First of all i want to make clear that Norn have different origins than the humans, because the gods brought the humans to tyria and the norn were already located in Tyria. But why do the norn look like humans, only just alot taller.
The kodan themeselves believe the norn are descendants from them. The wiki says this:
_Little is known of the history of the norn before they were first encountered in Guild Wars: Eye of the North.
The origin of the norn race isn’t known, but there are theories. The kodan believe the norn may be descended from a group of kodan hunters separated from the tribes during the great storm, becoming lesser creatures for giving up being bears_
further the wiki says this about the great storm:
The great storm was long-lasting blizzard and a pivotal point in the history of the kodan race. Lasting for many seasons, it caused starvation and discontent among the kodan, eventually resulting in a small group leaving to look for hunting grounds further south. Though the kodan may not actively claim to be related to the norn, some Voice believe that this early splinter group of kodan may be the origin of the norn people
What if this has happened to the group of kodan. They came down south, devasted from the storm, questioning their faith in their god Koda, because of the torment from the storm. Finally after a long and painfull journey they reached a place south with peace and they saw the fresh to tyria humans. The humas were being helped and taken care of by the six human gods. These new gods made them question their own god, why would they be tormented for no reason (the kodan religoin is strongly about balance) and why do these six gods help the humans. This made them abondon their old religion and trying to transform into human-like beings just to get into these six new gods favor.
After a long period of time the Kodan had found out that no matter what they did or tried, they would never be accepted or noticed by these new gods. The six gods only cared about their own (made) humans. Stuck In this permanent transformation there was no way back. Because they betrayed their old god koda, they would never be accepted again by the other kodan. Going further the transformation meant to give in by these new gods, because of the high pride level of the koda they would never be able to do that.
After the series of events a new race was born: the Norn.
It would be too shamefull to let the kodan and other races know of their mistakes. So the norn were hiding in the mountains, waiting till time passed, trying to forget their mistakes, hoping everybody else forget about them and what they really were.
In the meantime they started having faith in the spirits of the wild, because the animals on the hunting grounds had never abonded them and this is still their religion nowdays.
This theory would also explain some things about the norn and koda. For example why is bear the great spirit? (because they are still bear like creatures, but just with a more human form)
And why do only the Kodan voices (kodan priests) speak of norn as descandants of the kodan (because Koda was the only one besides the group of kodan that knew that the group of kodan had abandoned him and transformed into the norn. He only knows the thruth and tells it to his voices)
If somebody disagrees with my theory or has his own theory i’m glad to read it.
ps. sorry for the long post in combination with my bad english. It’s not my native language.