A quick search of my quotes files turned up the following ones
alluding to life on other worlds, the earliest being from the 1st
century B.C.E.
For no reasonable mind can assume that heavenly bodies which may be
far more magnificent than ours would not bear upon them creatures
similar or even superior to those upon our human earth.
Giordano Bruno (1548-1600)
A community of matter seems to exist throughout the visible universe,
for the stars contain many of the elements which exist in the Sun and
Earth. It is remarkable that the elements most widely dif-fused
through the host of stars are some of those most closely connected
with the living organisms of our globe including hydrogen, sodium,
magnesium, and iron. May it not be that, at least, the brighter
stars are like our sun, the upholding and energizing centres of
systems of worlds, adapted to be the abode of living beings?
Sir William Huggins (1865)
I know some will say, we are a little too bold in these Assertions of
the Planets, and that we counted hither by many Probabilities, one of
which, if it chanced to be false, and contrary to our Supposition,
would, like a bad Foundation, ruin the whole Building, and make it
fall to the ground. But...supposing the Earth, as we did, one of the
planets of equal dignity and honor with the rest, who would venture
to say, that nowhere else were to be found any that enjoy'd the
glorious sight of Nature's Opera? Or if there were any Fellow
spectators, yet we were the only one that had dived deep to the
secrets and knowledge of it?
Christiaan Huygens (ca. 1690)
For there is such a huge supply of atoms that all eternity would not
be enough time to count them; there is the force which drives the
atoms into various places just as they have been driven together in
this world. So we must realize that there are other worlds in other
parts of the universe, with races of different men and different
animals.
Lucretius (ca. 1st Century B.C.)
Heaven and earth are very large, yet in the whole of space they are
but a small grain of rice. It is as if the whole of empty space were
a tree, and heaven and earth were one of its fruits. Empty space is
like a kingdom, and heaven and earth no more than a single individual
in that kingdom. Upon one tree there are many fruits, and in one
kingdom many people. How unreasonable it would be to suppose that
besides the heaven and earth which we can see there are no other
heavens and no other earths?
Teng Mu (ca. 13th Century)
God is able to create particles of matter of several sizes and
figures... and perhaps of different densities and forces, and thereby
to vary the laws of Nature, and make worlds of several sorts in
several parts of the Universe. At least, I see nothing of
contradiction in this.