The first humans emerged about 2 million years ago. They still resembled apes but had begun to develop human characteristics. They walked upright and made simple stone tools.
The first humans did not have homes. They did not have livestock or raise crops. They lived by roaming the land searching for wild food.
When one of these people was alone he could think and act as he pleased.
When he was with others and did as he please it might cause trouble. If he tried to take food from another person it might start a fight. If he tried to take another person's spot it might cause a fight.
Fighting was dangerous. It was also disturbing to the group. People learned that it was better to respect another person's space. His spot and the things he had with him were "his." If he left his spot or laid down his stone tool, it became "not his" and could be taken by the person who grabbed it first.
Actual physical possession meant it was his.