Complex sentence: Human remains, sometimes accompanied by artifacts, have been found in or close to them, which could be scientifically dated, but it has been impossible to prove that these archaeological remains date from the time when the stones were originally set in place.
Split into simpler sentences: Human remains, sometimes accompanied by artifacts, have been found in or close to them, which could be scientifically dated. It has been impossible to prove that these archaeological remains date from the time when the stones were set in place.