It was like that before the male role in reproduction was discovered. One source I read said that early humans may have veiwed child birth as a natural body function, the same as taking a dump, and may have veiwed sex as "a nice way to say hello". In a situation like this, tribes would be baised around mothers, since it is the only known relitive, along side one's siblings and maternal relations. Some theories i read said that men would temporily join other bands in order to trade, and improve relations (i.e. knock them up with their "nice hellos") then return to their mother's band.