Certain writers have implied that it's not a coincidence. There are also other human-looking or near-human aliens in the DC universe, and it's sometimes implied that they're all somehow connected to the original (2 billion year old) humanoids, the Oans, who founded the Green Lantern Corp among other things.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort