Shadows of Forgotten Ancestors

The day is only a few hours long. Gravitational tides raised in the Earth’s oceans and interior by the Moon, and in the Moon’s solid body by the Earth, gradually slow the Earth’s rotation and lengthen the day. From the moment of its formation, the Moon has been drifting away from the Earth. Even now, it hovers over us, a baleful reminder that had the colliding world been much bigger, the Earth would have scattered in fragments through the inner solar system—a short-lived, unlucky world like so many others. Then humans would never have come to be. We would be just one more item on the immense list of unrealized possibilities.

——

Shortly after the Earth had formed, its molten interior was churning, great convection currents circulating, a world in a slow boil. Heavy metal was falling to its center, forming a massive molten core. Motions in the liquid iron began to generate a strong magnetic field.

The time came when the Solar System had pretty well been swept free of gas and dust and rogue worldlets. On Earth, the massive atmosphere—that had kept the heat in—dissipated. Indeed, the collisions themselves helped to drive that atmosphere into space. Convection still carried hot magma up to the surface, but the heat from the molten rock could now be radiated away to space. Slowly the Earth’s surface began to cool. Some of the rock solidified and a thin, at first fragile crust formed, thickened, and hardened. Through blisters and fissures, magma and heat and gases continued to pour out of the interior.

Punctuated by spasmodic flurries of worlds falling out of the sky, the bombardment slowed. Each large impact produced a great dust cloud. There were so many impacts at first that a pall of fine particles enveloped the planet, prevented sunlight from reaching the surface, and in effect turned off the atmospheric greenhouse effect and froze the Earth. There seems to have been a period, after the magma ocean solidified but before the massive bombardment ended, when the once molten Earth became a frozen, battered planet. Who, scanning this desolate world, would have pronounced it fit for life? What wild optimist could have foreseen that peonies and eagles would one day spring from this wasteland?

The original atmosphere had been ejected into space by the relentless rain of worldlets. Now a secondary atmosphere trickled up from the interior and was retained. As the impacts declined, global dust palls became more rare. From the surface of the Earth the Sun would have seemed to be flickering, as in a time-lapse movie. So there was a time when sunlight first broke through the dust pall, when the Sun, Moon, and stars could first be noticed had there been anyone there to see them. There was a first sunrise and a first nightfall.

In sunny intervals, the surface warmed. Outgassed water vapor cooled and condensed; droplets of liquid water formed and trickled down to fill the lowlands and the impact basins. Icebergs continued to fall from the sky, vaporizing on arrival. Torrents of extraterrestrial rain helped form the primeval seas.

Organic molecules are composed of carbon and other atoms. All life on Earth is made from organic molecules. Clearly they had somehow to be synthesized before the origin of life in order for life to arise. Like water, organic molecules came both from down here and from up there. The early atmosphere was energized by ultraviolet light and the wind from the Sun, the flash and crackle of lightning and thunder, auroral electrons, intense early radioactivity, and the shock waves of objects plummeting groundward. When, in the laboratory, such energy sources are introduced into presumptive atmospheres of the primitive Earth, many of the organic building blocks of life are generated, and with astonishing ease.

Life began near the end of the heavy bombardment. This is probably no coincidence The cratered surfaces of the Moon, Mars, and Mercury offer eloquent testimony to how massive and world-altering that battering was. Since the worldlets that have survived to our time—the comets and the asteroids—have sizeable proportions of organic matter, it readily follows that similar worldlets, also rich in organic matter but in much vaster numbers, fell on the Earth 4 billion years ago and may have contributed to the origin of life.

Some of these bodies, and their fragments, burned up entirely as they plunged into the early atmosphere. Others survived unscathed, their cargoes of organic molecules safely delivered to the Earth. Small organic particles drifted down from interplanetary space like a fine sooty snow. We do not know just how much organic matter was delivered to and how much was generated on the early Earth, the ratio of imports to domestic manufactures. But the primitive Earth seems to have been heavily dosed with the stuff of life⁴—including amino acids (the building blocks of proteins), and nucleotide bases and sugars (the building blocks of the nucleic acids).