As early as Oligocene times a forerunner of the great apes (Propliopithecus), most nearly resembling the gibbons, appears in the desert bordering the Fayum in northern Egypt. Early in Miocene times true tree-living gibbons found their way into Europe and continued throughout the Pliocene in the forms known as Pliopithecus and Pliohylobates, the latter being a true gibbon in its proportions; it ranged northward into the present region of Germany. Another ape which early reached Europe is the Dryopithecus; it is found in Miocene times in southern France; the grinding-teeth suggest those of the orang, the jaw is deep and in some ways resembles that of the Piltdown man. A third ape (Neopithecus) occurs in the Lower Pliocene near Eppelsheim, in Germany, and is known only from a single lower molar tooth, which recalls the dentition of Dryopithecus and more remotely that of Homo. In the Pliocene of the Siwalik hills of Asia is found Palæopithecus, a generalized form which is believed to be related to the chimpanzee, the gorilla, and the gibbon; the upper premolars resemble those of man.
None of these fossil anthropoids either of Europe or of Asia can be regarded as ancestral to man, although both Neopithecus and Dryopithecus have been placed in or near the line of human ancestry by such high authorities as Branco and Gaudry. When Dryopithecus was first discovered by Lartet, Gaudry[(1)] considered it to be by far the most manlike of all the apes, even attributing to it sufficient intelligence for the working of flints, but fuller knowledge of this animal has shown that some of the living anthropoids are more manlike than Dryopithecus. This animal is closely related to the ancestral stock of the chimpanzee, gorilla, and orang. The jaw, it is true, resembles that of the Piltdown man (Eoanthropus), but the grinding-teeth are much more primitive and there is little reason to think that it is ancestral to any human type.[N]
Fig. 16. The gibbon is primitive in its skull and dentition, but extremely specialized in the adaptation of its limbs to arboreal life. Photograph from the New York Zoological Park.
Among these fossil anthropoids, as well as among the four living forms, we discover no evidence of direct relationship to man but very strong evidence of descent from the same ancestral stock. These proofs of common ancestry, which have already been observed in the existing races of man, become far more conspicuous in the ancient Palæolithic races; in fact, we cannot interpret the anatomy of the men of the Old Stone Age without a survey of the principal characters of the existing anthropoid apes, the gibbon, the orang, the chimpanzee, and the gorilla.
Fig. 17. The orang has a high rounded skull and long face.
Photograph from the New York Zoological Park.
The gibbon is the most primitive of living apes in its skull and dentition, but the most specialized in the length of its arms and its other extreme adaptations to arboreal life. As in the other anthropoids, the face is abbreviated, the narial region is narrow, i. e., catarrhine, and the brain-case is widened, but the top of the skull is smooth, and the forehead lacks the prominent ridges above the orbits; thus the profile of the skull of the gibbon (Fig. 16) is more human than that of the other anthropoid apes. When on the ground the gibbon walks erect and is thus afforded the free use of its arms and independent movements of its fingers. In the brain there is a striking development of the centres of sight, touch, and hearing. It is these characteristics of the modern gibbon which preserve with relatively slight changes the type of the original ancestor of man, as noted by Elliot Smith.[(2)]
Fig. 18. The chimpanzee. This figure illustrates the walking powers of the chimpanzee, the great length of the arms, and the abbreviation of the legs. Photograph from the New York Zoological Park.