Over centuries tribes from western Arabia settled in Oman, making a living by fishing, farming, herding or stock breeding, and many present day Omani families trace their ancestral roots to other parts of Arabia. Arab migration to Oman started from northern-western and south-western Arabia and those who chose to settle had to compete with the indigenous population for the best arable land. When Arab tribes started to migrate to Oman, there were two distinct groups. One group, a segment of the Azd tribe migrated from the southwest of Arabia in A.D. 120/200 following the collapse of Marib Dam, while the other group migrated a few centuries before the birth of Islam from central and northern Arabia, named Nizari (Nejdi). Other historians believe that the Yaarubah from Qahtan which belong to an older branch, were the first settlers of Oman from Yemen, and then came the Azd.