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Rebuild the evolutionary history of apes and humans and determining the morphology of the last common ancestor from which they developed is an arduous task.
Some molecular techniques such as DNA sequence analysis they can only be applied to current or recently extinct species.
To address the more remote past, the scientific community has to turn to morphological data from fossils to evaluate the proximity between two species.
However, due to the large number of features that have evolved independently and that have not been inherited from a common ancestor, computer algorithms are not enough.
But not all anatomical parts have evolved independently.
The bony labyrinth of the inner ear
Between them, the bony labyrinth of the inner ear, located in the temporal bone, is an element fairly common in the fossil record due to its high mineralization.
Due to their relationships with balance and hearing, the components of the labyrinth (the vestibule and the cochlea) have been investigated to date mainly to make functional inferences in apes and humans, especially about their locomotion system.
But, in addition, this structure has proven to be very informative for analyzing the evolutionary history of different groups of mammals.
Now, an international research team led by Alessandro Urciuoli and David Alba from the Institut Català de Paleontologia Miquel Crusafont (ICP) has quantified the 'phylogenetic signal', that is, the degree to which the morphological similarities reflect the kinship relationships of the vestibular apparatus of current anthropoids (monkeys, apes and humans).
For this, the team analyzed the morphology of the enigmatic extinct hominoid Oreopithecus –Whose classification has traditionally been controversial among the scientific community– and the fossil hominin Australopithecus.
The results have been published in the journal eLife.
“Since this approach had been shown to be effective in analyzing the phylogenetic position of hominins and other mammals in previous studies, we wondered if it could also be applied to other fossil primates.”Says Urciuoli.
The researchers relied on a recently developed 3D geometric morphometric analysis technique.
Who were these hominoids?
According to the study, Oreopithecus was a basal representative that survived beyond the rest of the members of its group and converged in some aspects with the current great apes.
The conclusions are also consistent with the already known position of Australopithecus as an early representative of the human lineage.
“Our work confirms the potential of the morphology of the inner ear and, in particular, of the semicircular canals, to refine the phylogenetic relationships of Miocene apes, which are still highly controversial.”, Says Alba.
From these results the researchers were able to reconstruct the ancestral morphology of various hominoid lineages current using statistical methods and identify specific characters for each of them, useful for phylogenetic inferences.
“We have generated a testable hypothesis about the evolution of the inner ear in apes and humans, based on the analysis of other fossils, in particular the great apes of the Miocene, which will need to be subjected to a more detailed examination in the future.”, Urciyoli emphasizes.
“Unraveling the kinship relationships among the panoply of known Miocene apes is essential to improving our understanding of hominin evolution as a whole, and has implications in reconstructing the ancestral morphotype from which hominins and our more current relatives evolved close like chimpanzees and bonobos", Concludes the researcher
Urciuoli, A., Zanolli, C., Beaudet, A., Dumoncel, J., Santos, F., Moyà-Solà, S., & Alba, DM 2020. "The evolution of the vestibular apparatus in apes and humans" . eLife. DOI: 10.7554 / eLife.51261.