The Earth’s early atmosphere comprises of carbon dioxide, nitrogen, and water vapor, as well as a low amount of other substances, such as carbon monoxide, hydrogen sulfide, and hydrogen cyanide. Moreover, there is only a little amount of oxygen. According to scientists, this composition made the sky orange and the oceans brown.

According to the experiments conducted by Stanley Miller and Harold Urey, the organic compounds, which are responsible for sustaining life on Earth, were formed from the simpler compounds that are present in the Earth’s early atmosphere.

According to the endosymbiotic theory, the prokaryotes and eukaryotes had a symbiotic relationship in the past, wherein the prokaryotes lived inside the eukaryotes. Eventually, the prokaryotes evolved into mitochondria and chloroplasts and they became a part of the modern eukaryotic cell.

According to some scientists, evolution occurs at different rates. In this case, the first living cells evolved gradually as a result of many steps, such as the formation of microspheres, evolution of RNA and DNA and the origin of eukaryotic cells.

In addition, the endosymbiotic theory describes the symbiosis between prokaryotic and ancient eukaryotic cells that resulted in the complex eukaryotic cells that we have today. The prokaryotes that remained in the primitive eukaryotic cells became organelles, particularly the mitochondria and the chloroplast. Eventually, these cells became eukaryotes that reproduce sexually.

The atmosphere of the early Earth consists mainly of carbon dioxide and nitrogen, with little or no oxygen. In this case, the ancient prokaryotes are anaerobic, which evolved in the absence of oxygen. Eventually, when photosynthetic bacteria became common, they released oxygen and changed the Earth’s atmosphere. The new conditions in the Earth’s atmosphere led to the evolution of aerobic organisms.

The similarities between mitochondria, chloroplast, and bacteria support the endosymbiotic theory. The pieces of evidence that support the theory are listed below.

a. The DNA found in mitochondria and chloroplasts matches the DNA of bacteria.
b. The ribosome found in these organelles and the ribosome found in bacteria are alike.
c. The mitochondria, chloroplasts, and bacteria divide by fission.