From the Greek proteros, first and zoon, living being. Precambrian geological time period of about two billion years, predating the Phanerozoic and post-Archaic. At the beginning of this period about two and a half billion years ago, (million up, million down because here scientists can not tune much), the first multicellular forms of life appeared and the concentration of oxygen in the atmosphere increased. Single-celled forms began about four billion years ago. And how did life begin? Ah! Good question.
