the first photosynthesizers-the earliest ancestors of modern plants. For hundreds of millions of years these proto-plants released enough oxy- gen to transform Earth's atmosphere. Some bacteria "learned" how to both photosynthesize and respire.
But some microbes known as purple bacteria only developed re- spiratory systems. In a low-oxygen environment they might have died out. Atmospheric changes eventually made the dual system redundant. The respirers thrived and probably evolved into...