The first colonies on the red planet failed. Some quickly, some quietly, some violently. Something out there was fundamentally hostile - not to our biology, but to our psychology.
Then, out of the blue, one group succeeded. Not just marginally, but wildly. Researchers ran correlation after correlation - until they stumbled on the answer by chance.
The secret to living on Mars had been hidden in our DNA all along - written off as a quirk, not seen for the gold it was:
You don't miss green if you've never seen it.
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