What is a prokaryote, and when did prokaryotes arise?

A prokaryote is a unicellular organism that belongs to either the domain Bacteria or the domain Archaea. Therefore, unlike a eukaryote, the prokaryote lacks in a nucleus and other organelles. The first organisms that appeared about 3.5 billions years ago were prokaryotes. They developed primitive forms of photosynthesis without oxygen as a waste. The first photosynthesizing prokaryotes appeared about 2.7 billion years ago which led to the changes in Earth atmosphere and consequently organisms dependant on oxygen could arise.