Looking at the sky, we seen the moon, stars, sun and planets. Their motion have been plotted since Early Babylonia, and gods and godesses have been associated with them.
When, so we are told, men began to think about Astronomy rather than Astrology, and Materiality rather then Mythology in Hellenic Antiquity, did they then begin to suppose that these moving objects in the sphere of heaven may also contain life? If they did what reason did they put forward for that? Or did it happen much later, say in the late Renaissance, after Galileos discovery of the Moons of Jupiter. It certainly must have happened before Jules Verne novel of 1865 - de la Terre a la Lune (from the Earth to the Moon).
(Returning to the scene of mythology - did one then think, say that goddess of the moon, Selene lived on the moon? Or the god of the Sun - Helios lived on the Sun? Probably not - I suppose they were seen as embodiments, or the spirit of the Sun or Moon).
This isn't really metaphysics, as is usually understood, but I couldn't think of anymore appropriate tags. I suppose I see the question of life like us elsewhere, that is not on this planet being a fundamental question of natural philosophy (physikoi) as understood in these terms in Early Philosophy. After all, once it was deduced that the earth was a sphere, as was done in Hellenic Antiquity, and presumably hanging in space; and seeing that the Sun & Moon are also spheres - the natural question would have been are they like Earth. One might assume that the Sun was discounted as being too hot. But that does leave the moon as a possibility.