Conditions for life on Earth

• Atmosphere


  Gaseous envelope surrounding a planet or other celestial body
• Insolation


  The amount of solar radiation received by a surface
• Conditions for life on Earth

  • The mass of Earth and force of gravity retained an atmosphere
  • The atmosphere provided gaseous resources: carbon dioxide, methane, nitrogen
  • Atmospheric pressure and temperature maintained liquid water
  • A suitable temperature range was controlled by incoming insolation and its behaviour in the atmosphere, controlled by the surface albedo, absorption of infrared energy and the presence of the atmosphere
• Position in the solar system

  • Suitable temperatures were maintained by the distance from the Sun
• Orbital behaviour


  The rotation and tilt of the Earth on its axis and its orbit around the Sun, controlled daily and seasonal variations in insolation and temperatures
• Magnetosphere


  • The Earth's molten core produced a magnetic field (magnetosphere) that deflects solar radiation
• The Earth was formed about 4.6 billion years ago as gravity pulled rock fragments in space
• Sunlight together. The huge amount of energy absorbed as the rocks joined, created heat and produced a ball of molten rock. The surface gradually cooled to produce a surface crust of solid rock.
• Mass


  The mass of the Earth was great enough to prevent most gases from escaping into space
• The atmosphere included the elements essential for life: carbon, hydrogen, oxygen, and nitrogen. They were present in compounds such as methane, ammonia, and carbon dioxide.
• The atmospheric pressure was high enough to prevent all the liquid water from boiling. Water is vital for living organisms as it is the general physiological solvent in which most biological reactions take place. It is also important in transport and temperature regulation.
• Goldilocks zone


  The light emitted from the Sun and the distance from the Sun, were suitable to produce temperatures on Earth that would be suitable for life. Being too close or too far away from the Sun would prevent liquid water being present.
• The time taken for the Earth to rotate on its axis produced a day/night cycle that was fast enough to minimise excessive heating or cooling.
• Axis of rotation


  The axis of rotation is at an angle to its orbit around the Sun which produces seasonal variations in conditions as the Earth orbits the Sun.
• Zones on Earth


  • Arctic Circle
  • Tropic of Cancer
  • Equal
  • Tropic of Capricom
  • Antarctic Circle
• Speed of rotation


  The 24-hour period of rotation of Earth around its axis reduces temperature extremes.
• Magnetic field


  The molten layers beneath the crust produce the Earth's magnetic field which deflects the 'solar wind' and prevents biologically damaging radiation reaching the Earth's surface.
• Life first developed on Earth about 3.5 billion years ago. The conditions on Earth then were very different from those that exist now. The atmosphere contained some toxic gases, like ammonia, but no oxygen and the solar energy reaching the ground included high levels of ultra-violet radiation.
• How life first developed

  1. The chemical composition of the sea included increasingly complex organic molecules

  2. Simple single-cells formed, possibly around volcanic geothermal vents on the seabed where the warm temperatures and rich mix of chemicals made biological processes more likely

  3. These archaea were single-celled organisms similar to bacteria
• They still survive in many habitats, especially the oceans. Some are anaerobic, such as the methanogenic archaea that live in intestines and marshes.
• Oxygen production


  Oxygen was first produced by photosynthetic bacteria, then by algae and plants.
• Ozone layer


  Ozone was produced by chemical reactions involving oxygen and ultraviolet light in the stratosphere.
• Carbon sequestration


  Atmospheric carbon dioxide concentrations were reduced by photoautotrophs.
• Biogeochemical cycles


  The processes of biogeochemical cycles are linked by living organisms, preventing the build-up of waste products or shortages of resources.
• Key principles


  • The structure and movement of Earth, and its position in the Solar System, control the abiotic conditions on Earth that make life possible
  • The presence of life has changed the conditions on Earth and made it more suitable for life to become more varied and abundant
  • Living systems have responded to environmental changes, such as the increasing intensity of sunlight, to maintain the conditions that allow living organisms to survive
• Liquid water


  All living organisms require water for survival. It performs essential physiological functions and controls many environmental conditions.
• Properties of water


  • Solvent: the 'general physiological solvent. Most chemical reactions in living organisms involve reactants that are dissolved in water
  • Transport within organisms: water is the solvent in blood and sap where it transports dissolved gases, sugars, amino acids, mineral nutrients, waste products, etc.
  • Temperature control: the evaporation of water absorbs heat, causing temperatures to decline
  • Anomalous expansion on freezing: water is most dense at 40 degrees so water that is cooler than this floats stopping the convection current that may have cooled the whole water body
  • High specific heat capacity: water warms up and cools down slowly, which helps to moderate the rate and size of temperature changes
• Aquatic habitats: oceans, seas, lakes, marshes, and rivers.
• Absorption of UV radiation: this protected living organisms before the ozone layer developed and absorbed UV in the stratosphere.
• Temperature range

  Most areas of Earth have temperatures between 0.C and 35-C, so most areas are warm enough to have liquid water but not hot enough to denature proteins.
• Atmospheric gases required for life

  • Carbon dioxide for photosynthesis and the synthesis of carbohydrates, proteins, and lipids
  • Nitrogen for protein synthesis
• Solar insolation


  Sunlight provides the energy for photosynthesis. The heat produced by the absorption of sunlight provides the energy that drives the water cycle and warms the Earth's surface and the oceans.
• How life on Earth caused environmental change


  As life developed and became more abundant, it started to change the environmental conditions which eventually made it possible for new life forms to evolve and new habitats to be colonised.
• Atmospheric oxygen


  By 2.7 billion years ago, some of the Archaea in the oceans had developed the ability to photosynthesise and release oxygen. For millions of years, all the oxygen produced reacted with iron in the oceans. Once all the iron had reacted with oxygen, the surplus dissolved oxygen built up in the oceans. Much of this was released into the atmosphere where concentrations started to rise about 2.45 billion years ago. Oxygen in the atmosphere absorbed ultra-violet light, producing a dynamic equilibrium of reactions involving O, Ox, and O. The absorption of ultra-violet light made life on the Earth's surface possible.
• Carbon sequestration


  Carbon dioxide is a greenhouse gas and helps to retain heat energy in the atmosphere. Photosynthetic organisms absorbed carbon dioxide, some of which was stored in geological sediments such as carbonate rocks and fossil fuels. This reduction in atmospheric carbon dioxide levels helped to prevent a long-term temperature rise as the brightness of the Sun increases by about ten per cent every billion years.
• Biogeochemical cycles


  As a greater variety of organisms evolved, inter-connected biological processes developed which produced blogeochemical cycles. These meant that relatively small amounts of some nutrient elements could support life over long periods of time without the resources becoming depleted.
• Transpiration


  Once plants had evolved and colonised the land, transpiration returned water vapour to the atmosphere and increased the amount of rainfall in other areas, making the growth of even more plant life possible.
• It is important to understand how changes in monitoring methods have allowed more accurate estimation of past conditions on Earth.
• Limitations of early methods

  • Lack of ancient historical data
  • Limited reliability of proxy data for ancient conditions
  • Limited coordination between researchers
  • Lack of sophisticated equipment for accurate measurements
  • Inability to measure many factors
  • Lack of data collection in many areas
• Improved methods


  • Reliance on proxy data, eg dendrochronology, pollen analysis
  • Collection of long-term data sets
  • The use of electronic monitoring equipment
  • Gas analysis of ice cores
  • Isotope analysis of ice cores
  • Improved carriers for monitoring equipment, eg helium balloons, aircraft, satellites