12. With the increasing oxygen content of the atmosphere, respiration became more important. Oxygen-respiring bacteria evolved from purple nonsulfur photosynthetic bacteria by the loss of photosynthetic ability. This explanation of the origin of respiration would account for the remarkable similarity of the electron-transport chains of photosynthesis and O2. respiration, and their great difference from the processes involved in sulfate respiration in Desulfovibrio. It also would explain the near identity in molecular structure of cytochrome c in respiring eucaryotes and in respiring and photosynthetic bacteria.

13. Eucaryotes developed from procaryotes by a symbiotic relationship between a nonrespiring host, respiring bacteria that were the ancestors of mitochondria, and photosynthesizing blue-green algae that degenerated with time into chloroplasts. This step probably was complete about 1.6 billion years ago, judging from the Beck Springs fossil deposits.

14. In the interval between the development of the first eucaryotes and the beginning of the Cambrian era, plants and animals diverged, soft-bodied multicelled organisms developed, and most of the evolutionary lines arose that later would lead to the major classes of living organisms. We move solidly from chemical evolution and prehistory into the known fossil record.

This is the picture of life on Earth that we have been able to develop so far. Whether life on other planets would have the same chemistry is a question we cannot answer. We would assume it to be carbonbased and water-mediated, but whether nucleic acids are inevitable as genetic records, and proteins as structural materials and catalysts, is more than we can predict. The real understanding of the limits of chemical systems and their organization into living creatures has yet to begin.