Terry Harrison, Department of Anthropology, New York University, New York, NY 10003
Proconsul, from the early and middle Miocene of East Africa,
is widely regarded as the most primitive representative of the Hominoidea. However, a
review of the historical development of this viewpoint shows that the proposed
relationship of Proconsul to extant hominoids has long been an unquestioned assumption
without adequate evidence. There appear to be relatively few characters that can be
advanced in support of such a phylogenetic link, and in many cases these features are of
limited or uncertain significance for determining relationships. Critical scrutiny of some
of the key morphological features and complexes traditionally viewed as Proconsul+hominoid
synapomorphies (i.e., relative premolar cusp height, brain size, morphology of the distal
humerus, and absence of a tail) serves to illustrate the nature and complexity of this
problem. For example, based on new estimates of cranial capacity in KNM-RU 7290, relative
brain size in Proconsul heseloni can be shown to be close to the mean value for extant
anthropoids. Also, given the range of diversity seen in modern anthropoid clades it would
appear that the degree of encephalization is of limited utility as a character in
phylogenetic analysis, and can be shown to be much more intimately correlated with
behavioral or ecological attributes, such as diet. Similarly, vertebrae associated with
the partial skeletons of Proconsul heseloni from the Kaswanga Primate Site on Rusinga
Island can be interpreted as caudal vertebrae that demonstrate that Proconsul may have had
a relatively long tail.
On the other hand, there are a number of derived cranial and
postcranial characters shared by all extant catarrhines that are absent in Proconsul. An
important set of synapomorphies linking extant catarrhines to the exclusion of Proconsul
relate to the possession of a complex of anatomical specialization associated behaviorally
with a greater emphasis on the adoption of more orthograde postures and with sitting or
sleeping upright on relatively small diameter perches. These include relatively large
lumbar vertebrae and the development of ischial callosities. The primitive retention of
small lumbar vertebrae and the absence of ischial callosities in Proconsul demonstrate
that it lacked a major defining characteristic of modern catarrhines. In addition, extant
catarrhines share a number of important derived features of the cranial base and facial
skeleton, whereas Proconsul retains a more primitive platyrrhine-like morphology in these
respects. One can conclude from these new observations that Proconsul is not a primitive
hominoid, as has generally been inferred, but is rather a stem catarrhine that diverged
prior to the last common ancestor of all extant members of the Catarrhini. It is important
to note, however, that removal of Proconsul from consideration as a stem hominoid does not
dramatically alter the configuration or timing of the major cladogenetic events in
catarrhine and hominoid evolution. Morotopithecus, an East African early Miocene
contemporary of Proconsul, appears to be a demonstrable stem hominoid that establishes
that this clade originated prior to ~21 Ma.