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Milestones in the history of life.
By about 3.8 billion years ago, chemical and molecular evolution had produced the first living cells. They made ATP by anaerobic pathways.
The first major divergence gave rise to bacteria, and to the common ancestor of the archaea and eukaryotes.
A second major divergence set the archaea and the ancestors of eukaryotes on separate evolutionary roads.
Some of the bacteria evolved the ability to carry out photosynthesis by the cyclic pathway.
Later, the oxygen-releasing noncyclic pathway evolved in the cyanobacteria, and oxygen began accumulating in the atmosphere.
This opened the way for the evolution of aerobic respiration in many bacterial groups.
In the ancestors of the eukaryotes, infoldings of the plasma membrane gave rise to a nuclear envelope and other internal membranes.
Mitochondria evolved from aerobic bacteria that were taken up by other cells and became endosymbionts.
Similarly, chloroplasts evolved from prokaryotic photosynthetic autotrophs that became endosymbionts.
By 900 million years ago, all major lineages had evolved. Each has been shaped by different selective pressures and has its own unique characteristics.
