OXFORD UNIVERSITY PRESS, JULY 2014Viewed from outer space, our planet is a sea of blue and green. Dark blue oceans cover two-thirds of its surface; the rest is green—a bit of desert brown, some icy white at the poles, but mostly rich, luscious, magical green. It wasn’t always like this. Time and time again, from fiery beginnings up through the eras and epochs, our emerald planet has reinvented itself. A billion years ago, had we been here to see it, the Earth would have had a very different feel. Not only would the landmasses have been unrecognizable, they would have been various shades of brown and grey. And had we been around 700 million years ago, there would have been little more to see than solid sheets of ice and an equatorial ring of surface water—‘snowball Earth’ scientists call it. Geologically speaking, the greening of our planet took place decidedly late in the day, within the last 500 million years of planetary history.
But what a revolution it was! Mosses, ferns, and shrubs, grasses and cacti, flowering plants galore, hardwoods and softwoods, on and on. These organisms dominate the world we see. And what strange creatures too, these silent, slow-growing, sun-worshiping giants affixed to the surface of the Earth. They absorb nutrients from the soil and sow their seeds any way they can, on the wind, hidden within tasty fruit, or on the bodies of bees and moths. For all their abundance and diversity, the land plants are, genetically speaking, but tiny twigs on the evolutionary tree of eukaryotes. Using DNA sequence analysis their history can be traced back to a common ancestor they share with much smaller, simpler aquatic creatures, unicellular green algae. The green algae are ...
