HL 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 unit 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 compartmentalisation 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 can’t be replicated and the first protocells did not fossilise.

• 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, several hundred types of genes. 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.

• A.2.1.9

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

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

• A2.2.12

  Origin of eukaryotic cells by endosymbiosis

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  Evidence suggests that all eukaryotes evolved from a common unicellular ancestor that had a nucleus and reproduced sexually. Mitochondria then evolved by endosymbiosis. In some eukaryotes, chloroplasts subsequently also had an endosymbiotic origin. Evidence should include the presence in mitochondria and chloroplasts of 70S ribosomes, naked circular DNA and the ability to replicate.
  AOS: Students should recognise that the strength of a theory comes from the observations the theory explains and the predictions it supports. A wide range of observations are accounted for by the theory of endosymbiosis.

• A2.2.13

  Cell differentiation as the process for developing specialised tissues in multicellular organisms

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  Students should be aware that the basis for differentiation is different patterns of gene expression often triggered by changes in the environment.

• A2.2.14

  Evolution of multicellularity

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  Students should be aware that multicellularity has evolved repeatedly. Many fungi and eukaryotic algae and all plants and animals are multicellular. Multicellularity has the advantages of allowing larger body size and cell specialisation.

• A1.1.7

  Extraplanetary origin of water on Earth and reasons for its retention

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  The abundance of water over billions of years of Earth’s history has allowed life to evolve. Limit hypotheses for the origin of water on Earth to asteroids and reasons for retention to gravity and temperatures low enough to condense water.

• A1.1.8

  Relationship between the search for extraterrestrial life and the presence of water

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  Include the idea of the “Goldilocks zone”.