The habitat and nature of early life

Earth is over 4,500 million years old. Massive bombardment of the planet to ok place for the first 500-700 million years, and the largest impacts would have been capable of sterilizing the planet. Probably until 4,000 million years ago or later, occasional impacts might have heated the ocean over 100 degreesC. Life on Earth dates from before about 3,800 million years ago, a nd is likely to have gone through one or more hot-ocean 'bottlenecks'. Only hyperthermophiles (organisms optimally living in water at 80-110 degreesC) would have survived. It is possible that early life diversified near hydro thermal vents, but hypotheses that life first occupied other pre-bottleneck habitats are tenable (including transfer from Mars on ejecta from impacts there). Early hyperthermophile life, probably near hydrothermal systems, ma y have been non-photosynthetic, and many housekeeping proteins and biochemi cal processes may have an original hydrothermal heritage. The development o f anoxygenic and then oxygenic photosynthesis would have allowed life to es cape the hydrothermal setting. By about 3,500 million years ago, most of th e principal biochemical pathways that sustain the modern biosphere had evol ved, and were global in scope.