Complex sentence: Some microbes are capable of photosynthesis at concentrations of of a few parts per million and so the last life forms would probably disappear finally due to the rising temperatures and loss of the atmosphere when the sun becomes a red giant some four-billion years from now.
Split into simpler sentences: Some microbes are capable of photosynthesis at concentrations of of a few parts per million. Last life forms would probably disappear only because of rising temperatures and loss of the atmosphere when the sun becomes a red giant some four-billion years from now.