while the stone age existed in what we call modern times
the Europeans came to the Americas and found the Indian tribes using stone tools thus stone age, the European went to Africa found the people using stone tools.... aka stone knives stone axes
the trade of iron and steel to the Indians was the success the Europeans lived on. The Indians loved the "magic" axes and in the beginning they were limited to the chief and medicine man. this is where the myth the tomahawk was a sacred tool... the sacred tool was the European version.
But as pointed out , a scientist is limited in his ability .... like in the big bank theory when the guys were driving and the car broke down they asked who know about the internal combustion engine works YEAH sure easy ..... who know how to fix one,, silence
while a engineer would be better,, really a farmer blacksmith etc from the year 1900 would do much better
but realize any of them would not be able to speak to the humans they found
the likely most would just get by or die if stuck there on one way trip
I do like the answer of him inventing the time machine would be the pinnacle OF HUMAN ABILITY THEN ALL THINGS Future AND past could be utilized as he wished and he could bring who and what he needed
he would go back in time (stone age) to learn not dominate
as for time travel paradox,,, if time travel is possible there then in the same theory must be parallel time lines aka alternative co-existing time lines
if you change history in the past you are in in reality it is a new con-current history after the one you came from -- like bringing up a existing saved MS word doc and making additions and changes, then re saving with a new name .. they both exist.
since the time machine excludes you from the actions of time around you AKA you travel in a timeless envelope the future and past passing does not effect you., ONLY will in transit
the closest historical and best noted historical occurrence of stone age people meeting modern people would be Cortes lands in the new world (central america) - seen as a god
if modern multi skilled person wanted to go back in time to dominate ,, it would be best to go to the bronze age
Read a Connecticut Yankee in King Authors Court