For most of human history, only two celestial bodies were visible as anything more than a glowing speck of light: the Sun, which is too bright to look at, and the Moon, whose features were so distinctive and tantalizingly beyond explanation that it became in itself a symbol of mystery. We now know that the famously pockmarked Moon has five billion years of history written on its face, as this video from NASA's Goddard Space Center shows:
But figuring out exactly where the Moon came from is a little bit trickier. Some say it originated elsewhere because gravity trapped the Moon in its orbit, which still leaves unanswered the question of what the Moon was doing before it got trapped in Earth's orbit. Other theories suggest that the Moon was ejected from the Earth. But the most popular theory—known as the giant impact hypothesis—states that the Moon originated after something collided with Earth.
A team of French geologists has added weight to this theory: by applying mathematical models to the geophysical formation of Earth, they have shown that the collision of a planetary-sized object with Earth 95 million years or so after the formation of the solar system—a collision that could easily have knocked off a Moon-sized chunk of matter—would help explain Earth's chemical composition. It's not proof, but it's one more piece of evidence in favor of the giant-impact hypothesis.