As far as I am aware, large meetings of different tribes did occur during the paleolithic at regular intervals, although not very frequently. In any case, such

As far as I am aware, large meetings of different tribes did occur during the paleolithic at regular intervals, although not very frequently. In any case, such meetings don't have to be frequent for such a virus to spread once they do occur. The virus could have originated in a single tribe, killing some of the people and immunising others and then remained endemic in that tribe for years before they came into contact with others.