Prokaryotes were the first inhabitants on Earth, appearing 3.5 to 3.8 billion years ago. Prokaryotes are unicellular organisms that lack organelles or internal membrane-bound structures. Therefore, th...Prokaryotes were the first inhabitants on Earth, appearing 3.5 to 3.8 billion years ago. Prokaryotes are unicellular organisms that lack organelles or internal membrane-bound structures. Therefore, they do not have a nucleus. They generally have a single chromosome—a piece of circular, double-stranded DNA located in an area of the cell called the nucleoid. Most prokaryotes have a cell wall outside the plasma membrane.
