Humans and other great apes diverged from smaller, "lesser" apes like gibbons about 17 million years ago based on molecular data. It’s thought that our common ancestor was great ape-like, and that gibbons are a specialized, dwarfed lineage that evolved from big-bodied apes. However, a new fossil small-bodied ape discovered in Spain – and dating back just 11.6 million years – features characteristics of both groups. They named it Pliobates cataloniae. The work, published in Science this week, suggests that the last common ancestor of all apes looked more like lesser apes and less like great apes.

First, some names to keep track of. Today’s great apes (or hominids) consist of humans, bonobos, chimps, gorillas, and orangutans; meanwhile, gibbons and siamangs make up the small-bodied apes (or hylobatids). Together, we’re the hominoids and we’re all tailless. Hominoids and Old World monkeys like baboons and macaques split roughly 25 million years ago.

"The main implication of Pliobates is that small-bodied primates played a more important role in the evolution of living apes, including humans, than previously thought," David Alba from the Institut Català de Paleontologia Miquel Crusafont (ICP) tells IFLScience. "They have been discounted in this regard because they lack the newer, derived features of living hominoids, being generally considered as preceding the divergence between Old World monkeys and apes."

On January 3, 2011, a team of paleontologists unearthed the first of 70 bones and fragments – several teeth, most of a skull, and part of a left arm – from the Abocador de Can Mata stratigraphic series in Catalonia on the northeastern Iberian Peninsula. The deposits are 11.6 million years old, and back then, this modern-day landfill was a warm, humid forest. After analyzing the fossils and using CT scans to create virtual reconstructions, Alba and colleagues discovered that the remains belonged to a never-before-seen species of extinct primate. It has a mosaic of primitive characteristics and derived features resembling that of hominoids today.



Virtual reconstruction of the skull of Pliobates cataloniae based on high-resolution CT scans, in frontal (A), lateral (B), and basal (C) views. Institut Català de Paleontologia Miquel Crusafont (ICP)

"The origin of gibbons is a mystery because of the lack of fossil record, but until now most scientists thought that their last common ancestor with hominids must have been large, because all of the undoubted fossil hominoids found so far were large-bodied," Alba says in a statement. Until Pliobates, all the small-bodied, 5- to 15-kilogram fossils found had a body plan that was too primitive to be closely related to living hominoids, and some researchers thought that small-bodied apes are dwarfed versions of great apes. "This find overturns everything," he adds.

With an estimated body mass of 4 to 5 kilograms, Pliobates was about the size of a small gibbon. Its brain size implies a monkey-like degree of complexity similar to that of gibbons, but its brain-to-body-mass ratio is more similar to that of a great ape. Microscopic wear on its primitive teeth indicates a diet of mostly soft, ripe fruit. While Pliobates retains some primitive characteristics, its forearm anatomy – specifically the wrist bones and the elbow joint – boasts the basic design seen in living hominoids. It sported adaptations for cautious climbing and clambering through the canopy, and it could walk on the tops of tree branches as well as hang from below.

Furthermore, based on a phylogenetic analysis with more than 300 dental and skeletal characters, Pliobates is a stem hominoid close to the divergence between lesser and great apes. The new genus name, Pliobates, is a contraction of the genus names Pliopithecus ("more ape") and Hylobates ("the one that walks in the woods or in the trees"), and it’s an allusion to this mix of features. The species name refers to the location it was first discovered. The specimen is also nicknamed Laia, a diminutive of Eulalia the patron of Barcelona, whose name means “well spoken, eloquent."



Long bones from the left arm of Pliobates cataloniae. Humerus (A), radius (B), and ulna (C). Institut Català de Paleontologia Miquel Crusafont (ICP)