Earth is believed to have formed about 5 billion years ago. In the first 500 million years a dense atmosphere
emerged from the vapor and gases that were expelled during degassing of the planet's interior. These gases may
have consisted of hydrogen (H2), water vapor, methane (CH4) , and carbon oxides. Prior to 3.5 billion years ago
the atmosphere probably consisted of carbon dioxide (CO2), carbon monoxide (CO), water (H2O), nitrogen (N2),
The hydrosphere was formed 4 billion years ago from the condensation of water vapor, resulting in oceans
of water in which sedimentation occured.
The most important feature of the ancient environment was the absence of free oxygen. Evidence of such an
anaerobic reducing atmosphere is hidden in early rock formations that contain many elements, such as iron and uranium,
in their reduced states. Elements in this state are not found in the rocks of mid-Precambrian and younger ages,
less than 3 billion years old.
One billion years ago, early aquatic organisms called blue-green algae began using energy from the Sun to
split molecules of H2O and CO2 and recombine them into organic compounds and molecular oxygen (O2).
This solar energy conversion process is known as photosynthesis. Some of the photosynthetically created oxygen combined
with organic carbon to recreate CO2 molecules. The remaining oxygen accumulated in the atmosphere, touching off a massive
ecological disaster with respect to early existing anaerobic organisms. As oxygen in the atmosphere increased, CO2 decreased.
High in the atmosphere, some oxygen (O2) molecules absorbed energy from the Sun's ultraviolet (UV) rays
and split to form single oxygen atoms. These atoms combining with remaining oxygen (O2) to form ozone (O3) molecules,
which are very effective at absorbing UV rays. The thin layer of ozone that surrounds Earth acts as a shield, protecting
the planet from irradiation by UV light.
The amount of ozone required to shield Earth from biologically lethal UV radiation, wavelengths
from 200 to 300
nanometers (nm), is believed to have been in existence 600 million years ago. At this time,
the oxygen level was approximately 10% of its present atmospheric concentration.
Prior to this period, life was restricted to the ocean. The presence of ozone enabled
organisms to develop and live on the land. Ozone played a significant role in the evolution
of life on Earth, and allows life as we presently know it to exist.