The Khmers are one of the oldest ethnic groups in the area, having filtered into Southeast Asia around the same time as the Mon. Most archaeologists and linguists, and other specialists like Sinologists and crop experts, believe they arrived no later than 2000 BCE (over four thousand years ago) bringing with them the practice of agriculture and in particular the cultivation of rice. They were the builders of the later Khmer Empire which dominated Southeast Asia for six centuries beginning in 802 CE, and now form the mainstream of political, cultural, and economic Cambodia. The Khmers developed the first alphabet still in use in Southeast Asia which in turn gave birth to the later Thai and Lao scripts. The Khmers are considered by most archaeologists and ethnologists to be indigenous to the contiguous regions of Isan, southernmost Laos, Cambodia and Southern Vietnam. That is to say the Khmer have historically been a lowland people who lived close to one of the tributaries of the Mekong. The Khmers see themselves as being one ethnicity connected through language, history and culture, but divided into three main subgroups based on national origin. The Khmer of Cambodia speak a dialect of the Khmer language. The Northern Khmer (Khmer Surin) are ethnic indigenous Khmers whose lands once belonged to the Khmer Empire but have since become part of Thailand. The Northern Khmer also speak the Isan language fluently. Maintaining close relations with the Khmer of Cambodia, some now reside in Cambodia as a result of marriage. Similarly, the Khmer Krom are indigenous Khmers living in the regions of the former Khmer Empire that are now part of Vietnam. Fluent in both their particular dialect of Khmer and in Vietnamese, many have fled to Cambodia as a result of persecution and forced assimilation by Vietnam. All three varieties of Khmer are mutually intelligible. While the Khmer language of Cambodia proper is non-tonal, surrounding languages such as Thai, Vietnamese and Lao are all highly tonal and have thus affected the dialects of Northern Khmer and Khmer Krom.