Primate Genus Sheds Light On Great Ape And Human Origins

The ape family tree has dwindled to a few remaining twigs today, but during the Miocene era (23-5.5 million years ago), it was a thriving, bushy affair, with numerous species in Africa, Asia, and Europe. Researchers agree that the great ape and human lineage originated from one of these animals, but their vote for the most likely ancestor changes as new discoveries are made and old fossils are re-analyzed. The new fossil ape reported in Science prompted its discoverers to take another look at the Kenyapithecus genus, an ape that researchers had once placed in the key ancestor position. Their analysis reclassified a more primitive-looking species of Kenyapithecus within the new genus Equatorius, allowing the researchers to draw a clearer connection between the remaining Kenyapithecus species and the living apes and humans.

The research team, which includes senior author Steve Ward of the Northeastern Ohio Universities College of Medicine and Kent State University and project director Andrew Hill of Yale University, revisited the problem of Kenyapithecus' place in great ape ancestry after uncovering the skeleton at the site of Kipsaramon in Kenya. They compared features in the Kipsaramon fossil's teeth and jaws to dental and facial patterns in two Kenyapithecus species: Kenyapithecus africanus and Kenyapithecus wickeri. Their analysis revealed two distinct patterns in the sample: a more primitive pattern represented by Kenyapithecus africanus (and including the Kipsaramon specimen), and a more modern, great ape-like pattern represented by Kenyapithecus wickeri.

"Some researchers had previously suspected that there was more than one taxon represented in Kenyapithecus, but before the Kipsaramon fossil, the sample was too limited to draw any definite conclusions," co-author Jay Kelley said. Ward and his colleagues decided to formally recognize the two different groups by placing the more primitive-looking individuals in the new Equatorius genus, and retaining the name Kenyapithecus for the more advanced specimens.

Tantalizing evidence from a continent away provided the second key piece of data that convinced Ward and his colleagues to recognize the Equatorius genus. Kenyapithecus wickeri's overall dental pattern is also found in the teeth of an unnamed ape species from the middle Miocene site of Pasalar in Turkey. Being able to see the same anatomy in a group of animals from a completely separate part of the world convinced the authors that they were right to single out Kenyapithecus wickeri as distinct from Equatorius.

"The connection between the Pasalar material and the Kenyapithecus wickeri specimens gave us confidence that the differences we had found in the Kenyapithecus material were real and of generic level importance," Kelley said. The similarities also raise the intriguing possibility of a link between Eurasian and African apes during the middle Miocene. The Pasalar material is currently being analyzed, and Kelley stressed that the exact nature of this link will remain unclear until this work is complete.

Equatorius' exact relationship to later apes is still uncertain at this point, but the genus generally fits the description of a "stem hominoid", one of a cluster of species positioned somewhere on the evolutionary ladder near the origins of the ape group. Stem hominoids are considered too primitive to be the direct ancestors of modern apes and humans. However, Equatorius is more modern in some respects than earlier primitive apes, especially in its skeleton. "Equatorius is a fascinating animal. It shows evidence of using the ground more frequently than earlier ape species. As we continue to analyze the upper arm and shoulder of Equatorius, we expect to learn a great deal more about the behavior of this animal," said Steve Ward.

Removing the more primitive Equatorius specimens from the Kenyapithecus sample was like lifting a veil off the second genus, allowing the researchers to clearly identify the advanced features of Kenyapithecus that linked it to more modern apes. "One of the reasons for the conflicting opinions about the relationship of Kenyapithecus to the great ape and human clade was that this mixed sample was giving off mixed signals. Different things were thrown into a common pot, muddling up the real story," said Kelley. "Kenyapithecus' exact relationship to the origin of these later apes is still somewhat of a mystery at this point, but we think that the work reported in our paper at least establishes a clearer framework for interpreting how Kenyapithecus might fit into ape and human ancestry."

The site of Kipsaramon is located in Kenya's Baringo Basin, a focus of continuous paleoanthropological research for nearly 20 years by Yale University's Baringo Paleontological Research Project, led by co-author Andrew Hill. Geologic sediments in the basin cover the period between 15 million years ago to the present, and contain the only complete record in Africa for portions of this crucial time in ape and human history. Research in the basin has also produced a wealth of information about climate and the evolution of other animal species from this time period.

Ward, Hill, and members of the team from the National Museums of Kenya came upon the Kipsaramon skeleton after searching uphill from a large number of isolated teeth that had washed into a gully at the site. The skeleton was moved from the site to the museum in one consolidated block, where the fragile bones were painstakingly removed from their surrounding rock. The fossil find consists of most of a lower jaw with teeth, some upper incisors, and an unusual amount of skeletal material, including bones or parts of bones from the arm, shoulder, collarbone, chest, wrists and fingers, and vertebrae, all belonging to a single male individual. The researchers believe that the new fossil and genus, along with the sheer volume of other Miocene fossils currently being described by other scientists, will generate another round of intense interest and debate about our lineage's origins. "The real importance of Equatorius is how it clarifies the relationship of middle Miocene apes.

"The fact that Kenyapithecus can be definitely shown to be more a derived ape with possible affinities to fossils in Turkey opens the door to interesting discussions concerning the ancestry of great apes and humans," said Ward.