The early Earth was a vision of hell, all scalding rock and choking fumes. Since then, its surface has cooled, continents have drifted, mountains have risen and eroded, and life has emerged, benign and green. Nearly all traces of the planet as it was have been wiped away. But from clues in the oldest rocks, deepest magmas, and even the cratered face of the moon, scientists have traced the planet's beginnings. As those early days have come into focus, so have the rare scenes, found today in some of Earth's harshest places, that recall its ancient self.
Its birth pangs began some 4.6 billion years ago as rock and ice particles swirling around the young sun collided and merged, snowballing to produce ever larger planetary building blocks. In violent pileups, they smashed together to create planets, including the infant Earth. In the turmoil, another body, as big as Mars, struck our planet with the energy of trillions of atomic bombs, enough to melt it all the way through. Most of the impactor was swallowed up in the bottomless magma ocean it created. But the collision also flung a small world's worth of vaporized rock into orbit. Debris quickly gathered itself into a ball, and since then Earth history has unfolded beneath the blank stare of the moon.
After the moon's fiery birth, the Earth's surface cooled. Even so, our planet remained an alien world for the next 700 million years; scientists call this time the Hadean, after the Greek underworld. Rafts of solid rock drifted in the magma like dark ice floes. Gases hissed from the cooling rock—carbon dioxide, nitrogen, water vapor, and others—enveloping the planet in a scalding atmosphere devoid of oxygen. As the temperature dropped further, the steam condensed into rain that tell in primordial monsoons and filled the ocean basins.
These first oceans may have been short-lived. Space rubble left over from the birth of the planets—chunks of rock tens to hundreds of miles across—bombarded Earth throughout the Hadean. The greatest impacts might have boiled the oceans away, forcing the process of cooling and condensation to begin again.
By 3.8 billion years ago the impacts relented. Liquid water could persist. About that time, perhaps in the oceans, lifeless chemical reactions crossed a threshold, producing molecules complex enough to reproduce themselves and evolve toward greater complexity. Life was on a road that led, as early as 3.5 billion years ago, to single-celled, blue-green cyanobacteria that flourished in the sunlit parts of the oceans. By the trillions, these microscopic organisms transformed the planet. They captured the energy of the sun to make food, releasing oxygen as a waste product. Little by little they turned the atmosphere into breathable air, opening the way to the diversity of life that followed.
Those days are long gone, but the processes that turned our planet from a hell to a habitable world are still on view today. Primordial heat left over from the planet's formation still bursts out in volcanic eruptions, spilling lava that exudes gases like the young, cooling Earth. In the planet's harshest environments today, cyanobacteria reign as they have for billions of years. And each time a plant gains a toehold on newly cooled lava, the victory of life over lifeless rock—won so long ago on the young Earth—is affirmed again.