New palaeotological evidence bearing on the evolution of the Hominoidea

Abstract

Summary The discovery in recent years of a remarkable abundance of fossil hominoids in East Africa and South Africa has served greatly to amplify the palaeontological evidence bearing on the evolution of this group of primates. The opportunity has been taken in this lecture to review some of this evidence. It has been recognized for some time that the Anthropoidea as a whole had their origin in a tarsioid ancestry, and this thesis finds support in the Eocene genus Amphipithecus and the Oligocene genus Parapithecus. It has also been considered probable that the hominoid sequence separated from the cercopithecoid sequence at least in Oligocene times. It has now become clear that by the early Miocene this segregation was complete, for the cercopithecoids were then already well differentiated in East Africa. The gibbon–like primates from the early Miocene of Kenya undoubtedly belong to the Propliopithecus—Pliopithecus group, and may have been ancestral to the modern gibbon. The larger Miocene apes from the same region showed great variation in size and were represented by at least four species. The discovery by Dr. and Mrs. Leakey of a skull of Proconsul africanus is of exceptional interest, and serves to demonstrate that these early apes still retained many primitive features equivalent to a cercopithecoid level of evolution. The few specimens of limb–bones found in Kenya also indicate that the limbs had not yet undergone all those specializations which, in modern apes, are associated with a brachiating mode of progression. The relation of Proconsul to Dryopithecus remains uncertain, for while the lower dentition shows only small differences, the upper molars are rather strongly contrasted. In general, however, Proconsul was evidently more primitive in its morphological characters, but at the same time it shows evidence of a slight degree of specialization. A few very fragmentary specimens from Kenya indicate that another genus of ape, identified as Sivapithecus, was probably contemporaneous with Proconsul, a discovery of some interest since it suggests that the various species of Sivapithecus and allied genera known from later formations in India may have had an African origin. The question naturally arises whether any of the groups of Miocene and Pliocene apes so far known have any special significance for the evolutionary origin of the Hominidae. It is suggested that, on the present evidence, this possibility deserves serious consideration, in spite of the objections which have been raised by some palaeontologists regarding supposed specializations in their dentition. While the discoveries in East Africa throw light on the problem of the initial differentiation of the Hominoidea as a whole from a generalized catarrhine stock, those made in South Africa have an obvious bearing on the emergence of the Hominidae from a common hominoid ancestry. The Australopithecinae are now known by the remains of more than thirty individuals (including infants, juveniles and adults both young and aged) from several independent sites in South Africa, and the excellence of the preservation of some of these remains permits more accurate comparisons than were possible a few years ago. The study of an extensive series of adult and immature skulls of modern apes makes it clear that in a number of important characters the australopithecine skull shows significant differences wherein it approaches that of primitive hominids. The many examples of the dentition now available also serve to demonstrate that, while it presents sharp contrasts with the anthropoid apes, there appear to be no characters of a fundamental nature whereby it can be definitely distinguished from the dentition of the Pithecanthropus group. The combination of cranial and dental characters provides strong morphological evidence for placing the Australopithecinae in the hominid rather than the pongid sequence of evolution. This interpretation now appears to be conclusively demonstrated by the evidence of the limb skeleton, remains of which have been found at Sterkfontein and Makapansgat. The precise position of the Australopithecinae in relation to the evolution of the Hominidae can hardly be determined until the question of their geological age is settled. So far as the morphological evidence at present available can be assessed, it seems a reasonable inference that they either represent a group which occupied a position quite close to the main line of later hominid evolution, or else they represent only slightly modified descendants of such a group. The new discoveries in East Africa and South Africa, as well as those made in Java shortly before the war, suggest that collateral lines of evolution in the Hominoidea may have been far more numerous than had been suspected. This can only accentuate the need for caution in seeking to interpret the precise phylogenetic relationships between fossil types and modern types.