Space.com has an informative piece about the long history of debate over life on other planets, and early attempts to contact possible alien civilisations. It's an area a lot of people are surprised by - many seem to think the idea of life on other planets must be a relatively new concept. In fact, as the article points out, the "plurality of worlds" was a hot topic in the 16th century.
As the Space.com article points out as well, techniques used in the search for ETI have largely been dictated by the technology of the times:
The desire to contact intelligent life on other planets is much older than the UFO craze and the SETI movement. Several 19th century scientists contemplated how we might communicate with possible Martians and Venusians.
These early proposals - which predate by 150 years the first extraterrestrial message that was sent in 1974 - were based on visual signals, as the invention of radio was still decades away. In fact, as history shows, ideas for interplanetary communication have largely been driven by whatever the current technology allowed - be it lamps, radios or lasers.
"You go with what you know," said Steven Dick, NASA Chief Historian.
There's little discussion though of what this means for modern SETI - are we likely to look back in 50 years and think how archaic our thinking on this was, as new technology and means of communication are devised (just as radio was beyond the conceptual boundaries of people 150 years ago)?
Astrobiologist David Grinspoon covers much of this history in his excellent book Lonely Planets: The Natural Philosophy of Alien Life (which, unbelievably, seems to be out of print - though you can pick up a second hand copy cheap through Amazon), with liberal doses of humour. Grinspoon points that early arguments against alien life were often based simply on the Church's belief, through Aristotle, of an Earth-centric universe surrounded by the untouchable heavens. Also:
St. Augustine, widely recognized as one of the greatest thinkers in Christian antiquity, argued that if other worlds were inhabited by humanlike creatures, each would need a Savior, which was impossible because Christ was singular. Several scholars, however, found clever loopholes through which to admit intelligent extraterrestrials into a Christian universe. The most common argument was that other worlds would not need a redeemer because mankind's sin was so original. More specifically, aliens could not be sons of Adam and did not inherit his sin, so they were off the hook.
Aristotle's hold on the Christian imagination began to loosen when some scholars pointed out that a universe with only one world implied limits on the creative powers of God. In 1277, Etienne Tempier, the bishop of Paris, issued a proclamation declaring Aristotle's terrestrial/celestial dichotomy a heresy. This precipitated a sea change in attitudes toward other worlds and alien life. Many Christian scholars began breaking from Aristotle, and numerous treatises were published arguing that God could make as many worlds as he damn well pleased. He is, after all, God.
Was the existence of alien life forbidden by the uniqueness of Christ's incarnation or required by God's omnipotence. In 1440 Nicholas of Cusa, a German ecclesiastic, wrote Of Learned Ignorance, a widely celebrated book that exuberantly rejected Aristotle's hierarchical, Earth-centered cosmology, advocating in its place a universe bustling with life on every star. But Cusa was not scorned by the Church hierarchy for his belief in life elsewhere. On the contrary, after writing Of Learned Ignorance Cusa was made a cardinal. So why did the Church celebrate Cusa, and, 150 years later, condemn Galileo?
There are several reasons. First, Galileo was somewhat of a tactless boor - a quality often left out of the Galileo myth - and his obnoxiousness helped seal his fate.
Grinspoon also mentions Giordano Bruno's role in 'poisoning the waters' prior to Galileo, by "encouraging the Church authorities to associate Copernicanism with flagrant anti-Christian agitation".