Which era is the oldest and longest, characterized by the origin of unicellular organisms?

• A. Paleozoic Era
• B. Precambrian Era
• C. Mesozoic Era
• D. Cenozoic Era

Answer: (B)

Life began with unicellular organisms during the Precambrian, which is the oldest and longest stretch of Earth’s history. This enormous span runs from the formation of the planet about 4.6 billion years ago up to around 541 million years ago, encompassing most of the time before complex life appeared. In this era, the first life forms were simple single-celled organisms like bacteria and archaea, followed later by simple single-celled eukaryotes. Evidence such as ancient microfossils and stromatolites helps us see these early organisms. After this long interval, life becomes more complex and multicellular, leading into the Paleozoic, Mesozoic, and Cenozoic eras. Those later eras are younger and shorter because they occur after life had already begun and diversified, whereas the Precambrian contains the origin of life itself.