How the Earth formed, gained its Moon and its oceans, and gathered the ingredients of life, before there was any. 6 scenes.
A bright young Sun at the centre of a flat disc of dust and rock, with a small, growing Earth embedded in the disc.
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Scene 1 of 6 · About 4.57 billion years ago
A disc of dust and rock
The Sun formed from a collapsing cloud of gas and dust. What was left over flattened into a disc around it, and in that disc grains of dust stuck together into pebbles, pebbles into rocks, and rocks into bodies that kept colliding and growing. Over tens of millions of years, a few of them became the rocky planets, the Earth among them.
We know when from meteorites, leftovers of that disc that still fall to Earth. Some of the elements in them are radioactive and decay at a steady rate, uranium into lead for one. Measuring how much has decayed dates the rock. In 1956 Clair Patterson used lead in meteorites to date the Earth at about 4.55 billion years, and later measurements agree: about 4.54 billion years.
Scene 2 of 6 · Roughly 4.5 billion years ago (debated)
The collision that made the Moon
While the Earth was still growing, it was struck by another young planet, about the size of Mars, which scientists call Theia. The collision threw a great mass of rock into orbit around the Earth, and that rock gathered into the Moon.
The evidence comes largely from the rocks the Apollo astronauts brought back: the Moon's rock is remarkably close to the rock of the Earth's mantle, and the Moon has only a small iron core, as you would expect if it formed from the outer, rocky layers of a planet. When it happened is still debated; estimates run from about 50 to about 200 million years after the first solids of the solar system.
Scene 3 of 6 · After the impact
A molten world
The impact released so much energy that the Earth's surface melted into an ocean of magma, molten rock hundreds or even thousands of kilometres deep. Iron had been sinking toward the centre as the Earth grew, forming its core; the lighter rock stayed above.
The new Moon was far closer than it is today, and it has been drifting away ever since. It still is: lasers bounced off mirrors left on the Moon show it moving away by about 3.8 centimetres a year.
Scene 4 of 6 · By about 4.4 billion years ago
Water and the first crust
As the surface cooled, a crust formed, and water vapour in the air condensed and fell as rain, until there were oceans. How early? The oldest known pieces of the Earth are tiny crystals of zircon from the Jack Hills in Western Australia, about 4.4 billion years old. The oxygen in them suggests they formed from rock that had been in contact with liquid water near the surface, so there may have been oceans within about 150 million years of the Earth's birth.
The air was nothing like ours: mostly carbon dioxide, nitrogen and water vapour, with no free oxygen. Oxygen came much later, made by living things, and part 4 tells how.
Scene 5 of 6 · Through the first few hundred million years
Ingredients from the sky and the sea
Every living thing is built from a few kinds of small molecules: amino acids, which make proteins; sugars; the bases that spell out genes; and fatty molecules that make membranes. Could they form with no life to make them?
In 1953 Stanley Miller passed electric sparks, as from lightning, through a mixture of gases meant to imitate the early air, and within a week the water held amino acids. The early air is now thought to have been less favourable than his mixture, but experiments with other mixtures and energy sources still make some of life's small molecules.
They also fell from the sky. The Murchison meteorite, which landed in Australia in 1969, contains dozens of amino acids that formed in space. One way or another, the young Earth was gathering the ingredients.
Scene 6 of 6 · Before about 3.5 billion years ago
Where chemistry got going
Ingredients are not yet life. Somewhere, small molecules had to join into larger ones, and some of those had to start making copies of themselves. Where that happened is one of the great open questions, and two settings lead:
Hydrothermal vents on the seafloor, where warm water rich in dissolved minerals seeps out of the rock. The difference in chemistry between the vent water and the ocean is a natural source of energy.
Shallow ponds on land, which dry out and fill again. Each drying concentrates whatever is dissolved, and helps small molecules link up.
Nobody knows which, if either, it was, and both are tested in laboratories. What is known is the result: by about 3.5 billion years ago, the Earth had cells. The next part follows how the first ones might have formed.
Clair Patterson (1956). Age of meteorites and the earth. Geochimica et Cosmochimica Acta 10(4): 230–237.
James N. Connelly and others (2012). The absolute chronology and thermal processing of solids in the solar protoplanetary disk. Science 338: 651–655.
Robin M. Canup and Erik Asphaug (2001). Origin of the Moon in a giant impact near the end of the Earth's formation. Nature 412: 708–712.
Simon A. Wilde, John W. Valley, William H. Peck and Colin M. Graham (2001). Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409: 175–178.
John W. Valley and others (2014). Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography. Nature Geoscience 7: 219–223.
Kevin Zahnle, Laura Schaefer and Bruce Fegley (2010). Earth's earliest atmospheres. Cold Spring Harbor Perspectives in Biology 2: a004895.
Stanley L. Miller (1953). A production of amino acids under possible primitive Earth conditions. Science 117: 528–529.
Keith Kvenvolden and others (1970). Evidence for extraterrestrial amino-acids and hydrocarbons in the Murchison meteorite. Nature 228: 923–926.
Bruce Damer and David Deamer (2020). The hot spring hypothesis for an origin of life. Astrobiology 20(4): 429–452.
William Martin, John Baross, Deborah Kelley and Michael J. Russell (2008). Hydrothermal vents and the origin of life. Nature Reviews Microbiology 6: 805–814.
