An anatomical surprise
In the Miocene sediments of Nachola, a region in northern Kenya, paleontologists from a joint Kenya-Japan expedition unearthed one of the most complete fossil ape skeletons ever found. The specimen, KNM-BG 35250, belongs to a species named Nacholapithecus kerioi. Dated to approximately 15 million years ago, this skeleton presents a combination of features that challenges simple classifications. While modern great apes like chimpanzees and gorillas have short, stiff lower backs, Nacholapithecus had a long and flexible vertebral column with six lumbar vertebrae. This is more typical of monkeys than any living ape.
This anatomical arrangement suggests Nacholapithecus moved very differently from its modern relatives. It was likely a pronograde arboreal quadruped, meaning it walked on all fours along branches with its spine held horizontally. This monkey-like spinal structure, combined with powerful forelimbs and long feet, indicates a versatile locomotor repertoire. It lacked the specialized adaptations for below-branch suspension seen in modern apes, but its shoulder joints were highly mobile. This suggests an animal that was a capable climber but also spent a significant amount of time moving on the ground.
A world in transition
The world of Nacholapithecus, 15 million years ago during the Middle Miocene, was undergoing significant change. East Africa was not uniformly covered in dense tropical forest. Tectonic activity associated with the formation of the Great Rift Valley was beginning to alter the landscape, creating a mosaic of environments. Evidence points to a mix of woodlands and more open, seasonal habitats.
In this varied environment, an ape that could travel efficiently both in the trees and on the ground would have had a distinct advantage. The long, flexible back and robust limbs of Nacholapithecus would have been well-suited for moving across open ground between patches of forest. Its unique anatomy shows primate evolution at a time when new habitats were opening up, prompting adaptations that diverge from the paths taken by the ancestors of modern apes. Nacholapithecus is not a direct ancestor to any living ape but represents a successful side branch of the hominoid family tree.