Genes has nothing to do with it, grab a random untrained Mexican from the streets punch him and he drops as fast as anyone else. Its all about the training.
And boxing did came from north America, it doesn't matters whether Greeks fought or not in ancient times. Fist fighting died with the ancient world.
Fist-fighting in a box for money began in England and spread all over the English world. American sailors would organize prizefighting and Hispanics loving to see things fight each other (dogs, cocks, bulls) loved that sport and started doing the same.
Spain never really picked that up because they didn't traded much with the British, they had their own merchant ships.
Spain does has an excuse, boxing is unpopular there, just like baseball.