need does not produce a molecule; RNA is proposed because of its properties.

CED 7.12.A.2 · Read this in Fix

1. ALife arose at about 3.5 bya, as the age of the oldest fossils is when life began.
   A student who thinks the oldest fossil marks the origin of life picks this. Life must have existed by 3.5 bya, but it could have arisen earlier, after about 3.9 bya.
2. BLife arose between about 3.9 and 3.5 bya, as Earth was too hostile before 3.9 bya. Correct
   Life could not have arisen while the environment was too hostile, before about 3.9 bya, and must have existed by about 3.5 bya, the age of the earliest fossils. Together, these dates give the range in which life most likely arose.
3. CLife arose at exactly 3.9 bya, as dates from rocks give the exact time of each event.
   A student who thinks geological dates are exact picks this. The dates are estimates, and the evidence gives a range of dates for the origin of life, not an exact moment.
4. DLife arose soon after 4.6 bya, as Earth could support life from when it formed.
   A student who thinks Earth could support life as soon as it formed picks this. The timeline shows that the environment was too hostile for life until about 3.9 bya.

CED 7.12.A.1.ii · Read this in Fix

1. AThe earliest known organisms lived when the atmosphere held almost no O₂. Correct
   The earliest fossils date to about 3.5 bya, when the graph shows atmospheric O₂ at about 0% of its present level. O₂ began to rise only about 2.4 bya, so the earliest life lived without atmospheric O₂.
2. BLife could not have begun until O₂ started to rise, about 2.4 billion years ago.
   A student who thinks all life needs oxygen picks this. The earliest fossils, at 3.5 bya, are more than a billion years older than the rise in O₂.
3. CAtmospheric O₂ has stayed close to its present level since Earth formed.
   A student who thinks the atmosphere has always been like today's picks this. The graph shows almost no O₂ before about 2.4 bya and present levels only in the last few hundred million years.
4. DO₂ rose as soon as the first organisms appeared, as they released it.
   A student who thinks the first organisms released O₂ from the start picks this. The graph shows no rise in O₂ for more than a billion years after the earliest fossils.

CED 7.12.A.1.i · Read this in Fix

1. ASimple living cells, built from the gases, appear after the week of sparks.
   A student who thinks origin-of-life experiments produce living cells picks this. The hypothesis concerns organic molecules; such experiments produce molecules such as amino acids, not cells.
2. BNo difference in organic molecules is found between the two apparatuses.
   A student who confuses the predicted result with the null hypothesis picks this. 'No difference' is the null hypothesis; if the hypothesis is supported, organic molecules form in the apparatus with sparks.
3. COrganic molecules form only in an apparatus that microbes have contaminated.
   A student who thinks only living things can make organic molecules picks this. In a sealed apparatus with no organisms, organic molecules such as amino acids form from simple molecules when energy is supplied.
4. DAmino acids and other organic molecules form where the sparks were passed. Correct
   If organic molecules can form from simple molecules without living organisms when energy is supplied under early-Earth conditions, the apparatus with sparks should contain organic molecules such as amino acids, and far fewer should form without sparks. Experiments of this kind have produced amino acids.

CED 7.12.A.1 · Read this in Fix

1. AEach round of copying makes a strand with the same base sequence as the strand copied.
   A student who thinks copying makes an identical strand picks this. The model shows that the first copy, 3′-UACGGU-5′, differs from the original; only the second copy matches it.
2. BEach round of copying needs a protein enzyme, as all catalysts in living things are proteins.
   A student who thinks all catalysts are proteins picks this. The model shows no protein, and the hypothesis assumes that genetically encoded proteins were not involved; RNA can act as a catalyst.
3. CThe first copy is complementary to the original, and copying it gives the original sequence. Correct
   Each base in the first copy pairs with the base opposite it in the original (A with U, G with C), so the first copy is complementary. Copying the first copy gives a strand with the original sequence, 5′-AUGCCA-3′, so the information is passed on: genetic continuity by RNA replication.
4. DThe original strand is first copied into DNA, and that DNA then serves as the template.
   A student who thinks only DNA can be copied as genetic material picks this. The model shows RNA copied directly from RNA, with no DNA involved.

CED 7.12.A.2.i · Read this in Fix

1. ANew strand 3′-TAGCCT-5′
   A student who uses the DNA pairing rule A–T for RNA picks this. RNA contains uracil, not thymine, so A pairs with U.
2. BNew strand 3′-UAGCCU-5′ Correct
   In RNA, A pairs with U and G with C, so the bases opposite 5′-AUCGGA-3′ are U, A, G, C, C, U. The new strand is antiparallel, so its 3′ end lies opposite the template's 5′ end: 3′-UAGCCU-5′.
3. CNew strand 5′-AUCGGA-3′
   A student who thinks a copy has the same sequence as its template picks this. A new strand formed by base pairing is complementary to its template, not identical.
4. DNew strand 5′-UAGCCU-3′
   A student who thinks a new strand runs in the same direction as its template picks this. The bases are right, but the strands are antiparallel: the new strand's 3′ end lies opposite the template's 5′ end.

CED 7.12.A.2.ii · Read this in Fix

1. ATube 1 makes RNA identical to the template; tube 2 makes few or no copies.
   A student who thinks copying makes an identical strand picks this. A strand formed by base pairing is complementary to its template, not identical to it.
2. BBoth tubes make RNA of the same sequence, as the ribozyme sets the base order.
   A student who thinks the catalyst carries the sequence information picks this. The ribozyme joins nucleotides, but the order of bases comes from base pairing with a template.
3. CThe two tubes do not differ, as the null hypothesis states for this experiment.
   A student who confuses the predicted result with the null hypothesis picks this. The null hypothesis states that the template has no effect; the prediction, if base pairing is necessary, is a difference between the tubes.
4. DTube 1 makes RNA complementary to the template; tube 2 makes few or no RNA copies. Correct
   If copying depends on base pairing, the template sets the order of bases in the new strand, so tube 1 makes RNA complementary to the template, and tube 2, with no template, cannot copy any sequence.

CED 7.12.A.2.ii · Read this in Fix

1. AThe catalyst is RNA, as only RNase lowered activity significantly. Correct
   RNase treatment reduced activity to about 4%, and its error bar does not overlap the untreated bar. Protease treatment left activity at about 96%, with an error bar that overlaps the untreated bar widely, so no effect was shown. The catalyst is RNA: a ribozyme, with no protein needed.
2. BThe catalyst is a protein, as catalysts in living things are proteins.
   A student who thinks all biological catalysts are proteins picks this. Destroying the proteins had no significant effect, while destroying the RNA removed the activity.
3. CProtease treatment left the activity exactly the same, as the bars overlap.
   A student who thinks overlapping error bars show identical means picks this. Overlap shows that a difference has not been demonstrated, not that the activity was exactly unchanged.
4. DProtease treatment significantly reduced the activity, as its mean is lower.
   A student who treats any difference in means as real picks this. The 4-point difference lies well within the overlapping ±2 SE bars, so it may be due to chance.

Working No test statistic is calculated; the decision rests on the ±2 SE error bars. Untreated: 100, bar 91–109. Protease: 96, bar 85–107. RNase: 4, bar 1–7. Protease vs untreated: the bars overlap widely (91–107), so no significant difference is shown. RNase vs untreated: the bars do not overlap (7 < 91), so the loss of activity is likely real. Destroying RNA removes the activity and destroying protein does not, so the catalyst is RNA, a ribozyme. Overlap does not prove the two means are identical, and the 4-point difference between 96 and 100 has not been shown to be real.

CED 7.12.A.2.iii · Read this in Fix

1. AProteins came first, as the RNA could have evolved only to assist proteins that already existed.
   A student who thinks proteins came before nucleic acids picks this. The observation that RNA catalyzes the making of proteins fits RNA coming first, not later.
2. BCells evolved catalytic ribosomal RNA because they needed a faster way to make the proteins they required.
   A student who thinks features arise because organisms need them picks this. Need does not produce new molecules; the observation is relevant because it shows RNA acting as a catalyst.
3. CThe observation shows that DNA was the earliest genetic material, as ribosomal RNA is copied from it.
   A student who thinks only DNA can be genetic material picks this. That ribosomal RNA is transcribed from DNA in modern cells says nothing about the earliest genetic material; RNA catalysis supports the RNA world hypothesis.
4. DProtein synthesis could have begun in an RNA world, as RNA, not protein, catalyzes its central step. Correct
   The hypothesis assumes that genetically encoded proteins were not involved as catalysts in early life. Finding that RNA, not protein, catalyzes the central step in making proteins is consistent with protein synthesis having begun in an RNA-based world, with RNA catalysis retained in modern ribosomes.

CED 7.12.A.2.iii · Read this in Fix

1. AThe RNA will lose all of its catalytic activity, as every change in its base sequence will destroy its function.
   A student who thinks every mutation destroys function picks this. A single base change may have no effect, reduce activity or occasionally improve it.
2. BThe change may alter the RNA's shape and activity, and it will be passed on, as copies are made from this RNA. Correct
   Because the same RNA is both the genetic material and the catalyst, a change in its base sequence can change how it folds and so its catalytic activity. Because copies are made from it by base pairing, the change is passed on, as genetic continuity by RNA replication requires.
3. CThe change arose because the organism needed a better catalyst, so its activity is likely to be greatly improved.
   A student who thinks changes occur because organisms need them picks this. A copying error is random with respect to need, and its effect may be harmful, neutral or beneficial.
4. DThe change will not be passed on to copies of the RNA, as only changes in DNA can be inherited.
   A student who thinks only DNA can be genetic material picks this. In this organism, RNA is the genetic material, and copies made from the changed RNA by base pairing carry the change.

CED 7.12.A.2.i · Read this in Fix