Giuseppe Cocconi and Philip Morrison—two physicists at Cornell—began their 1959 article in Nature magazine quite frankly: we can't reliably estimate the probability of intelligent life out in the universe, but we can't dismiss the possibility of it either. We evolved and we're intelligent, so wouldn't it stand to reason that alien civilizations could arise on planets around other sun-like stars? In all likelihood, some of those civilizations would be older and more advanced than ours and would recognize our Sun as a star which could be host to life, with whom they would want to make contact. The central question of the paper was then: how would the beings send out their message?
Electromagnetic waves were the most logical choice. They travel at the speed of light and would not disperse over the tremendous distances between stars. But at which frequency? The electromagnetic spectrum is far too wide to scan in its entirety, so they made an assumption that has remained central to SETI research ever since. They would listen in at 1420 MHz, which is the emission frequency of hydrogen, the most abundant element in the universe. They reasoned it was the one obvious astronomical commonality we would share with an unknown civilization and that they would recognize it too.