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The Presence of the Frontal Opercula in Homo naledi

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When it comes to the evolution of the human brain, size isn’t everything. In fact, shape is a huge determinant. A new study from Hawks in PNAS suggests that morphology may proceeded size in the evolution of hominin brains. Hawks and team performed a comparative anatomy study of Homo and Australopithecus brains based on endocasts. Endocasts are the impressions of the brain that is made upon the skull. Many have done this before, but this study includes Homo naledi.

On surface value, Homo naledi has a brain volume about the same size as that of an Australopithecine—the 2-4 million year old genus of primates that lived in Africa. One of the differences that some members of the genus Homo exhibit are expanded frontal lobes into structures known as the frontal opercula. Within the frontal opercula, there are several structures that we believe function in language like Broca’s area as well as social behavior, as well as areas related to motor planning.

Early Neanderthals from the Sima de los Huesos in Spain and even in some Homo erectus (1.5 million years ago) have frontal operculum in their endocasts. Hominins earlier than 1.8, like hominins like Homo rudolfensis (1.8 million years ago) and Homo habilis (2.5 to 1.5 million years ago), and the Australopithecines do not have frontal operculums.

3D model of a Chimpanzee brain; B: 3D model of a human brain; C: endocast of an Australopithecus sediba skull; D: endocast of a Homo naledi skull. Hawks et al. 2018

3D model of a Chimpanzee brain; B: 3D model of a human brain; C: endocast of an Australopithecus sediba skull; D: endocast of a Homo naledi skull. Hawks et al. 2018

What about Homo naledi?

The Homo naledi endocasts showed frontal opercula. It’s really hard to infer they were chatting up a storm like us, but one skull had Brodmann’s area 47. This area assists recognizing emotion and syntax. Furthermore, one side of the frontal opercula is larger, an asymmetry we also see in modern humans where language-processing and producing centers of our left hemispheres being more developed. Lastly, Broca’s was also clearly visible. All of which suggests some rudimentary language abilities.

Having these advanced structures within the Homo naledi’s brain also implies the presence of these structures in the last shared ancestors with Australopithecines. To use the chicken or the egg analogy, the development of the frontal opercula wasn’t a sequelae of bigger brains. In other words, the fact that Homo naledi’s brain looks so much more like ours means these neurological structures occurred early in the history of our lineage.

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