A2.1 Origins of cells HL only

1. There was no free oxygen, so no ozone could form to absorb the radiation. — Ozone (O₃) is made from oxygen (O₂) by ultraviolet light in the upper atmosphere. Pre-biotic Earth lacked free oxygen and therefore lacked ozone, so ultraviolet radiation was not absorbed and reached the surface.
2. The methane and carbon dioxide in the atmosphere had destroyed the ozone layer. — A student who pictures the ozone layer as an independent shield that gases can damage picks this. There was no ozone layer to destroy: ozone is formed from oxygen, and pre-biotic Earth had no free oxygen.
3. Free oxygen was used up oxidizing methane before it could form ozone. — A student who assumes oxygen has always been in the air looks for a reason it failed to make ozone. In fact there was no free oxygen at all on pre-biotic Earth; it accumulated only after photosynthetic organisms evolved.
4. The high temperatures broke down ozone as fast as it was produced. — A student who uses the heat of early Earth to explain every early condition picks this. The higher temperatures were a consequence of greenhouse gases; the lack of ozone was a consequence of the lack of oxygen, not of temperature.

Syllabus statement A2.1.1 · Read this in Learn

1. They are cells that are too small to hold the organelles needed for metabolism. — A student who thinks of a virus as a very small cell picks this. A virus is not a cell of any size: it is genetic material in a protein coat with no cytoplasm, ribosomes or membrane-enclosed interior.
2. They are not whole organisms, and only complete organisms are living. — A student who regards only whole organisms as living picks this. Being an organism is not the criterion: a bacterium is a single cell and is alive. Viruses are non-living because they lack metabolism and cannot replicate independently, whatever their size or organization.
3. They carry out no metabolism and can only replicate using a host cell. — A virus has no enzymes for metabolism and cannot grow, respond or maintain itself. New virus particles are made by the host cell's machinery, so a virus is not self-sustaining, which is why it is considered non-living.
4. They have genes and can evolve, but they do not have a nucleus. — A student who counts viruses as living because they have genes and evolve looks for a structural reason and picks the missing nucleus, but bacteria also lack a nucleus and are alive. Viruses are non-living because they have no metabolism and cannot replicate without a host cell.

Syllabus statement A2.1.2 · Read this in Learn

1. Catalysis, self-replication of molecules, self-assembly and the emergence of compartmentalization — These four processes are the requirements named in the IB guide: reactions had to be speeded up, information-carrying molecules had to be copied, structures had to organize themselves, and a membrane-bound compartment had to separate internal chemistry from the outside.
2. A single improbable chance event assembling a complete cell with all of its parts at once — A student who reads 'spontaneous' as 'all at once' picks this. No scientist proposes a complete cell assembling by chance; the origin of cells is explained as a sequence of processes that could each occur spontaneously.
3. Formation of DNA, then transcription and translation, before any membrane had formed — A student who assumes DNA had to come first picks this. The first genetic material is presumed to have been RNA, and compartmentalization is one of the requirements, not something that came only after a full DNA-based system.
4. Continuing spontaneous generation of whole cells from non-living matter in pond water — A student who thinks cells still arise from non-living matter picks this. Cells are currently produced only by division of pre-existing cells; the requirements listed in the guide describe how the first cells could have emerged under conditions that no longer exist.

Syllabus statement A2.1.3 · Read this in Learn

1. Living cells can form from inorganic molecules given an energy source and enough time. — A student who has heard that the experiment 'created life' picks this. No cells, polymers or self-replicating molecules were formed; the products were small carbon compounds such as amino acids.
2. The twenty amino acids used in proteins are the only products that such conditions make. — A student who thinks 'amino acids' means only the twenty protein building blocks picks this. The products were an undirected mixture that included some protein amino acids, amino acids not used in proteins, and other carbon compounds.
3. Amino acids and other carbon compounds can form abiotically under such conditions. — The water in the apparatus came to contain amino acids and other carbon compounds, showing that compounds needed by life can form from inorganic molecules by chemical processes, with lightning-like discharges as the energy source, under conditions that may have existed on pre-biotic Earth.
4. Carbon compounds can form only where living organisms are already present. — A student who holds that organic compounds need a living source picks this, but the apparatus was sterile and contained no organisms. The result shows the opposite: carbon compounds form by chemistry alone.

Syllabus statement A2.1.4 · Read this in Learn

1. It shielded the enclosed molecules from the ultraviolet light that reached the surface. — A student who carries over the GCSE idea that a membrane 'protects the cell' picks this. A thin bilayer is no shield against ultraviolet radiation; the importance of the compartment lies in what it does to the chemistry inside.
2. It allowed the chemistry inside the compartment to become different from that of the outside. — A membrane keeps self-replicating molecules and their products together at higher concentrations and separates them from the surroundings, so a distinct internal chemistry can develop – the reason the IB guide gives for why a membrane-bound compartment was needed.
3. It provided a solid droplet of lipid inside which the first chemical reactions could occur. — A student who imagines fatty acids forming an oil droplet picks this. A vesicle is a hollow sphere of bilayer enclosing water; the reactions occur in the aqueous interior, not in a solid mass of lipid.
4. It allowed a complete cell to assemble in a single step from the molecules dissolved in the water. — A student who thinks the first cell appeared in a single event picks this. Compartmentalization is one of several requirements that combined gradually; enclosing molecules does not make a cell.

Syllabus statement A2.1.5 · Read this in Learn

1. RNA formed as single-stranded copies made from the very first DNA molecules to exist. — A student who assumes DNA is the original genetic molecule and RNA its copy picks this. The hypothesis is the reverse: RNA carried information first and DNA took over storage later.
2. RNA is simpler than DNA, so it is more stable and survived for longer in the early oceans. — A student who equates simplicity with durability picks this. RNA is less stable than DNA; the case for RNA rests on its ability to be replicated and to catalyse reactions, not on stability.
3. RNA can be replicated and is catalytic, so it could serve as both genes and enzymes. — RNA can be copied by complementary base pairing and some RNA molecules, ribozymes, are catalytic. One type of molecule could therefore have acted as both the genetic material and the enzymes of the earliest cells, avoiding the problem of which came first, genes or enzymes.
4. RNA was needed to code for the proteins that acted as the very first enzymes. — A student who believes all enzymes are proteins picks this. The point of the RNA hypothesis is that RNA itself catalysed the first reactions; protein enzymes evolved later, and ribozymes still catalyse peptide bond formation in ribosomes.

Syllabus statement A2.1.6 · Read this in Learn

1. The same genetic code and some shared genes are found in all organisms. — The IB guide names the universal genetic code and shared genes across all organisms as the evidence for LUCA. Both are features that are best explained by inheritance from a single common ancestor.
2. All organisms carry identical DNA sequences in each of their genes. — A student who takes 'universal genetic code' to mean identical DNA picks this. Genes differ widely between organisms; it is the code translating codons into amino acids that is universal.
3. Fossils of LUCA itself have been identified in the oldest rocks on Earth. — A student who thinks of LUCA as a specific first organism that could be found picks this. No fossil has been identified as LUCA; the evidence comes from comparing organisms alive today.
4. DNA extracted from the oldest known fossils matches the genomes of modern organisms. — A student who believes DNA can be recovered from any fossil picks this. DNA does not survive for billions of years; genomic evidence comes from the genomes of living organisms.

Syllabus statement A2.1.7 · Read this in Learn

1. Carbon-14 dating of the carbon left in the fossilized cells themselves — A student who knows carbon dating as 'the' dating method picks this. Carbon-14 decays away within about 50 000 years, so it is useless for rocks billions of years old.
2. Sequencing the DNA preserved in the fossils and applying a molecular clock — A student who believes DNA survives in ancient fossils picks this. No DNA remains after billions of years; molecular clocks use the genomes of living organisms, not fossil DNA.
3. Identifying the fossils as LUCA and reading its age from the family tree — A student who thinks LUCA is a specific fossil organism picks this. No fossil has been identified as LUCA, and a family tree does not supply dates; the age comes from dating the rock.
4. Radiometric dating of the rock using isotopes with very long half-lives — The rock containing the fossils is dated from the decay of long-lived isotopes such as uranium-238, giving an age of about 3.5 billion years and so a minimum age for life on Earth.

Syllabus statement A2.1.8 · Read this in Learn

1. Photosynthetic microfossils in rocks formed in shallow sunlit water, and light-harvesting genes shared by all organisms — A student who assumes the first life was photosynthetic and lived in sunlight picks this. Genes for photosynthesis are not shared by all organisms, and the evidence for LUCA points to a dark, vent environment.
2. Microfossils in ancient seafloor vent precipitates, and conserved genes across all life that fit vent conditions — Iron-oxide tubes and filaments in rocks formed from seafloor hydrothermal vent precipitates, at least 3.77 billion years old, resemble vent bacteria, and genomic analysis shows that genes conserved across bacteria and archaea encode a metabolism suited to hot, anaerobic, hydrogen-rich vent conditions.
3. DNA sequenced from vent microfossils, and its close match to the genomes of bacteria living at vents today — A student who believes DNA can be recovered from any fossil picks this. DNA does not survive for billions of years; the genomic evidence comes from comparing the genomes of living organisms.
4. A fossil of LUCA itself recovered from a vent chimney, together with the age of the surrounding rock — A student who thinks LUCA is a particular organism that could be found picks this. No fossil has been identified as LUCA; the vent microfossils are evidence of early life in that environment, not of LUCA specifically.

Syllabus statement A2.1.9 · Read this in Learn

1. Carbon compounds can only be produced by the metabolism of living cells. — A student who believes organic compounds need a living source picks this. Carbon compounds form by ordinary chemistry given reactants and energy; the pre-biotic conditions supplied both, which is the point of the statement.
2. The atmosphere now contains free oxygen, which oxidizes such compounds as they form. — The pre-biotic atmosphere lacked free oxygen and was rich in methane and carbon dioxide, with ultraviolet light reaching the surface. Today free oxygen oxidizes carbon compounds and the reducing gases are scarce, so these chemical processes no longer occur.
3. Oxygen was present then as now, so something else must have stopped the processes. — A student who assumes oxygen has always been part of the air picks this and looks for some other explanation. In fact the pre-biotic atmosphere had no free oxygen at all; oxygen accumulated only after photosynthetic organisms evolved, and its presence today is precisely why these chemical processes no longer occur.
4. The same reactions occur today, but living organisms consume the products first. — A student who thinks the pre-biotic processes are still running picks this. The IB guide states that these compounds formed by chemical processes that do not now occur; the oxygen-rich atmosphere prevents them, whether or not organisms are present.

Syllabus statement A2.1.1 · Read this in Learn

1. A whole multicellular animal — A student who regards only whole organisms as living picks this. Cells are alive in their own right, and unicellular organisms show that a single cell can sustain all the functions of life.
2. A virus inside a host cell — A student who counts viruses as living because they reproduce picks this. A virus does not sustain itself: the host cell's metabolism and ribosomes make the new particles, so the virus is not a self-sustaining unit.
3. A mitochondrion in a cell — A student who thinks respiring organelles with their own DNA are alive picks this. A mitochondrion depends on the cell for most of its proteins and cannot survive or reproduce outside it, so it is not self-sustaining.
4. A free-living bacterium — A bacterial cell carries out metabolism, growth, response, homeostasis and reproduction using its own enzymes, genetic material and membranes. The cell is the smallest unit of self-sustaining life.

Syllabus statement A2.1.2 · Read this in Learn

1. The claim is correct, because an event that cannot be directly observed or repeated lies outside science. — A student who equates 'testable' with 'directly observable' picks this. Scientific claims must be testable, but testing can be indirect, through predictions checked by simulations, model systems and evidence from living organisms.
2. It is testable indirectly, though hard to test because the exact conditions cannot be replicated. — Claims in science must be testable, and this hypothesis makes testable predictions – that carbon compounds, vesicles and catalytic RNA form under plausible conditions. Scientists struggle with it because the exact pre-biotic conditions cannot be replicated and the first protocells did not fossilize, but difficulty is not untestability.
3. The claim is wrong, because the Miller–Urey experiment has already confirmed how the first cells formed. — A student who believes Miller and Urey produced life picks this. Their experiment provided evidence only that carbon compounds can form abiotically; it did not test the later steps or confirm the origin of cells.
4. The claim is wrong, because the spontaneous formation of cells can still be observed in ponds today. — A student who thinks cells still arise from non-living matter picks this. Cells are currently produced only by division of pre-existing cells; the origin of the first cells cannot be watched happening now.

Syllabus statement A2.1.3 · Read this in Learn

1. The experiment has been disproved, so it provides no evidence about the origin of carbon compounds on early Earth. — A student who treats any flaw in a method as overturning the conclusion picks this. The revised atmosphere limits how well the original mixture modelled early Earth, but experiments with other plausible mixtures also yield amino acids, so the evidence still stands.
2. The composition of the mixture does not matter, because the experiment proved how life began in any case. — A student who believes the experiment produced life picks this. It produced carbon compounds only, and the realism of the conditions matters when judging how relevant those products are to early Earth.
3. The conditions on pre-biotic Earth cannot be known, so no laboratory experiment can tell us anything about them. — A student who thinks an unrepeatable past event cannot be studied scientifically picks this. The exact conditions cannot be replicated, but hypotheses about them make testable predictions, and simulation experiments are one way of testing them.
4. The mixture was a limitation, but the finding that such compounds can form abiotically still stands. — Evaluation weighs limitations against strengths. The gas mixture is a genuine limitation, but the experiment's central result – that amino acids and other carbon compounds form from inorganic molecules given an energy source – has been repeated with revised mixtures, so it remains evidence for the pre-biotic origin of carbon compounds.

Syllabus statement A2.1.4 · Read this in Learn

1. The molecules are amphipathic: hydrophobic tails are excluded from the water while hydrophilic heads stay in contact with it. — A fatty acid has a hydrophilic carboxyl head and a hydrophobic hydrocarbon tail. Water molecules hydrogen-bond to each other and to the heads and exclude the tails, so the molecules arrange themselves into a bilayer with the tails inside, and the bilayer closes into a sphere. No enzymes or energy input from cells are needed.
2. The molecules are entirely hydrophobic, so they separate from the water and pack into a solid droplet of lipid. — A student who pictures fatty acids as oil picks this. The carboxyl group is hydrophilic, so the molecules do not form a solid droplet: they form a bilayer enclosing water, with water on both sides.
3. Strong bonds form between neighbouring hydrocarbon tails, pulling the molecules together into a closed layer. — A student who hears 'hydrophobic interactions' as an attraction between tails picks this. The forces between hydrocarbon chains are weak; the tails end up together because water excludes them, not because they bond strongly.
4. Enzymes present in the water link the fatty acids together into a membrane, in the same way as happens in cells. — A student who thinks membranes can only be built by cells picks this. Vesicle formation is self-assembly: it needs no enzymes, which is why it could have occurred before any cell existed.

Syllabus statement A2.1.5 · Read this in Learn

1. It shows that RNA is more stable than DNA, which is why it survived from the earliest cells. — A student who thinks RNA came first because it is more durable picks this. RNA is less stable than DNA; the ribosome's ribozyme is significant because it shows RNA can catalyse reactions, not because RNA is long-lasting.
2. It shows that rRNA must be a protein, since only proteins can act as catalysts. — A student who holds that all enzymes are proteins picks this. Ribosomal RNA is a nucleic acid; its catalytic activity is precisely what shows that enzymes need not be proteins.
3. It shows that DNA came first, because the rRNA is transcribed from genes in DNA. — A student who reads the order DNA to RNA to protein as an evolutionary order picks this. That rRNA is transcribed from DNA in modern cells says nothing about which molecule came first; the catalytic ability of RNA points to RNA.
4. It shows that RNA can catalyse reactions, as presumed for the earliest cells. — The ribosome's peptidyl transferase is a ribozyme still used by every cell today. Its survival is evidence that RNA can catalyse reactions, supporting the presumption that RNA served as both genetic material and enzymes before proteins took over most catalysis.

Syllabus statement A2.1.6 · Read this in Learn

1. The code is the only one that is chemically possible, so every organism that evolved had to use it. — A student who assumes chemistry fixes the codon assignments picks this. If the code were chemically inevitable, sharing it would not indicate common descent; the minor variant codes in mitochondria show that other assignments are possible.
2. Codon assignments are largely arbitrary, so a shared code is best explained by a single ancestor. — Which codon specifies which amino acid is set by tRNAs and their loading enzymes, not by chemical necessity, and other assignments would work. That all organisms use the same convention is therefore best explained by their descent from a single ancestor, LUCA.
3. It shows that all organisms have the same genes, inherited without change from LUCA. — A student who confuses the code with the genome picks this. A shared codon assignment says nothing about which genes an organism has; genes differ greatly between organisms.
4. It shows that LUCA was the first cell, because the code must have existed from the start. — A student who equates LUCA with the first cell picks this. The shared code shows that all living organisms descend from LUCA; it does not show that no other forms of life existed before or alongside it.

Syllabus statement A2.1.7 · Read this in Learn

1. They were wiped out by an asteroid impact before LUCA had appeared on Earth. — A student who associates extinction only with catastrophes picks this. No catastrophe is required: the guide attributes their extinction to competition from LUCA and its descendants.
2. It cannot be known, because unobserved events on early Earth lie outside science. — A student who thinks an event that nobody observed cannot be studied scientifically picks this. Claims about the past are testable indirectly: the universal code and shared genes show that all living organisms descend from LUCA, and the extinction of other early lineages is inferred, attributed to competition from LUCA and its descendants.
3. They became extinct through competition from LUCA and its descendants. — LUCA and its descendants outcompeted other early forms of life for resources, so those lineages became extinct. This is why every organism alive today traces back to LUCA even though it was not the only or the first form of life.
4. They did not exist, because LUCA was the very first living cell to exist on Earth. — A student who takes LUCA to be the first cell picks this. LUCA is the last common ancestor of living organisms, not the first organism; the guide includes the likelihood that other forms of life evolved and became extinct.

Syllabus statement A2.1.7 · Read this in Learn

1. The dates are compatible: fossils give a minimum age for life, while molecular clock dates carry wide uncertainty. — A fossil shows that cells existed at least 3.5 billion years ago, so life could well be older. A molecular clock date is an estimate with an error range of hundreds of millions of years. Together they indicate that cells arose within the first billion years of Earth's history and that life has been evolving for over three billion years.
2. The molecular clock date must be wrong, because the fossil gives the exact date at which life began on Earth. — A student who treats fossil dates as exact and complete picks this. A fossil gives a minimum age, not the date life began, and neither approach yields an exact figure for events this ancient.
3. The fossil date is unreliable, because carbon-14 dating cannot be used to measure ages of 3.5 billion years. — A student who assumes fossils are dated by carbon-14 picks this. The rocks are dated radiometrically using long-lived isotopes such as uranium-238, which are suited to ages of billions of years.
4. Both dates show that life is a recent arrival, occupying only a small fraction of Earth's history. — A student whose sense of the past begins with the familiar fossils picks this. 3.5 billion out of 4.5 billion years is around 80% of Earth's history; life appeared within the first billion years.

Syllabus statement A2.1.8 · Read this in Learn

1. LUCA lived in sunlit surface water and obtained its energy by photosynthesis. — A student who assumes life must start with photosynthesis picks this. The conserved genes point to chemical energy from hydrogen and carbon dioxide in the absence of light; photosynthesis evolved later in one lineage.
2. LUCA lived in an atmosphere already rich in oxygen, since these genes are still in use today. — A student who assumes oxygen was always present picks this. The genes encode metabolism for life without oxygen, and free oxygen did not accumulate until photosynthetic organisms had evolved.
3. LUCA lived in a hot, anaerobic environment such as a hydrothermal vent on the seafloor. — Genes conserved in both bacteria and archaea were probably present in their common ancestor, so their functions reveal LUCA's way of life: anaerobic, thermophilic and using hydrogen and carbon dioxide, all of which match conditions at hydrothermal vents.
4. LUCA's genome was read directly from DNA that was preserved in the ancient vent fossils. — A student who thinks ancient DNA can be sequenced from any fossil picks this. The analysis compared the genomes of living organisms; DNA does not survive billions of years, and no fossil of LUCA is known.

Syllabus statement A2.1.9 · Read this in Learn