Prokaryotic Life

Examine the upper line of the geological chart in table 38-1. The span from the left edge to the right edge represents the vast stretch of time from the formation of the Earth’s crust to present day-a span of approximately 4.5 billion years, Just more than seven-eighths of that time comprises the Precambrian era-from 4.5 billion years ago to somewhat fewer than 600 million years ago. Within this era and approximately 3.2 billion years ago, prokaryotic organisms (bacteria and cyanobacteria) came into being. For approximately five-sixths of the time since then, bacteria and the blue-green algae were the only organisms on Earth. The oxygen in the atmosphere was progressively increased by the photosynthetic activity of Cyanophyta (the blue-greens).

Table 38-1 The Geologic Time Chart.