The First Living Cells were Prokaryotes
Scientists believe that early life most likely developed in warm ponds or near deep-sea hydrothermal vents, which is where the chemicals and energy needed to form the molecules of life were located. The oldest evidence of life has been dated to 3.5 billion years ago inside fossilized stromatolites, rocks containing cells that resemble modern day prokaryotes (see picture to the upper right hand corner) (PALEOBIOLOGY). Protobionts, as described in the previous section, are the pre-cursor to the first "living prokaryotes" without some key adaptions. The main switch from protobionts to prokaryotes occurred as the cells acquired the ability to produce all of their own compounds from only the molecules present in the environment. This is unlike protobionts, which only made some of the compounds they needed. These first prokaryotes were autotrophic, meaning they produced all of their own food using simple inorganic molecules that were not produced by other organisms. Many of these autotrophs used light energy to produce their compounds, and the development of photosynthesis occurred very early in prokaryote history. Even before photosynthesis arose, prokaryotes began using electron transport systems of membrane bound proteins to produce their energy in the form of ATP. Earth at this time had many high energy compounds like methane and and hydrogen sulfide to use as an energy source, and it was only when these resources became scarce that photosynthesis developed. Prokaryotes ruled the earth for over 1.5 billion years, from 3.5 to 2 billion years ago, which was around when the first eukaryotes evolved. Over time, prokaryotes diversified and adapted to many different environments; the current prokaryotes are so numerous that they consist two out of the three domains (archaea and bacteria) (AP BIOLOGY TEXTBOOK).