Life on Earth began in a harsh, oxygen-free environment billions of years ago. From simple molecules, the first cells emerged, leading to a diverse array of organisms. This incredible journey showcases life's adaptability and resilience in the face of changing conditions.

Earth's atmosphere has undergone dramatic changes, from a reducing environment to our current oxygen-rich air. This evolution, driven by biological and geological processes, has shaped our planet's climate and created the conditions necessary for complex life to thrive.

The History and Evolution of Life on Earth

Origins of Earth's biodiversity

• Early Earth conditions
  • Reducing atmosphere composed of methane, ammonia, hydrogen, and water vapor
  • Absence of free oxygen in the atmosphere
  • Frequent volcanic activity and impacts from celestial bodies
• Formation of organic compounds
  • Miller-Urey experiment simulated early Earth conditions in a laboratory setting
  • Produced amino acids and other essential organic molecules necessary for life
• First life forms emerged as prokaryotes
  • Anaerobic, single-celled organisms that thrived without oxygen
  • Earliest evidence of life dates back to 3.5-3.8 billion years ago (stromatolites)
• Photosynthesis led to the rise of oxygen in the atmosphere
  • Cyanobacteria were the first photosynthetic organisms to evolve
  • Oxygen accumulated in the atmosphere as a byproduct of photosynthesis
  • Great Oxygenation Event occurred 2.4-2.1 billion years ago, significantly increasing atmospheric oxygen
• Eukaryotic cells and multicellularity emerged
  • Endosymbiotic theory suggests mitochondria and chloroplasts originated from prokaryotic cells
  • First eukaryotic cells appeared around 2.1 billion years ago (protists)
  • Multicellular life evolved approximately 1.5 billion years ago (algae, fungi)
• Cambrian Explosion marked a rapid diversification of animal life
  • Most modern animal phyla appeared during this period
  • Occurred around 541 million years ago (trilobites, brachiopods)

Chemical Evolution and Early Life

• Abiogenesis: the process by which life arose from non-living matter
• RNA world hypothesis suggests RNA molecules were precursors to current life forms
• Extremophiles demonstrate life's ability to adapt to harsh environments, providing insights into potential extraterrestrial life

Earth's Atmosphere and Climate

Atmospheric evolution through time

• Early atmosphere was composed of reducing gases
• Oxygen buildup resulted from photosynthesis by cyanobacteria
  • Banded Iron Formations (BIFs) provide evidence of oxygen in ancient oceans
  • Oxygenation of the atmosphere occurred gradually over time
• Carboniferous Period (359-299 million years ago) had high oxygen levels
  • Extensive forests contributed to increased oxygen concentrations
  • Formation of coal deposits resulted from the burial of organic matter
• Geological processes affect the atmosphere's composition
  • Volcanic eruptions release gases such as CO2 and SO2 into the atmosphere
  • Weathering of rocks removes CO2 from the atmosphere through chemical reactions
• Plate tectonics influences long-term climate patterns by altering continental positions and ocean currents

Greenhouse effect in climate change

• Greenhouse gases include CO2, methane, and water vapor
  • These gases absorb and re-emit infrared radiation, trapping heat in the atmosphere
• Natural greenhouse effect maintains Earth's habitable temperature range
• Anthropogenic greenhouse effect is caused by human activities
  • Fossil fuel combustion releases CO2 into the atmosphere (coal, oil, natural gas)
  • Deforestation reduces carbon sequestration by removing trees
  • Agriculture contributes through livestock farming (methane) and rice cultivation (methane)
• Consequences of enhanced greenhouse effect include:
  1. Global temperature rise
  2. Sea level rise due to thermal expansion and melting ice (glaciers, ice sheets)
  3. Changes in precipitation patterns (droughts, floods)
  4. Increased frequency and intensity of extreme weather events (hurricanes, heatwaves)
• Carbon cycle plays a crucial role in regulating atmospheric CO2 levels and climate
• Milankovitch cycles influence long-term climate variations through changes in Earth's orbit and axial tilt

Human impacts on atmosphere

• Fossil fuel combustion increases CO2 levels and causes air pollution
  • Particulate matter, nitrogen oxides, and sulfur dioxide are harmful pollutants
• Deforestation reduces carbon sequestration and leads to habitat loss and biodiversity decline
• Agriculture contributes to methane emissions from livestock and nitrous oxide emissions from fertilizer use
• Ozone depletion is caused by chlorofluorocarbons (CFCs) breaking down stratospheric ozone
  • Montreal Protocol is an international agreement to phase out ozone-depleting substances
• Ocean acidification occurs due to increased CO2 absorption by oceans
  • Lowering of ocean pH impacts marine ecosystems, particularly calcifying organisms (corals, mollusks)