...firstly, no, these people do not go directly to hell. they
will be judged based on their conscience, and how they respond to
it.
secondly, to those tribes that have not heard of Jesus, sometimes
God interferes supernaturally to let them know about Jesus. For
example, i read of a case whereby a muslim in a middle eastern
country had a dream where an angel appeared to him, and asked him
to search for Jesus. When he woke up, he travelled to a village, to
a certain house and knocked on the door, asking the family there if
they had heard of Jesus. Lo and behold! turns out the family were
the few christians in a muslim-majority country.
and yes, the muslim became a christian.
