• In addition to serving as a template for protein assembly, some RNA molecules can act as enzymes. Like proteins, RNA molecules can assume different shapes. These shapes depend on areas of attraction between the RNA nucleotides. For these reasons, the first molecule that held hereditary information may have been RNA rather than DNA.

• The first cells that formed on Earth were probably heterotrophic prokaryotes.

• The first autotrophic cells probably used chemosynthesis to make food. Chemosynthesis produces energy through the oxidation of inorganic substances.

Vocabulary ribozyme (p. 288) archaea (p. 289)

chemosynthesis (p. 289) cyanobacteria (p. 289)

• Most modern autotrophic cells use photosynthesis to make food. An important byproduct of photosynthesis is oxygen.

• Once oxygen began to accumulate on Earth, cells would need to bind oxygen to other compounds in order to prevent damage to cell enzymes. This binding function may have been a first step toward aerobic respiration in cells.

• Eukaryotic cells may have evolved from large prokaryotic cells that engulfed smaller prokaryotic cells. The engulfed prokaryotic cells may have become the ancestors of organelles such as mitochondria and chloroplasts.

ozone (p. 289) endosymbiosis (p. 290)

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