Evidence for expansion of the precuneus in human evolution

Abstract

The evolution of neurocranial morphology in Homo sapiens is characterized by bulging of the parietal region, a feature unique to our species. In modern humans, expansion of the parietal surface occurs during the first year of life, in a morphogenetic stage which is absent in chimpanzees and Neandertals. A similar variation in brain shape among living adult humans is associated with expansion of the precuneus. Using MRI-derived structural brain templates, we compare medial brain morphology between humans and chimpanzees through shape analysis and geometrical modeling. We find that the main spatial difference is a prominent expansion of the precuneus in our species, providing further evidence of evolutionary changes associated with this area. The precuneus is a major hub of brain organization, a central node of the default-mode network, and plays an essential role in visuospatial integration. Together, the comparative neuroanatomical and paleontological evidence suggest that precuneus expansion is a specialization of Homo sapiens that evolved in the last 150,000 years that may be associated with recent human cognitive specializations.

Human Paleoneurology and the Evolution of the Parietal Cortex

“…Interestingly, if we compare the midsagittal brain morphology in humans and chimpanzees, we also find that their main difference concerns the extension of the precuneus, which is much larger in our species [Bruner et al, 2017c]. This difference seems, again, to be localized in the dorsal and anterior part of the precuneus, a region that matches the area 7a according to Scheperjians et al [2008].…”

Identification of in vivo Sulci on the External Surface of Eight Adult Chimpanzee Brains: Implications for Interpreting Early Hominin Endocasts

“…It is also hypothesized that L was displaced caudally during hominin evolution, and recent evidence confirms that its "true" homologue eventually disappeared [Allen et al, 2006;Alves et al, 2012]. The newly described sulcal variations described above for chimpanzees suggest that, among other factors [Bruner et al, 2017], L may have been lost in conjunction with the full emergence of two annectant gyri (1 and 2), one of which entailed expansion of the angular gyrus (BA 39) -a region that is also important for human language. Significantly, the new sulcal variations reported here for both frontal and parietooccipital regions in chimpanzee brains are consistent with comparative diffusion tensor imaging findings from studies of a white-matter tract, the arcuate fasciculus, which arches around the Sylvian fissure linking temporal and frontal lobe areas.…”

“…Frontal lobe sulcal configurations that were previously believed to be derived in australopithecines (compared to apes) are represented in some of the newly described chimpanzee brains, and this finding overturns certain published hypotheses about frontal lobe evolution in small-brained hominins. Newly described variations in the superficial representation of two gyri that connect parietal and occipital cortices in chimpanzees and humans contribute to hypotheses about the specific mechanisms that may have been involved in evolutionary reorganization of the caudal part of the brain [Bruner et al, 2017]. Although it is beyond the scope of the present paper, future researchers who apply geometric morphometric approaches to the study of chimpanzee sulci [e.g., Gómez-Robles et al, 2015;Bruner et al, 2017] or wish to construct probability maps that illustrate variation in the locations of various sulci for chimpanzees [e.g., Schenker et al, 2010], as has been done for humans [Iaria and Petrides, 2007], will, of course, require accurate identifications of sulci as an initial step and should, thus, find the above descriptions useful.…”

“…Connolly did not characterize this feature as part of an annectant gyrus (instead describing it as "a triangular area slightly depressed below the level of the neighboring cortex") and it is not clear whether Laz's are medial extensions of 2 (a possibility suggested by the right hemisphere of Iyk, Fig. 5) or simply superficially exposed parts of the cuneus [Bruner et al, 2017]. As noted, Laz's Ls appear to be located relatively caudal compared to the other chimpanzees' in Fig.…”

“…Although it is now known that, contrary to what was reported by Smith [1903], human ancestors lost L at some point during hominin brain evolution [Allen et al, 2006], so far the fossil record of endocasts has been mute on exactly how and when this happened. Was a crescentshaped L pushed caudally by expanding parietooccipital regions, possibly in conjunction with bulging of the parietal region and expansion of the medially located precuneus [Bruner et al, 2017], over an extended period of evolution until it reached a point near the occipital pole and then disappeared? Or did L lose its opercular lid and disappear during an earlier period of hominin evolution as 1 and 2 became fully exposed (recall Laz)?…”