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The caveman would also not be as used to fighting humans. There were a lot more predators to fight and a lot less people.
And this would make it harder to fight an astronaut, how?
Firstly, cavemen didn't never fight each other. Beast fights were more common, sure, but the Stone Age wasn't an era of peace, enlightenment, and free love, you know.
Secondly, the astronaut would be just about as useless in melee combat, assuming that it was in fact an "average" astronaut who never got in a brawl to the death or even injury and hasn't thought about such fights since he was trained.
Thirdly, if a caveman can beat a bear with a spear and a friend, chances are he can best a rusty-with-melee, atrophied astronaut using a knife or club.
I'm not convinced he has an edge in intelligence or training. Sure, the astronaut's training is more formal, but the caveman's is more practical. Intelligence is also rather difficult to assert unless you measure it in education level, which isn't useful in a fight. And while the astronaut may be taller, he also has an atrophied musculoskeletal system, unlike the caveman.
Not neccessarily. We don't know when he was last in space
It's more recently than the caveman was. And, remember, the caveman presumably gets into fights with animals or people regularly, not to mention other heavy labor that would come up. Cavemen weren't wimps, and astronauts...well.
Besides, referring to him as an "astronaut" implies a certain recency to the space expedition.