Final answer:
The Precambrian occurred before the origin of eukaryotes, with prokaryotic cells emerging first approximately 3.5 billion years ago, while eukaryotic cells originated around 2.1 billion years ago within the Proterozoic Eon of the Precambrian.
Step-by-step explanation:
The correct answer to this question is a. Precambrian. The Precambrian time spanned from the formation of Earth around 4.6 billion years ago to approximately 541 million years ago. During this vast period, the earliest life forms appeared. The fossil record indicates that prokaryotic cells, which are simpler and lack a nucleus, emerged approximately 3.5 billion years ago. It was not until around 2.1 billion years ago that eukaryotic cells, which have a nucleus and other complex structures, came into existence. The oldest fossil clearly related to modern eukaryotes, a red alga, has been dated to be approximately 1.2 billion years old. However, genetic evidence and sophisticated molecular clock analyses suggest that the emergence of eukaryotic cells might be placed at about 2 billion years ago. This sequence of events firmly places the origin of eukaryotes within the Proterozoic Eon of the Precambrian.
Therefore, we can conclude that the Precambrian era, which encompasses the Proterozoic Eon, came first, before the origin of eukaryotic cells. The development from prokaryotic to eukaryotic life marks a significant evolutionary milestone within the Precambrian.