An Overview of the Genus ProconsulBiology
An Overview of the Genus Proconsul
By: Creighton Smith
This paper is intended as an overview of the fossils and related data concerning the Genus Proconsul. In 1927 a settler digging limestone from a quarry in western Kenya found the first fossil that would come to be labeled Proconsul. The specimens were sent to the British Museum where A. Tindell Hopwood identified them as the left maxilla and a tooth of a hominoid primate. Hopwood spent a few years raising money and in 1931, he went to the limestone quarries in Kenya and recovered several more fossils. The site was dated at 18 million years old based on some of the other fossils recovered. Two years later he published that the fossils represented a new Genus ancestral to the modern Chimpanzee (Walker and Teaford, 1989).
During the early 1930’s in London there was a chimpanzee named Consul was entertaining the vaudeville crowds by wearing a suit and hat, riding a bicycle and smoking a pipe. Hopwood named his discovery after this chimpanzee and it was dubbed Proconsul africanis (Walker and Teaford, 1989). Over the next several years other specimens of Proconsul were discovered, many of them from Rusinga Island in Lake Victoria, and a total of four species within the Genus Proconsul have been identified.
The four different species of Proconsul are very similar morphologically with the main difference being body mass. P. heseloni is the smallest of the four species with a mean weight, based on numerous fossils, of just 10.9 kgs ( Rafferty, 1995). P. africanus weighs in at 27 kgs (Fleagle, 1999). P. nyanzae specimens from the Rusinga / Mfangano sites in Uganda have a mean weight of 35.6 kgs ( Rafferty, 1995). The estimated mean body weight of P. major is 75.1 Kgs. P. major’s mean is derived from a series of distal tibia measurements. One fossil tibia from Napak, Uganda gives measurements that put P. major’s upper limit at 86.7 Kgs ( Rafferty, 1995). It is interesting to note that Fleagle and Rafferty have quite different weight estimates for the different Proconsul species.
The most fossil material recovered belongs to Proconsul heseloni (see Figure. 1 on the cover page). For this reason P. heseloni is often referred to as being indicative of the whole Genus. With this in mind, I will refer to the Genus of Proconsul as a whole for the purposes of this paper.
It is important to reconcile the position of Proconsul in the evolutionary tree. Proconsul lived 18 million years ago during the lower Miocene. Although it shares some features with modern gorillas and chimpanzees, suggesting a closer relationship to those species, most of the features are relatively unspecialized. This suggests that Proconsul represents the last common ancestor of all great apes and humans as shown in figure 2
(Walker and Teaford, 1989).
Figure 2. Evolutionary tree of primates showing Proconsul’s relative position to extant species (Walker and Teaford, 1989).
There does exist conflicting evidence that suggests the position of Proconsul, as depicted by figure 2. may not be entirely accurate. Carol V. Ward of John Hopkins University said at the 2002 American Anthropological Association, “Analysis of the rare remnant of Proconsul’s pelvic anatomy now indicates it resembles the hip bones of living monkeys more than those of living apes ” (Bower, 2002). Ward goes on to say “The Proconsul specimen generally looks monkey-like, sharing many features with the hipbones of baboons.”(Bower, 2002). Looking at Figure’s 3 and 4 one can see not only a similar bauplan, but also the hipbones are indeed similar. The major difference in these hipbones is the presence ischial callosity ridges on the baboon hips. This would seem to be a problem with Ward’s statements to the contrary. However, lack of ischial callosities is the primitive condition. The presence of these structures serves to unite lesser apes and old world monkeys but not Proconsul and chimpanzees who have also retained the primitive condition (Walker and Teaford, 1989).
Another difference between Proconsul and the hominoid group is a matter of the intermembral index. All of the apes, excluding humans, have an intermembral index greater than 100 meaning longer arms than legs. While Proconsul has an index of 89 (Fleagle, 1999) much like that of a terrestrial baboon. This would suggest a terrestrial lineage for Proconsul, however there are many factors that suggest otherwise.
The most obvious is the derived condition of tail loss that is specific to hominoids. This point is significant for acceptance of Proconsul as a human or at least hominoid ancestor. Another commonality with the great apes is the presence of large feet and hands with highly a divergent hallux on both. The comparison of both hands and feet in figures 3 and 4 will show the stark contrast of the adaptations in those extremities that suggest Proconsul is an arboreal quadruped like the great apes.
Figure 3. Skeleton of Proconsul (Walker and Teaford, 1989).
Figure 4. Skeleton of baboon, Papio (Fleagle, 1999).
Another anatomical correlate that suggests arboreal quadrapedalism in Proconsul is the shape of the olecranon process at the elbow joint between the humerus and ulna. The height of the olecranon process is consistent with a flexed arm position. This coupled with an intermembral index of 89, that is hind limbs longer than fore limbs, even suggests some possible leaping adaptation. Yet the distal phalanges show a bit of curvature like apes adapted for suspensory locomotion.
Also, like apes, Proconsul has a larger brain to body weight ratio than Old World Monkeys of comparable size (Walker and Teaford, 1989). Le Gros Clark observed that Proconsul possesses a frontal air sinus, or air space within the frontal bone of the skull. This is interesting because frontal sinuses are found in humans and African apes but not in Orangutans or lesser apes (Walker and Teaford, 1989).
Many arguments can be made for or against the relevance of these features. Walker, Teaford, Rafferty, and Fleagle would all agree that Figure 2 is a correct depiction, whereas Carol Ward would probably place Proconsul further down the line perhaps even to left of Old World Monkeys. The hybrid or mosaic pattern of Proconsul fossil evidence led Michael D. Rose of the University of Medicine and Dentistry of New Jersey to say “Miocene Hominoids (including Proconsul) are neither like Old World Monkeys nor like apes; rather, they are like Miocene Hominoids”(Walker and Teaford,1989).
Bower, Bruce 2002 Science News Vol. 138 Ebsco Publishing, Birmingham Al.
Fleagle, John G. 1999 Primate Adaptation and Evolution 2nd Edition
Academic Press, NY
Rafferty, Katherine L. 1995 Postcranial Estimates of Body Weight in Proconsul, With a Note on a Distal Tibia of P. major From Napak, Uganda Wiley – Liss Inc. NY
Walker, Alan and Teaford, Mark 1989 Scientific American January New York, NY