Like the Manobo, the Aeta or Negrito groups of the Philippines are extremely complex in terms of their dispersal and relationships. Two major branches apparently made their appearance in the archipelago 30,000 to 20,000 years ago: one traveling up the eastern flank of the islands to end up on the Pacific side of the Sierra Madre and comprising the Alta, Arta and Agta groups; the second branch appears to have moved up the western side, with some groups similarly ending up in northern Luzon; this branch includes the Pinatubo Negrito, Dumagat, Ata, Ati, Atta, Sinauna and Batak. The Aeta are physically characterized by shortness of stature (1.35-1.5 meters), dark skin, kinky hair, and broad noses. No true Aeta languanges remain; all have adopted the tongues of nearby dominant groups. They are widely distributed, with the greatest concentrations in northern Luzon and the Visayas, and generally live in highland areas or places that are difficult of access. At least 25 groups are known, many sharing the same name (Ita, Aeta, Ata, Atta, Agta, etc. are thought to come from the general filipino word "Itom," meaning "black"). Generally migrants, they often find trade and employment with more settled groups near their encampments. The social group is a small band that has fluid membership based on bilateral kinship.