Long recognized as an important genus for understanding the ancestry of great apes and humans, Kenyapithecus has been the subject of fierce taxonomic debate since its original discovery by noted paleoanthropologist Louis Leakey in 1961. Once seen as a direct ancestor of modern humans, Kenyapithecus is currently viewed as lying close to the origin of the great ape-human clade; however, controversy over the taxonomy and phylogeny of this genus continues, with researchers commonly arguing over the proper relationships of the two included species, K. wickeri and K. africanus.
Although fossils of Kenyapithecus are limited to isolated teeth, facial material, and other fragmentary bones, this primate is recognized as the most common large-sized hominoid known from the Middle Miocene of Eastern Africa. Louis Leakey discovered the first remains of this genus in 1961 when he excavated an upper jaw and isolated teeth from the 14-million-year-old sediments of Fort Ternan in western Kenya. One year later he described these specimens as Kenyapithecus wickeri, which he reconstructed as an early direct ancestor of man. In 1967 Leakey named a second species, K. africanus, which he based on fragmentary teeth and jaw material found at Maboko Island in Lake Victoria, 100 kilometers from the Fort Ternan site.
The discovery and description of both Kenyapithecus species drew the attention of the paleoanthropological community, and several additional excavations were launched in western Kenya in hopes of finding new material. These excavations recovered numerous important new specimens, many of which proved controversial. In the years immediately following Leakey’s early descriptions, it was commonly argued that Kenyapithecus was identical to Ramapithecus. While this view has fallen out of favor, few modern researchers support the validity of K. africanus. Based largely on the discovery of a nearly complete skeleton unearthed in the Tugen Hills in 1993, a team of scientists led by Steve Ward has advocated the transfer of K. africanus to its own genus, Equatorius; however, this proposal has also garnered debate, as other researchers have argued for a link between K. africanus and Griphopithecus. This taxonomic debate has yet to be settled, but regardless of the arguments, most researchers view Kenyapithecus wickeri as lying close to the origin of the great ape-human clade, and Equatorius (K. africanus) as a more primitive form.
Much of the aforementioned taxonomic debate can be blamed on the fragmentary nature of the known Kenyapithecus fossils, which are mostly limited to teeth and facial bones. Postcranial remains, which are often most phylogenetically informative, are poorly known; hence much about the anatomy and habits of Kenyapithecus are questionable. Based on the limited material, however, Kenyapithecus is known to have been a large, sexually dimorphic hominoid characterized by a robust lower jaw and thickly enameled teeth. These dental features have suggested to some researchers that Kenyapithecus fed on hard or abrasive foods such as nuts and coarse fruits.
Substantially more anatomical information is known for Equatorius (K. africanus). Based on the 1993 skeleton, which at the time of its discovery was the first Middle Miocene hominoid fossil preserved with associated teeth and postcranial remains, Ward and his coworkers have determined that Equatorius is more primitive than K. wickeri. Additionally, they have suggested that Equatorius was semiterrestrial, and thus the earliest known ape to occasionally leave the treetops for the ground. This transition, which was later paralleled in the immediate ancestors of humans, occurred about 15 million years ago when the rain forests of Africa were gradually replaced by open woodland. This environmental change may also explain the specialized diet of K. wickeri, which lived about one million years after Equatorius.
Fossils of Kenyapithecus and Equatorius have also been used as key evidence to support the controversial “Return from Eurasia” hypothesis which posits that hominoids went extinct in Africa during the Early-Middle Miocene while simultaneously diversifying in Eurasia. Later, during the Middle-Late Miocene, these Eurasian hominoids, which included the direct ancestors of the great ape-human clade, returned to Africa, where they subsequently evolved into modern great apes and humans. The ages of Kenyapithecus and Equatorius, along with their similarities to taxa known from Europe and Asia and the dearth of hominoids from the Middle-Late Miocene in Africa, have been used to support this hypothesis. Other researchers have argued that this scarcity of hominoid fossils is the result of an imperfect fossil record, and that the dating techniques used by the proponents of the reentry hypothesis are imprecise. Ward and his colleagues argue against this hypothesis, but do support a linkage between K. wickeri and a currently unnamed species known from the Middle Miocene site of Pa§alar, Turkey. This connection, which is manifested by similarities in canine and incisor morphology, represents the earliest known link between African and Eurasian Miocene large hominoids; however, Ward and his colleagues view this linkage as indicative of an African-Eurasian migratory relationship, not a full-scale extinction and reentry event.
The fossil record of African Middle-Late Miocene hominoids has long been recognized as representing a transition from species retaining primitive characteristics to more derived forms, including several species close to the origin of the speciose great ape-human clade. Although it was originally described as a hominid closely related and perhaps directly ancestral to humans, Kenyapithecus is now viewed as either a primitive member of the great ape-human clade or a genus lying immediately outside it. Largely due to the dearth of fossil material, Kenyapithecus jumps around many modern cladograms, but paleoanthropologists recognize this controversial genus as important for understanding the early evolution of many human characteristics.
References:
- Leakey, L. S. B. (1962). A new lower Pliocene fossil primate from Kenya. Annals of the Magazine of Natural History (Series 13), 4, 689-696.
- Ward, S., Brown, B., Hill, A., Kelley, J., & Downs, W. (1999). Equatorius: A new hominoid genus from the Middle Miocene of Kenya. Science, 285, 1382-1386.
- Ward, S. C., & Duren, D. L. (2002). Middle and Late Miocene African hominoids. In W. C. Hartwig (Ed.), The primate fossil record (pp. 385-397). Cambridge, UK: Cambridge University Press.
