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When the Earth was formed about 4,500 million years ago, it was a very different place. The surface was extremely hot, possibly even molten, there were no oceans and little or no atmosphere. It is believed that the early atmosphere was formed from volcanic gasses and was highly toxic. It is not known for sure but one theory suggests that during the first billion years of its existence, the Earth’s atmosphere was mainly carbon dioxide and there would have been little or no oxygen, rather like the atmospheres of Mars and Venus today. There may also have been water vapor which eventually condensed to form the oceans, and small proportions of methane and ammonia.
Somewhere around 3.5 and 2.7 billion years ago, bluish-green microscopic organisms called cyanobacteria (commonly called blue-green algae) appeared in Earth’s oceans. They made oxygen from carbon dioxide, water, and sunlight by photosynthesis. As cyanobacteria created more free oxygen, the amount of oxygen in the atmosphere slowly increased. Earth scientists (geologists) have discovered rocks in Canada that are about 2,500 million years old and contain iron oxide. This is good evidence that there was definitely free oxygen in the atmosphere at that time.
For the last 200 million years, the proportions of different gasses in the atmosphere have been about the same as they are today (about four-fifths nitrogen, about one-fifth oxygen with small proportions of various other gasses, including carbon dioxide, water vapor and noble gasses). Since the industrial revolution began, human activity has started to change the composition of the atmosphere - burning fossil fuels releases carbon dioxide into the air. This comes from carbon that was locked away in rocks hundreds of millions of years ago.
No one knows how life began on Earth but there are several theories. The scientist Charles Darwin suggested that it had arisen in warm ponds, hundreds of millions of years ago. That was in the 19th century and science was not advanced enough to be able to offer any possible mechanism. It is still not possible to prove or disprove theories about how life started on Earth, but we know much more about it now. In the 1950s, two scientists (Miller and Urey) carried out an experiment and managed to produce some simple organic chemicals that could possibly eventually have given rise to life by using a mixture of gasses similar to what they thought were in the early atmosphere. Since then, other scientists have come up with other theories including one that suggests that life developed around hydrothermal vents on the sea floor and that the first food chains and webs were based on chemosynthesis instead of photosynthesis. In your exams, you may be given information or data about these theories and asked to interpret it.