Elizabeth Kolbert in The New Yorker:
This story begins about two billion years ago, when the world, if not young, exactly, was a lot more impressionable. The planet spun faster, so the sun rose every twenty-one hours. The earliest continents were forming—Arctica, for instance, which persists as bits and pieces of Siberia. Most of the globe was given over to oceans, and the oceans teemed with microbes.
Some of these microbes—the group known as cyanobacteria—had mastered a peculiarly powerful form of alchemy. They lived off sunlight, which they converted into sugar. As a waste product, they gave off oxygen. Cyanobacteria were so plentiful, and so good at what they did, that they changed the world. They altered the oceans’ chemistry, and then the atmosphere’s. Formerly in short supply, oxygen became abundant. Anything that couldn’t tolerate it either died off or retreated to some dark, airless corner. One day, another organism—a sort of proto-alga—devoured a cyanobacterium. Instead of being destroyed, as you might expect, the bacterium took up residence, like Jonah in the whale. This accommodation, unlikely as it was, sent life in a new direction. The secret to photosynthesis passed to the alga and all its heirs.
A billion years went by. The planet’s rotation slowed. The continents crashed together to form a supercontinent, Rodinia, then drifted apart again. The alga’s heirs diversified.
One side of the family stuck to the water. Another branch set out to colonize dry land. The first explorers stayed small and low to the ground. (These were probably related to liverworts.) Eventually, they were joined by the ancestors of today’s ferns and mosses. There was so much empty space—and hence available light—that plants, as one botanist has put it, found terrestrial life “irresistible.” They spread out their fronds and began to grow taller. The rise of plants made possible the rise of plant-eating animals. During the Carboniferous period, towering tree ferns and giant club mosses covered the earth, and insects with wingspans of more than two feet flitted through them.