Ear study argues for new members of the human family

As our closest animal relatives, the biology of apes like chimpanzees, bonobos, and orangutans can shed light on our own human condition. Our closely related evolutionary history helps explain why both apes and humans share food, for example, or forge strong social bonds.

But despite our intertwined ancestry, we humans consider apes as quite different to ourselves, whether we think of them as vicious monsters in Hollywood films, or gentle giants to be protected at all costs.

A new study could alter that perception. Research published Monday in the journal Proceedings of the National Academy of Sciences takes a deep dive into the inner ear canals of two extinct European ape species — Hispanopithecus and Rudapithecus, from the Miocene era (5.3-23 million years ago) — to better understand how these creatures evolved.

The findings indicate both species likely belonged to the hominid family, which today includes all living 'great apes,' including chimpanzees, gorillas, orangutans — and humans.

Here's the background — To understand ape-human evolution, scientists honed in on a portion of the bony inner ear, known as the semicircular canals. Semicircular canals are tubes in the inner ear which evolved to help hominids keep their balance, and, ultimately, enable them to walk upright — a distinguishing physical feature which separates apes from other animals.

Because of these canals' unique relationship to movement in hominids, researchers in this study looked specifically at the volume of semicircular canals in the two ancient apes Hispanopithecus and Rudapithecus to better understand how they fit in the family tree connecting living great apes and humans.

How they did it — Unlike previous studies, which relied on measurements of the semicircular canal itself, researchers in this study took their methodology one step further.

The researchers conducted computer scans and 3-D analyses of the two extinct ape species' skulls, and then compared them to living apes and humans.

What's new — The inner ear study allowed scientists to interpret the evolutionary relationship between the ancient ape fossils Hispanopithecus laietanus — hailing from Spain — and Rudapithecus hungaricus — located in Hungary.

These two ancient apes are different enough from each other that they constitute two separate genus, the results suggest. But more intriguingly — they may also be our distant ancestors.

The study revealed some telling similarities between the semicircular canals in the ancient apes and modern hominids, including humans.

On the whole, Hispanopithecus' inner ear canal bears closer resemblance to living great apes and humans than does Rudapithecus, a finding the researchers argue may speak to a hidden twist in the primate family tree.

"Hispanopithecus and Rudapithecus appear to have diverged in opposite directions from their [last common ancestor]," the researchers write in the study.

But despite the differences between the two ancient apes, both share primitive similarities with living Great Apes and humans — in particular, moderately "stout" semicircular canals.

Ultimately, the study makes the case for these ancient apes deserving a place in the hominid family, adding another piece to the evolutionary puzzle of apes and humans.

The study states:

Our results show that the vestibular morphology of both Hispanopithecus and Rudapithecus more closely resembles that of extant great apes and humans than that of hylobatids, in agreement with the current consensus that they belong to the great-ape-and-human clade.

The researchers also used their findings to better chart the evolutionary history of other apes, including orangutans, which they found diverged from living African great apes and humans because of their inflated semicircular canals.

Why it matters — The new findings suggest there's more to the hominid family than meets the eye — specifically, they show how a tiny physical tell can shed light on human-ape evolution.

Moreover, these findings could reshape our understanding of human evolution. Most scientists believe humans and great apes originated in Africa, then migrated to Eurasia and beyond.

But older research on these two ancient apes — also known in the literature as dryopiths — supports an entirely different origin story, with roots in Europe.

This hypothetical scenario favors "a European origin and subsequent back-to-Africa dispersal for the African and human clade."

Now these ancient apes have been identified as belonging to the hominid family, this study could have widespread implications for how scientists understand ape and human evolution.

The study states, "Resolving the phylogenetic position of dryopiths has important implications for the evolution of the great ape and human clade..."

What's next — This finding provides an exciting update to the great ape family tree, but there's much we don't yet know.

For example, could there be other Miocene-era, ancient apes which fall within the hominid lineage? If so, we may still be missing pieces of the evolutionary puzzle that led to humans. Further research is required to determine whether or not there are other relatives lurking in our distant pasts.

"In the future, the inclusion in the analyses of additional extinct hominoids will hopefully clarify further the evolutionary history of these hominoids during the Miocene," the study authors write.

Abstract: Late Miocene great apes are key to reconstructing the ancestral morphotype from which earliest hominins evolved. Despite consensus that the late Miocene dryopith great apes Hispanopithecuslaietanus (Spain) and Rudapithecus hungaricus (Hungary) are closely related (Hominidae), ongoing debate on their phylogenetic relationships with extant apes (stem hominids, hominines, or pongines) complicates our understanding of great ape and human evolution. To clarify this question, we rely on the morphology of the inner ear semicircular canals, which has been shown to be phylogenetically informative. Based on microcomputed tomography scans, we describe the vestibular morphology of Hispanopithecus and Rudapithecus, and compare them with extant hominoids using landmark-free deformation-based three-dimensional geometric morphometric analyses. We also provide critical evidence about the evolutionary patterns of the vestibular apparatus in living and fossil hominoids under different phylogenetic assumptions for dryopiths. Our results are consistent with the distinction of Rudapithecus and Hispanopithecusat the genus rank, and further support their allocation to the Hominidae based on their derived semicircular canal volumetric proportions. Compared with extant hominids, the vestibular morphology of Hispanopithecus and Rudapithecus most closely resembles that of African apes, and differs from the derived condition of orangutans. However, the vestibular morphologies reconstructed for the last common ancestors of dryopiths, crown hominines, and crown hominids are very similar, indicating that hominines are plesiomorphic in this regard. Therefore, our results do not conclusively favor a hominine or stem hominid status for the investigated dryopiths.