Current knowledge of the history and origin of life indicates that the first cells (self-replicating genetic systems enclosed in lipid membranes) arose more than 3.8 billion years ago. Although we do not know for certain how these cells came to be, it is clear that the molecular precursors of life were present on the early Earth, and that many of these molecules (like lipids, for example) spontaneously form structures similar to those seen in living cells. The universal nature of the genetic code indicates that these self-replicating genetic systems arose only once in the history of life. The first cells then gave rise to the archaebacteria, eubacteria, and eukaryotes (Figure 17.9).

Where did the molecules that made up the first living cells come from in the absence of life? The atmosphere on the early earth, in the absence of photosynthetic organisms, was composed of hydrogen, nitrogen, and carbon dioxide, and water. Oxygen, an important constituent of Earth's atmosphere today was not present. Under these conditions, and in the presence of heat or electrical discharge (lightening), amino acids, nucleotides and lipids form from gasses present in the atmosphere. It is likely that metabolic pathways were present even before the first cells (see figure 17.2). Remember that even though the actual events that led to the first living cells (figure 17.6) are not known in detail, many of the steps in the process (formation of biological molecules, lipid self-assembly, RNA catalysis) have been demonstrated experimentally.

Once noncyclic (oxygen-releasing) photosynthesis (chapter 5) evolved (about 2.5 billion years ago), oxygen began to accumulate in the atmosphere. Our oxidizing atmosphere is a product of life and requires the continual action of living things to exist. If noncyclic photosynthesis were to cease, the atmosphere would rapidly be depleted of oxygen. The presence of oxygen allowed aerobic metabolism (chapter 6) to evolve.

Eukaryotic organelles (membrane-bound compartments inside the cell) arose either from membrane infolding (nucleus and cytomembrane system) or from endosymbiosis (mitochondria and chloroplasts). Organelles that originated as endosymbionts have their own chromosomes and membranes, which are more similar to those of bacteria than to other DNA and membranes in the eukaryotic cell. These organelles reproduce by binary fission, like bacteria. Taken together, the similarities between endosymbiotic organelles and bacteria indicate that these organelles originated as free-living bacteria that took up residence in early eukaryotic cells. Chloroplasts occasionally retain the ability to live outside the host cell.

The history of life on earth has been characterized by adaptive radiation followed by mass extinctions due to climate change or asteroid impact. Figure 17.22 shows the approximate number of species present during the past three to four billion years. We will see later in the course that we are currently experiencing a human-triggered mass extinction.