Which of these major events occurred during the precambrian time? a. landmasses were all below water. b. cyanobacteria filled the water and air with oxygen. c. trilobites became extinct. d. people lived on earth.

The beginning of the Precambrian period starts with the formation of Earth about 4.5 billion years ago and ends at the first sign of complex life about 540 million years ago. Though the Precambrian Period is often referred to as a period, it's actually the only supereon, which means that it spans multiple eons.