c. 400 BCE - Metrodorus of Chios - "It is unnatural in a large field to have only one shaft of wheat and in the infinite universe only one living world."

c. 50 BCE - Lucretius - "Nothing in the universe is unique and alone, and therefor in other regions there must be other Earths inhabited by different tribes of men and breeds of beasts."

Following the Copernican Revolution, which dethroned the Earth from any special central position in the universe, and relegated it to being simply another planet, individuals increasingly speculated about other worlds as possible abodes of life. However, such speculations were not without risk! On February 17, 1600, the Italian monk Giordano Bruno was burned at the stake by the Inquisition for exposing the Copernican theory and the idea that there must be "an infinite number of suns with planets and life around them".

c. 1609 - Christian Huygens (in his work "Cosmotheoras") argued for the similarity of other worlds to Earth and their life forms.

c. 1900 - Percival Lowell claimed to have observational evidence for large-scale engineering structures on Mars (the "canals") and even described what he thought their physiology and social structure must be like!

Running parallel with these speculations were actual suggestions of methods to communicate with extraterrestrials.

c. 1820 Karl Gauss suggested that we could send a message to possible extraterrestrials by planting as huge forest of trees (in a barren area) in the form of a right triangle, thereby demonstrating our knowledge of the Pythagorean Theorem to the ETs. He thought such a symbol might be visible from Mars using a telescope.

c. 1840 Joseph von Littrow suggested digging trenches in Egypt in that form, filling them with kerosene, and lighting them, producing a symbol visible at night for very long distances.

1899 - N. Tesla attempts to send a powerful burst of radio noise as a "message" and then listen for a reply.

1922 - G. Marconi, inventor of wireless communication, tries to listen for radio signals from a boat in a remote oceanic location.

Over lunch one day in the 1950s, the physicist Enrico Fermi was discussing the possibility of life in the galaxy, and pose the following probelem. It was believed that, since there was nothing that would, in principle, prevent life from developing elsewhere in the galaxy, it ought to be abundant. Yet, if life were abundant, we should have been visited by them, yet we had not. Where are They? This is commonly referred to as the Fermi Paradox.

In the 1970's, Michael Hartwas interested in this dilemma, which some now refer to as the Fermi-Hart Paradox, and decided to carry out the first series of detailed calculations relevant to whether planets with suitable conditions should be common or rare.