Iban people
One other theory suggests that these Apo Duat people were once natives of old Brunei, but were pushed upriver into the highlands by the invading tribes such as Kayan, Kenyah, and Iban people. The ones that remained downriver (Lun Bawang people) were isolated from the ones who migrated to the highlands (Kelabit), causing their culture and language to slightly diverged.