Strange Swamp-Dwelling Prehistoric Ape That Moved Unlike Anything Alive Today Poses Evolutionary Puzzle

A peculiar species of ancient ape that has posed an evolutionary puzzle for decades could not walk on two legs like early hominins—but it also couldn't climb trees like other prehistoric primates, scientists have discovered.

The swamp-dwelling creature, Oreopithecus bambolii, lived between 8.3 and 6.7 million years ago on an isolated island that is now Tuscany and Sardinia, Italy. It was first described in 1872 and is considered a hominoid—the primate superfamily that includes modern humans. However, because of its unusual physical features, where it sits in the evolutionary family tree is not known. It does not appear to be closely related to any living species of ape.

It also has a pelvis that was more like those of early hominins. In the 1990s, a group of researchers suggested that O. bambolii was an ape that walked on two legs. This was later dismissed, with a team finding it would have walked on all fours. But major questions over the species' bizarre features remained.

In a study published in PNAS, researchers led by Ashley Hammond, from American Museum of Natural History, have now examined the skeleton of O. bambolii and compared it with other fossils to get a better idea of how this peculiar ape would have moved. They also examine evolutionary scenarios in which its physical traits could have emerged.

"Oreopithecus has been interesting to paleoanthropologists for years because it is known from a very complete skeleton (IGF 11778) and is dated to a key period of ape and human evolution—the time when the earliest hominins or their immediate ancestors would have been evolving," Hammond told Newsweek. "The IGF 11778 skeleton is flattened, though, so this has led to a variety of interpretations about locomotor behavior.

"Where Oreopithecus fits on the tree of life has been a point of contention for years. It is certainly closer to living species of great apes than it is to hominins. Although we know it is a fossil ape, we don't know how it is related to other apes based on it's unusual morphology."

Their findings showed its lower torso is unlike any known hominoids. It did not have any of the features needed for it to walk upright on two legs. It also lacked the pelvic and torso stiffness seen in modern apes that allows them to climb trees with such success. The team say O. bambolii's lower torso has some similarities with a siamang—a black-furred gibbon found in southeast Asia.

"This ape was probably quite versatile," Hammond said. "It had long arms, so it would have been good at climbing trees. The lumbar region of the spine shows that Oreopithecus did not have a stiff lower spine like living great apes, so it probably moved with more spinal flexibility than chimpanzees, gorillas, and orangutans. One of our conclusions is that this ape did not have the torso features of bipedality associated with hominins, but it likely could have still moved bipedally at times."

How it ended up with these features is unclear. Researchers say O. bambolii evolved from an unknown ancestor on an island with limited resources and a lack of land-based predators. Because of this, the team said the species would have developed an "array of adaptations," including changes to its hands and feet that allowed it to stand and move on two legs more efficiently than other apes living today.

"Oreopithecus was an insular, swamp-dwelling hominoid subjected to a very different array of selective pressures than African apes and hominins," the scientists wrote.

Oreopithecus bambolii
An Oreopithecus bambolii skeleton. The unusual, extinct species does not appear to be closely related to any living species of ape. S. Bambi University of Florence, Florence, Italy

Previous research suggested that because it had no predators, it may have shifted away from climbing—a risky behavior—in favor of lower energy, land-based activities such as foraging. It had been suggested the species went extinct after a land bridge emerged, connecting its island home to the mainland and large predators.

"We do not know why Oreopithecus went extinct, but it was an ape living in a period of time when the Mediterranean region was undergoing significant changes in climate and it may have been unable to cope with the environmental change happening regionally," Hammond said.

In the latest study, the team say O. bambolii provides an insight into the body plan of a hominoid that is unlike the apes and humans that exist today.

They say there are three evolutionary scenarios that led to O. bambolii. One is that it is a member of the hylobatidae family, which includes gibbons. Another possibility is that the features seen emerged at the origin of the hominoid crown group, 17 million years ago, with O. bambolii then being a stem member of the Hominidae family.

Finally, the researchers say O. bambolii could have developed its hominin-like pelvis through convergent evolution. They say there is a trend towards walking upright within fossil hominoids, and natural selection for this trait could explain the species' features.

"Future work incorporating other regions of the skeleton might clarify which of these scenarios is the most credible," they conclude. "All of these scenarios reaffirm late Miocene O. bambolii as an important morphological and behavioral point of comparison with models of hominin origins."

This article has been updated to include quotes from Ashley Hammond.

Correction 12/24 5.05 a.m. The headline has been updated to more accurately represent the findings.