Atmospheric Composition & Evolution of Life (HL) (DP IB Environmental Systems & Societies (ESS))

Atmospheric Composition & Evolution of Life

• The composition of the atmosphere has changed significantly over time

  • The evolution of life has been largely responsible for these changes.

• Early life forms on Earth influenced changes in the atmosphere, which impacted the evolution of life

Composition of the pre-biotic atmosphere

• Earth’s early atmosphere was very different from the atmosphere we have today

  • It is sometimes referred to as the pre-biotic (before life) atmosphere

• It is likely to have contained:

  • High levels of carbon dioxide (CO₂) and methane (CH₄)

  • No Oxygen (O₂), or only trace amounts

  • Nitrogen (N₂) was probably present in high concentrations, similar to today

• The early atmosphere had no protective ozone layer

  • This exposed Earth to harmful UV radiation

Changes in atmospheric composition due to evolution of life

Photosynthesis and oxygenation

• Around 2.5 billion years ago, cyanobacteria developed photosynthesis, altering Earth's atmosphere

  • These photosynthetic organisms absorbed CO₂ and released O₂

  • This led to a gradual decrease in CO₂ levels and an increase in O₂ levels

• The rise in O₂ allowed for the development of aerobic life forms that rely on oxygen

Formation of the ozone layer

• Increased oxygen in the atmosphere led to the formation of ozone (O₃) in the stratosphere

  • Ozone absorbs harmful UV radiation, creating a protective layer around Earth

  • This allowed for more complex life forms to evolve on land, protected from UV damage

Oxidation and mineral formation

• The process of iron ore formation occurred in ancient oceans, where photosynthetic organisms released oxygen into the seawater

• Oxygen reacted with dissolved iron, forming iron oxides like hematite and magnetite

• These iron oxides settled on the ocean floor in distinct layers

  • This created what are known as banded iron formations (BIFs)

  • Banded iron formations are significant geological evidence of early oxygenation of the oceans and atmosphere

• This oxidation process changed Earth’s surface and influenced the types of minerals present

• The presence of oxygen allowed for the process of aerobic respiration to evolve

  • This was more energy-efficient than anaerobic processes

  • This enabled the evolution of more complex and energy-demanding organisms

• The protective ozone layer made life on land viable

  • This allowed the spread of diverse terrestrial species