Intelligent live is common in the Universe

The belief that intelligent life might exist elsewhere in the universe was not subjected to a truly scientific enquiry until recently.  Yet it undoubtedly antedates recorded history …

To consider the Earth as the only populated world in infinite space is as absurd as to assert that in an entire field of millet, only one grain will grow.

— Metrodorus of Chios

In space there are countless constellations, suns and planets; we see only the suns because they give light; the planets remain invisible, for they are small and dark. There are also numberless earths circling around their suns, no worse and no less than this globe of ours. For no reasonable mind can assume that heavenly bodies that 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

The Milky Way is nothing else but a mass of innumerable stars planted together in clusters.

— Galileo Galilei

Our sun is one of a 100 billion stars in our galaxy. Our galaxy is one of billions of galaxies populating the universe. It would be the height of presumption to think that we are the only living thing in that enormous immensity.

— Wernher Von Braun

There are in fact 100 billion galaxies, each of which contain something like a 100 billion stars. Think of how many stars, and planets, and kinds of life there may be in this vast and awesome universe.

— Carl Sagan

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Metrodorus of Chios (4th c. BC)

Metrodorus of Chios was a Greek mathematician and philosopher. His cosmological philosophy was advanced for the ancient world.
It isn’t known exactly when he lived; it may have been as early as the third century AD or as late as the early sixth century, although the former seems more likely.
His cosmological philosophy was advanced for the ancient world. The following quote is attributed to him:

To consider the Earth as the only populated world in infinite space is as absurd as to assert that in an entire field of millet, only one grain will grow.

Search in Google on the quote produces a number of versions:

Popular Science in 1984 gives us this:

The belief that intelligent life might exist elsewhere in the universe … was not subjected to a truly scientific enquiry until recently.  Yet it undoubtedly antedates recorded history and was stated as early as the fourth century B.C.  It was then that Metrodorus of Chios wrote in his book On Nature that

to suppose that earth is the only populated world in infinite space is as absurd as to believe that in an entire field sown with millet, only one grain will grow.

Another version from the Internet Encyclopedia of Science:

To consider the Earth as the only populated world in infinite space is as absurd as to assert that in an entire field of millet, only one grain will grow.

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Epicurus (341–270 BC)

Epicurus was a Greek philosopher who was born on the Ionian island of Samos. Eventually, he moved to Athens where he opened a school of philosophy in a garden that he had bought and laid out for the purpose. He developed further the doctrine of atomism, as expounded by Leucippus and Democritus, including the idea that life exists elsewhere in the universe. In his Letter to Herodotus, he writes: 

There are infinite worlds both like and unlike this world of ours … we must believe that in all worlds there are living creatures and plants and other things we see in this world…

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Giordano Bruno (1548 – 1600)

Giordano Bruno (1548 – 1600) was an Italian philosopher, priest, cosmologist, and occultist. He is known for his system of mnemonics based on organized knowledge, his ideas on extrasolar planets and extraterrestrial life, and his support of Nicolaus Copernicus’s heliocentric model of the solar system.

He is likely to be the first thinker since antiquity to integrate a metaphysics, physics, psychology and ethics into an original, if unsystematically presented, philosophy, one that aspired to go beyond the reelaborations of Platonism, Aristotelianism or scepticism within a Christian context that had hitherto prevailed. The outcome was a radical alternative to medieval and Renaissance interpretations of human nature, the cosmos and God.

Before Galileo even picked up a telescope, Giordano Bruno made remarkably accurate philosophical observations about the universe and called for scientific investigation:

There are countless suns and countless earths all rotating round their suns in exactly the same way as the seven planets of our system. We see only the suns because they are the largest bodies and are luminous, but their planets remain invisible to us because they are smaller and non-luminous. The countless worlds in the universe are no worse and no less inhabited than our earth. For it is utterly unreasonable to suppose that those teeming worlds which are as magnificent as our own, perhaps more so, and which enjoy the fructifying rays of a sun just as we do, should be uninhabited and should not bear similar or even more perfect inhabitants than our earth. The unnumbered worlds in the universe are all similar in form and rank and subject to the same forces and the same laws. Impart to us the knowledge of the universality of terrestrial laws throughout all worlds and of the similarity of all substances in the cosmos! Destroy the theories that the earth is the centre of the universe! Crush the supernatural powers said to animate the world, along with the so-called crystalline spheres! Open the door through which we can look out into the limitless, unified firmament composed of similar elements and show us that the other worlds float in an ethereal ocean like our own! Make it plain to us that the motions of all the worlds proceed from inner forces and teach us in the light of such attitudes to go forward with surer tread in the investigation and discovery of nature! Take comfort, the time will come when all men will see as I do.

Like Spinoza after him, he emphasized knowledge as a path to freedom and panned the ignorance of superstitious people:

The fools of the world have been those who have established religions, ceremonies, laws, faith, rule of life. The greatest asses of the world are those who, lacking all understanding and instruction, and void of all civil life and custom, rot in perpetual pedantry; those who by the grace of heaven would reform obscure and corrupted faith, salve the cruelties of perverted religion and remove abuse of superstitions, mending the rents in their vesture. It is not they who indulge impious curiosity or who are ever seeking the secrets of nature, and reckoning the courses of the stars. Observe whether they have been busy with the secret causes of things, or if they have condoned the destruction of kingdoms, the dispersion of peoples, fires, blood, ruin or extermination; whether they seek the destruction of the whole world that it may belong to them: in order that the poor soul may be saved, that an edifice may be raised in heaven, that treasure may be laid up in that blessed land, caring naught for fame, profit or glory in this frail and uncertain life, but only for that other most certain and eternal life.

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Wernher Von Braun ( 1912 – 1977)

Wernher von Braun ( 1912 – 1977) was a German aerospace engineer and space architect. He was the leading figure in the development of rocket technology in Germany and the father of rocket technology and space science in the United States.

 

Our sun is one of a 100 billion stars in our galaxy. Our galaxy is one of billions of galaxies populating the universe. It would be the height of presumption to think that we are the only living thing in that enormous immensity.

 

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Carl Sagan ( 1934 -1996)

Carl Edward Sagan ( 1934 -1996) was an American astronomer, cosmologist, astrophysicist, astrobiologist, author, science popularizer, and science communicator in astronomy and other natural sciences. He is best known for his work as a science popularizer and communicator.

There are in fact 100 billion galaxies, each of which contain something like a 100 billion stars. Think of how many stars, and planets, and kinds of life there may be in this vast and awesome universe.

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We live on a hunk of rock and metal that circles a humdrum star that is one of 400 billion other stars that make up the Milky Way Galaxy which is one of billions of other galaxies which make up a universe which may be one of a very large number, perhaps an infinite number, of other universes. That is a perspective on human life and our culture that is well worth pondering.

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National boundaries are not evident when we view the Earth from space. Fanatical ethnic or religious or national chauvinisms are a little difficult to maintain when we see our planet as a fragile blue crescent fading to become an inconspicuous point of light against the bastion and citadel of the stars.

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