depends largely on what they're equipped with, but if they're both lightly clothed and unarmed, I'd say 95% chance that the astronaut wins, assuming they have some combat training, which I reckon most of them do. Modern combat training can allow almost any person to physically overcome almost any other person, no matter how strong or savage. If the astronaut doesn't have combat training, probably the caveman, due to lots of factors that we can basically sum up by saying that primitive humans were far more savage and experienced in violence than almost anyone in modern society. Depending on the circumstances of the combat, the vastly superior intelligence of the astronaut might allow them to win, but not in an enclosed 1v1 setting.

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if you think I took the question too seriously, I invite you to consider concluding your existence.