HL Only: Fatty-acid vesicles and compartmentalization

• Fatty acids can spontaneously coalesce into spherical bilayer vesicles.

• This spontaneous organization illustrates the importance of self-assembly during early cell evolution.

• A vesicle creates a membrane-bound compartment separated from its surroundings.

• Compartmentalization allows internal chemistry to become different from chemistry outside the compartment.

• Formation of such compartments was therefore a necessary step toward the evolution of the first cells.

Use the diagram to visualize fatty acids contributing to a membrane around a protocell. The key syllabus idea is that a membrane-bound compartment permits internal chemistry to differ from the external environment. Source

HL Only: Evidence for LUCA

• The last universal common ancestor (LUCA) is supported by characteristics shared across all living organisms.

• A universal genetic code provides evidence for common ancestry.

• Shared genes among organisms provide another line of evidence.

• Other forms of life may also have evolved but later become extinct through competition with LUCA and its descendants.

• LUCA therefore represents the common ancestor of surviving life, not necessarily the only early form of life.

HL Only: Checklist: can you do this?

• Can you explain how early-Earth conditions could permit spontaneous formation of carbon compounds?

• Can you distinguish self-sustaining life from non-living entities and explain why viruses are considered non-living?

• Can you explain why catalysis, self-replication, self-assembly and compartmentalization were necessary for the first cells?

• Can you evaluate the Miller–Urey experiment, including its evidential value and principal limitation?

• Can you explain how fatty-acid vesicles provide compartmentalization?

• Can you explain why RNA could function as both genetic material and a catalyst?

• Can you identify evidence for LUCA and explain why other early forms of life may have disappeared?

• Can you explain the evidence linking LUCA to hydrothermal vents and why dates for early life are estimates?

Use the apparatus diagram to visualize how the Miller–Urey experiment investigated pre-biotic formation of carbon compounds. Its results support the plausibility of spontaneous chemical formation, while uncertainty about exact early-Earth conditions limits the conclusions that can be drawn. Source

HL Only: RNA as the presumed first genetic material

• RNA was a plausible first genetic material because it can be replicated and also has catalytic activity.

• RNA may therefore initially have acted as both genetic material and the enzymes of the earliest cells.

• Modern ribozymes provide an example of RNA retaining catalytic activity.

• Ribozymes in the ribosome catalyse peptide bond formation during protein synthesis.

• This dual capacity links heredity and catalysis, two key requirements in hypotheses for early cellular life.

HL Only: Estimating dates for early life and LUCA

• Scientists use evidence-based approaches to estimate dates for both the first living cells and LUCA.

• These dates are estimates of events that occurred extremely far back in Earth's history.

• Students should appreciate the immense length of time over which life has been evolving on Earth.

HL Only: LUCA and hydrothermal vents

• Evidence supports the hypothesis that LUCA evolved in the vicinity of hydrothermal vents.

• One line of evidence is fossilized evidence of life in ancient seafloor hydrothermal vent precipitates.

• A second line is conserved sequences identified through genomic analysis.

• The hypothesis is therefore supported by both geological evidence and molecular evidence.

• These evidence types help investigate early life despite the difficulty of reproducing ancient pre-biotic conditions.

Use the diagram to visualize the hydrothermal-vent environment proposed for early cellular evolution. Pair this setting with the syllabus evidence from ancient vent precipitates and conserved genomic sequences. Source