Whether life existed anywhere, other than an Earth inhabited by ancient philosophers, was a hot topic of discussion at a time when scientists had no concept of the shape of their own planet, nor the scope of the universe.
It was the Atomist tradition of Greek philosophy, which would establish a foundation for the understanding of what could exist outside our own terrestrial boundaries. The school of thought founded by Leucippus and Democritus (460-370BC), practiced a great deal of philosophical assumption with some practical observations as back up. Their theory was that the cosmos was comprised of atoms moving constantly through the void of space. This led to a vision of multiple worlds existing all at once, having suns, moons, expanding in their spaces, declining into collapse, and generally behaving as modern science now shows the universe does. The statement that “there are some worlds devoid of living creatures, or plants or any moisture.” confirmed that they thought other such worlds did exist.
Plutarch (46-120AD) questioned the then current idea that the Moon was an earth-like world, and inhabited. To have 12 summers a year, with the sun vertically overhead every full moon, seemed not to be conducive to habitation. And if it were as hot as Earth summers, how would plant life survive, without the wind, rain, and clouds, which would not likely be able to form in that atmosphere? However, he didn’t eliminate the possibility entirely, since he did admit that even here on Earth, there were also deserts and fruitless areas as well.
It was left to Aristotle (384-322BC) to bring thinking around to a single world of intelligent beings. He divided the basic elements into earth, water, fire, air, and placed above them, the ethereal, or perfect world. Like element called to like, explaining why water and earth existed on the surface of the planet, since they were heavy elements. Air and fire being lighter, rose above them. And above all, was the ethereal or perfect world. What it came down to was, Earth was the only abode of life, since anything of earthly elements was drawn here.
The ongoing argument between proponents of the plurality of worlds, and followers of Aristotle, reached a peak in the 13th century, when the church weighed in with its position that God made all, and made more than man knows of. Pluralists conceded that God could indeed make other worlds like ours, and even unlike ours, and that Aristotle couldn’t prove any differently. But of course, there is still only one corporeal world.
The German cardinal Nicholas of Cusa (1401-1464) broadened the religious school of thought on extra-terrestrial life when he declared that “Life, as it exits here on earth in the form of men, animals and plants, is to be found, let us suppose, in a higher form in the solar and stellar regions. Rather than think that so many stars and parts of the heavens are uninhabited and that this earth of ours alone is peopled…”
Despite this, by the late Middle Ages, church and other doctrines firmly adhered to the beliefs of Aristotle that Earth was the one inhabited planet, and man, the one intelligent life form. This attitude lasted well into the late 1600s, but by the beginning of the 19th century, the pluralist school of thought had undergone a few minor changes, and re-emerged with the view that while other planets may not currently be inhabited, they were capable of supporting life.
Galileo and astronomer Christian Huygens had already stated that from their observations, the moon was not inhabited, and not capable of supporting life. No matter. Visions of other-worldly beings had been replaced by the prospect of new frontiers to conquer, at a time when the world had been well-explored, and science was ready for new territories.
However, the pluralists, whose viewpoint included things like “nature makes nothing in vain”, were still struggling against plain logic. German philosopher Ludwig Feurerbach, in a written work from 1830, noted that he personally saw useless, purposeless existence all around him, here on Earth. Therefore, if nature created other worlds the same as ours, they would be superfluous, since Earth already existed. Lower life forms would also be worthless, since Earth life was so far advanced. And if there were higher life forms, than Man himself was superfluous.
By 1853, the argument was back in the theological court, when Cambridge University geologist William Whewell put forth that anyone who believed in Christianity, could not accept that Earth was on a level with any other planet, since Earth alone was the scene of God’s mercy and Man’s salvation. He went on to surmise that since Earth was located mid-way between the fiery sun on one side, and the cold vapor of space on the other, that it alone was suited to become the home of civilisation.
Modern evolutionary biology, fathered by Darwin, and A.R. Wallace, would also have something to say, as the debate entered the twentieth century. Wallace, having examined planet conditions and the requirements to support life, concluded that Earth alone was the only planet capable of developing an advanced life series. Habitation of other worlds was speculation not based on astronomical, physical or biological proof, but in spite of it. He then went on to lay out his rationale, by saying that the endless changing and diversity of the Universe, argued against all planets developing the same life forms. As for Earth, the birth, growth, and death of various life forms, culminating in Man, was the highest achievement of evolution. Were it not so, then Man would be reduced to only one of numerous similar life forms, which would not only be redundant and “boring”, but speaking from an evolutionary point, not very likely.
Percival Lowell, an American astronomer, was the first to make an “assault” on proving the existence of life on Mars. He established an observatory in Arizona, and from May 1894- April 1895, made nearly a thousand maps and graphic representations of the planet. His observations, published in Mars (1895), Mars and Its Canals (1906), and Mars As the Abode of Life (1908), led him to declare that there was evidence of canals created by a higher life form, to channel water from the melting polar ice cap for the use of vegetation, patches of which could be seen, alongside areas of desert.
Lowell’s theories were, (as it would turn out) justifiably refuted by Wallace in 1903, when he observed that Mars receives less than half the sun-heat per unit surface, as Earth. His conclusion that the Martian polar ice cap was formed from gases and not water, meant that the planet would be incapable of supporting anything but the lowest life forms. Smaller, and with only one-ninth the density of Earth, Wallace suggested that what little heat absorbed by the surface during the day was lost at night, and the average surface temperature would be below the freezing point of water.
The theorizing, while based on more scientific observation than ever before, would continue unverified until the last quarter of the century, when the Viking missions would give Man his first “look” at who and what was on the planet, Mars.