Chapter 26

• Spontaneous generation


  The idea that nonliving material could give rise to living organisms
• Spontaneous generation was proposed by Aristotle and was accepted for almost 2,000 years
• Spontaneous generation was disproven by Louis Pasteur (pasteurization) in 1862
• Since the time spontaneous generation was disproved, there have been several proposals to explain how life started
• None of the hypotheses can actually prove the origin of life
• The "Big Bang" theory

  1. A cosmic explosion occurred billions of years ago

  2. Resulting in the formation of clouds of hydrogen and helium atoms
• Formation of organic molecules

  1. Over a long period, gravitational forces collapsed the clouds of hydrogen and helium

  2. This created carbon, oxygen, and other elements

  3. Organic molecules and monomers could then be formed
• Hypotheses on how life started

  • Reducing atmosphere on early Earth had an oxygen-poor environment, leading to the formation of organic molecules
  • Exogenic delivery of organic molecules came from other planets
  • Deep-sea vents were a source of organic molecules
  • Creation by a supreme being or supernatural attention
• Formation of cells


  1. Stage 1: Formation of nucleotides and amino acids

  2. Stage 2: Formation of RNA, DNA, and proteins

  3. Stage 3: Molecules being enclosed in membranes

  4. Stage 4: Acquisition of living properties
• Stage 1: Nucleotides and amino acids were produced prior to the existence of cells
• Stage 2: Simple organic molecules became polymerized to form nucleic acids and proteins on the surface of clay
• Stage 3: Polymers became enclosed in membranes, forming cell-like structures
• Stage 4: Enclosed membranes acquired living properties
• Once protobionts came into existence and had genetic material, they could mutate and evolve
• Fossils


  Preserved remains of past life on Earth, such as bones, shells, and impressions
• Fossils are formed when organisms get buried quickly and the sediment hardens into rock
• Paleontologists are scientists who study fossils
• The relative ages of fossils can be determined by their locations within the rock layers
• Radiometric dating can be used to estimate the absolute age of fossils
• Human populations are still evolving due to natural selection and environmental factors
• Examples of human evolution

  • Lactose tolerance
  • Blue eye color
  • Red hair color
• Which kind of cells were created first?

  Prokaryotes
• What kind of cells were created second?

  Eukaryotes