Scientists may have unearthed a fossil representing a species very closely related to the last common ancestor of all apes, which consists of humans, gorillas, chimpanzees, gibbons and orangutans. The fossil, which has been called Pliobates cataloniae, is surprisingly small. Given that most of the genuses in the ape family are large bodied animals, the fossil is a lot smaller than expected.
The fossil was found in Catalonia, Spain in 2011 and is thought to be about 11.6 million years old. Only fragments were found, but the animal it came from is estimated to be about 11 pounds.
Analysis of physical traits suggests it evolved before any of the apes diverging from a species that was the last common ancestor of all apes. Seen in the fossil are several features that have been observed in both great apes (the humans, gorillas and chimps) and of lesser apes (the gibbons and siamangs). Pliobates’ small size is similar to that of lesser apes. It also has similar ‘goggle-like’ orbital structure around its eyes. These traits suggest that Pliobates is similar to the ancestor of the lesser apes.
Also seen, however, are several traits seen in the greater apes. Pliobates has a very large cranial capacity in proportion to its small body size, a trait also seen in great apes. It also has a wrist structure seen in many great apes that allows rotation greatly aiding in climbing ability. These traits seem to point to a relationship between Pliobates and modern greater apes.
Strangely though, there are several features seen in Pliobates that are also seen in some of the older linages that evolved before the apes. Pliobates lacks a bone structure found in the elbow joints of greater apes that helps them stabilize their arms when hanging. This feature separates apes from other primates like the groups known as Pliopithicus and the Victoriapithecus, which are older lineages of primates. Also seen in Pliobates is wide external opening in the ear very similar to that seen in Pliopithicus.
These primitive traits seen in Pliobates have lead other scientists to believe that Pliobates is not related to the ancestor of great apes at all and that it evolved in the Pliopithicus lineage. Convergent evolution (the evolution of similar traits in different groups) may be the reason Pliobates exhibits traits that are seen in apes.
In the image above, the red line points to two areas where Pliobates could have evolved. It could, in fact be closely related to the common ancestor of all ape species or possibly closely related to Pliopithicus, a more primitive primate.
A third hypothesis is that Pliobates is actually only related to gibbons apes. The fossil is only 11.6 million years old and molecular evidence from previous studies suggests that the divergence between gibbons that the rest of the apes occurred 14 million years ago leaving enough time for Pliobates to emerge within the gibbon lineage. Once again, the traits seen in greater apes but not in gibbons may have evolved independently in Pliobates.
If Pliobates is closely related to the common ancestor of apes, then the independent evolution of a constricted ear tube would have had to occur in apes. This is because Pliobates has a very open ear cavity, which is inconsistent with the ear structure of apes.
The fossil will likely cause a large debate in the scientific community as to where Pliobates belongs. Further studies will be needed before Pliobates can put definitively in its proper place in the history of primate evolution.