Oxygen in the atmosphere is important for life for two main reasons. First, oxygen makes up
the ozone layer. The ozone layer is in the upper part of the atmosphere, and is made of O 3
molecules — a particular type of oxygen molecule. It blocks harmfulradiationfrom the sun
and keeps it from reaching Earth’s surface. Without an ozone layer, intense radiation from
the sun reached the early Earth’s surface, making life almost impossible. Secondly, oxygen
in the atmosphere is necessary for animals, including humans, to breathe. No animals would
have been able to breathe in Earth’s early atmosphere. However, there were probably several
typesofbacteriathatlivedonEarthduringthisearlytime. Theywouldhavebeenanaerobic,
meaning that they did not need oxygen to live.
Figure 12.14: Bacteria capable of photosynthesis first appeared on Earth about 2.4 billion
years ago. Photosynthesis takes sunlight, carbon dioxide, and water and produces sugar and
oxygen. Photosynthesis contributed oxygen to Earth’s early atmosphere and helped change
it from one rich in carbon dioxide to one rich in oxygen. ( 8 )
Very simple cells lived on Earth for the first few billion years of Earth’s history. Some of the
oldest fossils of more complex organisms are from about 2 billion years ago. They are found