Earth began without a Moon. The young planet, barely formed, had no companion in orbit, and how it came by one is a question that took most of a century, and a set of returned Moon rocks, to answer with any confidence.

The leading explanation is a collision. Around 4.5 billion years ago, the account goes, a body roughly the size of Mars struck the proto-Earth a glancing blow, throwing a great deal of molten and vaporised rock into orbit. That debris settled into a disc, and the disc gathered itself into the Moon. The impactor even has a name, Theia, although no piece of it has ever been held in a hand.

Why the impact won

The giant-impact idea, worked out in the 1970s, became the dominant theory because it accounts for things other explanations could not. It fits the combined spin of the Earth and Moon. It explains why the Moon has only a small iron core and is short on the easily vaporised elements that would have boiled away in a hot collision.

Much of the confidence rests on the rocks. Apollo astronauts brought back 382 kilograms of lunar samples between 1969 and 1972, and half a century of study has tied their chemistry closely to Earth’s. That link is the strongest evidence that the Moon was, in large part, made from our own planet.

The puzzle in the isotopes

The same link is also the theory’s biggest problem. The oxygen in lunar rock comes in very nearly the same mix of isotopes as the oxygen on Earth. Most bodies in the Solar System carry their own distinct isotopic signature, so a near-perfect match between two of them is odd.

In a straightforward impact, the Moon should be built mostly from Theia, and should carry Theia’s signature rather than Earth’s. That it does not has pushed modellers towards more energetic versions of the collision, violent enough to vaporise both bodies into a single hot cloud that mixed thoroughly before the Moon condensed out of it.

The broad story survives, even if the details are still being argued.

What the Moon does for Earth

Whatever its origin, the Moon’s effects on Earth are easier to measure than its birth. The clearest is the tides. The Moon’s gravity, with a smaller contribution from the Sun, drives the twice-daily rise and fall of the seas, and that same pull has gradually slowed Earth’s rotation over geological time.

The grander claim is about the planet’s tilt. Earth spins at an angle of about 23 degrees, which is what gives us seasons, and that angle wanders only slightly. One influential idea holds that the Moon is the reason it stays so steady.

The claim about life, handled with care

In 1993, the astronomer Jacques Laskar and colleagues published calculations in Nature suggesting that without the Moon, Earth’s tilt could swing chaotically, by many tens of degrees, over long stretches of time. Such swings would throw the climate into extremes, and the argument ran that the Moon, by holding the tilt steady, helped keep Earth calm enough for complex life to take hold.

It is a persuasive story, and it may be partly right. It is also not settled. Later modelling, including work led by Jack Lissauer, found that a moonless Earth’s tilt would vary more than it does now, but might stay within a fairly limited range over hundreds of millions of years rather than lurching wildly. On that reading the Moon probably helped, without being the line between a living world and a dead one.

The impact and the tides sit on firm ground. The idea that the Moon was necessary for life does not, and it is fairer to call it a hypothesis under question than a fact.

What to watch

The next round of evidence is coming from the Moon itself. NASA’s Artemis programme aims to return people to the surface, and fresh samples, particularly from regions Apollo never reached, could test the competing versions of the impact and settle details the existing rocks cannot.

The Moon’s precise age and the isotope puzzle both remain open.

For a world we can see on almost any clear night, a surprising amount about where it came from is still being worked out.