Origin and continuation of life. Energy is required to overcome the natural tendancy of large, complex molecules to break down into smaller ones. The energy used to make large biomolecules, for the most part, comes from the sun. The energy source of the sun, under reducing conditions, may have driven the formation of the first macromolecules by literally cooking up the a mixture of smaller molecules. However, the appearance of photoautotropic organisms permitted these cells to use the less energetic sunlight which could pass through the atmosphere of Earth.
Cells are prokaryotic or eukaryotic.
The endosymbiont hypothesis accounts for the evolution of eukaryotes. The endosymbiont hypothesis found some powerful support in the results of an article in the scientific journal Nature . This article showed striking similarities between the genome of Rickettsia prowazekii , which causes a form of typhus, and that of the mitochondrial DNA from some eukaryotes. Here is a link showing the evolutionary relationships between some organisms and mitochondria. The kinds of cells existing today.