Earth formed between 4.5 and 6 billion years ago. Conditions initially remained inhospitable for the potential development of life. By about 3.0 billion years ago, however, an atmosphere that contained the appropriate blend of nitrogen, oxygen, carbon, and hydrogen allowed life to commence. The formation of proteins and nucleic acids led to the generation of the genetic code, contained in deoxyribonucleic and ribonucleic acids, and the protein machinery to translate the information into a tangible product.

Fossil evidence indicates that one of the first life forms to arise were bacteria. The planetary conditions that were the norm four to six billion years ago were much different from now. Oxygen was scarce, and extremes of factors such as temperature and atmospheric radiation were more common than now. Although the exact origin of bacteria will likely never be known, the present-day bacteria that...