AHL Content Statements

• A2.1.1

  Conditions on early Earth and the pre-biotic formation of carbon compounds

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  Include the lack of free oxygen and therefore ozone, higher concentrations of carbon dioxide and methane, resulting in higher temperatures and ultraviolet light penetration. The conditions may have caused a variety of carbon compounds to form spontaneously by chemical processes that do not now occur.

• A2.1.2

  Cells as the smallest units of self-sustaining life

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  Discuss the differences between something that is living and something that is non-living. Include reasons that viruses are considered to be non-living.

• A2.1.3

  Challenge of explaining the spontaneous origin of cells

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  Cells are highly complex structures that can currently only be produced by division of pre-existing cells. Students should be aware that catalysis, self-replication of molecules, self-assembly and the emergence of compartmentalization were necessary requirements for the evolution of the first cells.
  NOS: Students should appreciate that claims in science, including hypotheses and theories, must be testable. In some cases, scientists have to struggle with hypotheses that are difficult to test. In this case the exact conditions on pre-biotic Earth cannot be replicated and the first protocells did not fossilize.
  

• A2.1.4

  Evidence for the origin of carbon compounds

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  Evaluate the Miller–Urey experiment.

• A2.1.5

  Spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers

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  Formation of a membrane-bound compartment is needed to allow internal chemistry to become different from that outside the compartment.
  

• A2.1.6

  RNA as a presumed first genetic material

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  RNA can be replicated and has some catalytic activity so it may have acted initially as both the genetic material and the enzymes of the earliest cells. Ribozymes in the ribosome are still used to catalyse peptide bond formation during protein synthesis.
  

• A2.1.7

  Evidence for a last universal common ancestor

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  Include the universal genetic code and shared genes across all organisms. Include the likelihood of other forms of life having evolved but becoming extinct due to competition from the last universal common ancestor (LUCA) and descendants of LUCA.
  

• A2.1.8

  Approaches used to estimate dates of the first living cells and the last universal common ancestor

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  Students should develop an appreciation of the immense length of time over which life has been evolving on Earth.
  

• A2.1.9

  Evidence for the evolution of the last universal common ancestor in the vicinity of hydrothermal vents

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  Include fossilized evidence of life from ancient seafloor hydrothermal vent precipitates and evidence of conserved sequences from genomic analysis.