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Science is a very funny thing. Nothing is discovered until it is discovered. For many centuries, mankind did not even know where babies came from. They would have scoffed at the idea that there were tiny invisible sperm, and tiny invisible "eggs" (ovum) that magically merged to create the babies in the womb . Even after the invention of the microscope, it took a long while before they learned all the secrets of human reproduction.
How do we know if there is life on another planet, until we go there? So far, we have only gone to the Moon & Mars. And both of them are uninhabitable, for any more than a couple of weeks, at best. (On MARS, there is NO water, NO oxygen, & NO vegetation for food. -- & the temperature is way too high for humans.
-- but I still give a lot of credit to the hundreds of brave people who volunteered to go there (on a "one way mission") anyway.
Someday, in the distant future, maybe some intergalactic space men will be checking out this solar system. And meanwhile, they will ask --- was there ever life on planet Earth? And they will look around to examine this dead planet, to find that War, Disease, & massive Pollution brought this once-thriving world to an end.
They will find that the ocean is filled with oil sludge, - bodies of rotting animals & fish species,- & rusted mangled wreckage of factories, Then the aliens will ask -- have there ever been any survivors? With hearts full of hope, they blast off to our nearest orbiting satellite -- (the Moon) -- to search for clues. --- It will not be long until they see -- a few ancient footprints (of rubber astronaut boots) -- & an old tattered United States FLAG -- it says: "ONE GIANT LEAP for MANKIND."