Origins of Cells

• These slides provide additional background and contextual information beyond that required of the IB content statement. Students are only required to know about the conditions of early Earth and the prebiotic formation of carbon compounds
• Formation of the Earth

  4567 million years ago
• Earth formed around 4.5 billion years ago when gravity pulled swirling gas and dust in to become the third planet from the Sun. During accretion, the proto-Earth was compressed by gravity, causing an increase in temperature. This rise in temperature, along with additional heat supplied by radioactive decay, caused the proto-Earth to become a hot molten mass
• As the proto-Earth cooled, it stratified according to density leading to a layered composition
• Origin of Water on Earth

  4400 million years ago
• Studies of rocks from this time suggest that water may have begun to exist on Earth as early as 4.4 billion year ago. The most favored explanation is that Earth acquired water from extraplanetary objects, meaning object from outside Earth's orbit (A1.1.7*)
• Evidence suggests that the early Earth had frequent volcanic eruptions, meteorites, lighting and high temperatures. High energy UV radiation would have penetrated the Earth's surface. For that reason, the period from 4.5 to 4.0 billion years ago is called the Hadean Eon, after Hades, the hell of the ancient Greeks!
• One of the Earth's oldest dated rocks was discovered in Canada. This sample has been dated at 4.03 billion years old
• Earth's first atmosphere


  • It likely began as a region of escaping hydrogen and helium, because these were the main gases in the dusty, gassy disk around the Sun from which the planets formed
  • Molecules of hydrogen and helium move really fast, especially when warm. They eventually all escaped Earth's gravity and drifted off into space
• Earth's second atmosphere


  • It is generally thought that ammonia (NH3), methane (CH4), carbon dioxide (CO2) and water vapour (H2O) were present sometime after the crust cooled due to volcanic outgassing
  • There was very little oxygen (O2) in the atmosphere because it quickly reacted with other elements
• Summary of Conditions on Early Earth


  • Atmosphere: Reducing atmosphere with higher proportion of reactive gasses such as ammonia (NH3) and hydrogen (H2). Very little oxygen (O2)
  • Temperature: Significantly warmer than today due to heat from asteroid collisions and accretion (coming together, formation) of the planet
  • UV Radiation: No ozone (O3) layer to block radiation from the sun so intense UV radiation reached the surface
  • Volcanic Activity: Volcanic eruptions released gasses (H2O and CO2) into the atmosphere
  • Asteroid Bombardment: Constant bombardment by asteroids as the solar system was forming
• There is no "standard model" of the origin of life and there are challenges in explaining the spontaneous origin of cells (A2.1.3)
• However, most currently accepted models draw at least some elements from the framework laid out in the 1920s by Alexander Oparin and J. B. S. Haldane
• Oparin and Haldane independently proposed nearly identical hypotheses for the steps of how life could have originated on Earth

  1. Formation of simple organic molecules from inorganic compounds (the focus of this slide set)

  2. Assembly of carbon compounds into polymers (B1.1.2)

  3. The formation of a polymer that can self replicate (enabling inheritance and variation, A2.1.6)

  4. Packaging of molecules into compartments with an internal chemistry different from the surroundings (A2.1.5)
• Inorganic molecules


  • Do not have both carbon and hydrogen within the same molecule
  • Usually have a small number of atoms
  • Often associated with non-living matter
• Organic molecules


  • Contain carbon and hydrogen
  • Can be large molecules with many atoms
  • Usually associated with living organism
• Simple organic molecules form from inorganic precursors is the first step in Oparin-Haldane origin of life
• Inorganic precursors


  • H20
  • NH3
  • H2
  • N2
  • CO2
• Organic molecules formed


  • Amino Acid (B1.2.1)
  • Nitrogenous Base (A1.2.4)
  • Fatty Acid (B1.1.10)
  • Monosaccharides (B1.1.4)
• Converting simple molecules into complex organic molecules requires an input of energy (C1.1.3)
• Today, most living things get the energy to synthesize molecules by oxidizing nutrients in cellular respiration (C1.2.4) or directly from the sun by photosynthesis (C1.3.1). But, before there was life, how did organic molecules form from inorganic precursors?
• Oparin and Haldane hypothesized that early Earth provided the conditions necessary for the formation of carbon compounds. These conditions do not exist on Earth today
• Conditions on early Earth


  • The molecules thought to be present in the early Earth atmosphere were chemically reducing in nature, meaning they are reactive gasses that easily transfer electrons electrons when they react
  • In such a reducing atmosphere, energy from volcanic eruptions and lightning can catalyze the creation of certain basic small molecules of life, like amino acids
• Evidence for the origin of carbon compounds was demonstrated in the experiment by Miller and Urey in 1953 (A2.1.4)
• Once formed, small organic molecules may have been able to spontaneously form polymers on early Earth
• Evidence that monomers formed polymers

  • Amino acids can spontaneously link together to form proteins (B1.2.2)
  • Fatty acids can fatty acids could have been concentrated sufficiently to assemble into membranes (A2.1.5)
  • RNA nucleotides can link together when exposed to a catalyst found in clay or in water (A2.1.6)
• Prebiotic formation of carbon compounds


  1. Formation of biological MONOMERS: amino acids, nitrogenous bases, fatty acids and simple sugars (A2.1.4)

  2. Formation of a nucleic acid polymer (likely RNA) that can encode hereditary information, self replicate and catalyze chemical reactions (A2.1.6)

  3. Packaging of molecules into membranes with an internal chemistry different from the surroundings. Fatty acids can spontaneously form spherical bilayers (A2.1.5), perhaps encapsulating the self-replicating molecules

  4. Formation of protocells that carry out metabolic reactions within an enclosed system
• Cells are considered to be the smallest unit of life on Earth (A2.1.2*)
• Chemical Evidence of Life

  3800 million years ago
• There is evidence that life existed on Earth by 3.8 billion years ago
• There are various approaches used to estimate the dates of the major events in the evolution of life on Earth (A2.1.8). Regardless, 3.8 billion years is an immense length of time over which life has been evolving on Earth!
• Fossil Evidence of Life

  3500 million years ago
• The first cells were prokaryotes (A2.2.5)
• Photosynthesis


  3200 million years ago
• One early photosynthesis evolved, it caused enormous changes to the Earth's atmosphere. Photosynthesis releases oxygen to the atmosphere (C1.3.2). During a period called the "Great Oxygenation Event" oxygen concentration in the atmosphere increased from 0% to around 20%
• Bacteria similar to those found in stromatolites were likely the first organisms to perform photosynthesis (B4.2.3)
• Other milestones in the evolution of life on Earth


  • First Land Plants 425 million years ago
  • First Animals 635 million years ago
  • Origin of Chloroplasts (B2.2.5) 1600 million years ago
  • Origin of Eukaryotic Cells (A2.2.6 and A2.2.12) 2400-2000 million years ago
  • Dinosaur Extinction 66 million years ago
  • Humans 0.2 million years ago
• There is a difference between living and the non-living. How are we able to know the difference?
• For centuries, scientists and philosophers have debated the difference between living and non-living matter and proposed hundreds of definitions of life. None have been widely accepted
• Textbooks traditionally describe life with a list consisting of: Physical structures of life such as cells and DNA (A1.2.1), Physiological processes of life like growth and reproduction (A2.2.7)