The Khmers developed the Khmer alphabet, the earliest alphabet still in use in Southeast Asia, which in turn gave birth to the later Thai and Lao alphabets. The Khmers are considered by archaeologists and ethnologists to be indigenous to the contiguous regions of Isan, southern Laos, Cambodia and South Vietnam. That is to say the Cambodians have historically been a lowland people who lived close to one of the tributaries of the Mekong River. The reason they migrated into Southeast Asia is not well understood, but scholars believe that Austroasiatic speakers were pushed south by invading Tibeto-Burman speakers from the north as evident by Austroasiatic vocabulary in Chinese, because of agricultural purposes as evident by their migration routes along major rivers, or a combination of these and other factors.