Origins of Prokaryotes, Eukaryotes, Mitochondria, and Chloroplasts

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Abstract
If current estimates of the antiquity of life on earth are correct, bacteria very much like Clostridium lived more than 3.1 billion yr ago. Bacterial photosynthesis evolved nearly that long ago and it seems reasonable, in view of our composite tree, to attribute the most ancient stromatolites (formed nearly 3 billion yr ago) to early photosynthetic bacteria. Blue-green algae appear to have evolved later. The tree shows that by the time O2-releasing photosynthesis originated in the blue-green algal line, there must have been a great diversity of morphological types, including bacteria that are ancestral to most of the major groups pictured on the composite tree. This time probably corresponded to the great increase in complexity of the fossil record about 2 billion yr ago. Aerobic respiration probably preceded O2-releasing photosynthesis. This may mean that the formation of O2 from H2O in the upper atmosphere was important to evolving prokaryotes prior to 2 billion yr ago. O2-releasing photosynthesis arose later and was largely responsible for the final transition to the present-day O2 level in the atmosphere. Judging from the relative branch lengths on the tree, the mitochondrial invasion occurred during this transition. Finally, perhaps 1.1 billion years ago, several independent symbioses between protoeukaryotes and various blue-green algae gave rise to photosynthetic eukaryotes; some of these developed into modern eukaryote algae, whereas a single line, possibly from an ancestral green alga, appears to have evolved into the higher plants. By combining the information from evolutionary trees based on several types of sequences, a broad outline of early events in the emergence of life that can be refined as new sequence infromation becomes available is developed.