Oxygen has been essential in the formation of life on Earth. Oxygen has not always been plenty in the early Earth. Low oxygen has delayed evolution billions of years ago.
For a long time in the Earth's history, there was a low level of oxygen. Even when early life was already producing oxygen through photosynthesis, oxygen levels on Earth did not rise. It wasn't until oxygen began to interact with sediments did oxygen level began to rise and support more life on Earth.
This has been the conclusion made on a research by Professor Tim Lenton and Dr. Stuart Daines from the University of Exeter Geography department. Organic material coming from dead living forms began to accumulate in sediments. The sediments were then pushed to the surface through plate tectonics. When that happened, oxygen levels began to rise as a result of the sediments coming into contact with oxygen.
This then created a cycle. As more sediment goes to the surface, it then gets exposed and adds more oxygen. Oxygen then comes into contact with sediment, which repeats the whole process. As this process happened, it also helped in increasing photosynthesis in early plant life. More oxygen then began to be added until such time that oxygen levels increased to its present state.
For a time though oxygen has been trapped at low levels, according to the University of Exeter's site. At that time oxygen levels was around 10 percent of the levels known today. Only when plants began to flourish more did oxygen level began to rise gradually.
The first bacteria is said to have come around 3.8 billion years ago, as Science Daily reports. It wasn't until some 2.7 billion years ago that plants that can make photosynthesis came in the ocean. Life on land came at only 600 million years ago. From there the gradual appearance of more complex life forms evolved.
Life on Earth has been gradual. Low oxygen delayed evolution billions of years ago. Emotional empathy can help in connecting with dogs better.