I may be totally off here but it seems as though every continent on Earth has developed swords. They all look different but in essence are still swords. Same with spears, arrows, etc. Is there some common ancestor here? Was the first sword made and then copied or did all these different societies come up with the same idea completely independent of one another?
The laws of physics are the same here as they are over there, you would expect people to gravitate towards the same solutions to problems. Of course it is possible the idea just spread from place to place.