What Do We Know about the Evolution of Gibbons?

Abstract

The extant hylobatids are relatively small primates, and often called lesser apes. The body weight is 5-8 kg for the majority of the hylobatid species, and 10-12 kg for the largest species, Hylobates syndactylus. Recent taxonomy classifies the hylobatids into a single genus Hylobates with four subgenera (Hylobates, Nomascus, Bunopithecus, & Symphalangus) (Groves, 2001).The present geographical distributions of the subgenera are almost perfectly bordered by three (or possibly four) huge rivers in Eastern Eurasia, that is, the Brahmaputra, Salween, Mekong (and Yangtze) Rivers. All these rivers originate from the Tibetan Plateau. Although van Gulik (1967) reconstructed the historical distribution of gibbons in China as being expanded northwards over the Yangtze River up to the Yellow River, the Pleistocene fossil evidence suggests that the northern limit of the gibbon distribution in China was probably the Yangtze River.Recent genetic studies suggest that these subgenera began to diversify around several to ten million years ago. In this period, the Tibetan Plateau reached a considerable altitude. The uplift of the Plateau probably influenced the development of the above mentioned huge rivers, strengthening the function of these rivers as zoogeographical barriers. It seems likely that the proto-hylobatid populations were then isolated from each other, and evolved into recent subgenera, though the diversification between the subgenera Hylobates and Symphalangus needs another explanation.At present, the fossil record of the hylobatids is very poor. There are some Pleistocene gibbon fossils discovered from southern China and Southeast Asia, but no Neogene fossil catarrhines, small or large, are thought to be the direct ancestor of extant gibbons. In Southeast Asian countries, except for a few findings such as the Chiang Muan hominoids discovered from Thailand by the Thai and Japanese scientists, we know nothing about the Neogene fossils of both large and small hominoids. No doubt further field works are necessary to reveal the evolutionary history of gibbons.