There is controversy around the mechanisms that guided the change in brain shape during the evolution of modern humans. It has long been held that different cortical areas evolved independently from each other to develop their unique functional specializations.
Some recent studies suggest that high integration between different cortical areas could facilitate the emergence of equally extreme, highly specialized brain functions. This study analyses the evolution of brain shape in primates using three-dimensional geometric morphometrics of endocasts.
The aim is to determine,
- firstly, whether modern humans present unique developmental patterns of covariation between brain cortical areas; and
- secondly, whether hominins experienced unusually high rates of evolution in brain covariation as compared to other primates.
The analyses including modern humans and other extant great apes at different developmental stages demonstrates that, unlike our closest living relatives, Homo sapiens retain high levels of covariation between cortical areas into adulthood. Among the other great apes, high levels of covariation are only found in immature individuals.
Secondly, at the macro-evolutionary level, the analysis shows that strong covariation between different areas of the brain in H. sapiens and Homo neanderthalensis evolved under distinctly higher evolutionary rates than in any other primate, suggesting that natural selection favoured a greatly integrated brain in both species.
These results hold when extinct species are excluded and allometric effects are accounted for.
These findings demonstrate that high covariation in the brain may have played a critical role in the evolution of unique cognitive capacities and complex behaviours in both modern humans and Neanderthals.
a, Postnatal growth stages for H.sapiens, P.troglodytes, G.gorilla (only stages 4 and adult) and r-PLS values per ontogenetic stage. The meshes (lower left corner) refer to the average and are coloured according to the magnitude of integration.
b, Pairwise r-PLS values between brain modules in adult H.sapiens, P.troglodytes and G.gorilla.
c, Comparison of r-PLS values per ontogenetic stage between H.sapiens and P.troglodytes calculated using the NR-PLS (r-PLS calculated between N-Core and R-Core modules) approach, that does not require the a priori definition of brain modules.
a, CR (covariance ratio; a measure of the overall covariation between modules divided by the overall covariation within modules) Distribution of CR evolutionary rates within Hominoidea. The black vertical line represents the average rate of CR evolution calculated over the entire primate tree, orange dots indicate internal nodes in the phylogeny. Australopithecus africanus represents the Australopithecina subtribe.
b, Evolutionary patterns of morphological integration within Homo. Magenta shades indicate a slowdown in the CR rate of evolution, the cyan shade indicates acceleration. Brain meshes represent the average shape for H. sapiens plus H. neanderthalensis and all Homo species, respectively. The CR values are mapped over the endocast mesh.
Within Hominoidea, H.neanderthalensis and H.sapiens show the highest rate of evolution of brain covariation. Interestingly, Australopithecina (A.africanus + P.boisei, grouped as one) was characterized by evolutionary rates comparable to those of P.troglodytes suggesting a graded trend for increased rate of CR evolution among hominins, leading to the highly integrated brain of Homo, especially evident in the parietal area.
H.sapiens and the other great apes share high covariation between different cortical areas of the brain throughout most postnatal development. However, only H.sapiens retains such strong morphological integration into adulthood. This finding is consistent with other reports indicating that the cortical areas of the human brain are tightly integrated throughout the adult life. Connectome analysis suggests an evolutionary shift in the human brain to enhance global network integration over that of the chimpanzee, indicating that humans evolved strong covariation even among spatially distant brain regions. This evolutionary pattern seems to have deep evolutionary roots. Hominins show a trend for an increased magnitude of covariation between different brain regions, escalating through Middle to Late Pleistocene human species (H.sapiens and H.neanderthalensis).
This finding contradicts the common perception that functional specialization in the modern human brain arises from a modular architecture (for example, semi-independent evolution of different cortical areas) but is in agreement with studies of encephalized non-mammalian vertebrates suggesting that high integration may drive functional specialization in the brain, even among distantly related taxa and under very different selective scenarios. These findings similarly suggest that coordinated changes in brain shape may have played a major role in maintaining the functional association between brain subunits, ultimately leading to the derived cognitive specialization observed in Homo.
The conclusion proposes that the persistence of high levels of morphological covariation into adulthood in modern humans and Neanderthals is linked to the evolution of derived cognitive abilities.
In addition, modern humans show high levels of integration between cortical areas throughout development. Strong covariation in adult brains shared by Neanderthals and H.sapiens, suggests Neanderthals followed the same developmental path as ours.
Neural plasticity and innovative–explorative behaviours are typically associated with juvenile life stages, as well as the extension of childhood learning and are central to Mithen’s theory of cognitive fluidity which postulates that only modern humans are capable of fully integrating diverse dominions of knowledge. Our evidence supports the argument that juvenilization of the human brain (and possibly to some extent the Neanderthal brain) was driven by prolonged brain growth, mediated by the retention of an unusually high degree of covariation between the different brain units into adulthood.
This remarkable conclusion is further discussed in “Extended youth identified in the human brain, but we are not alone“. It states: the brains of humans and Neanderthals evolved much faster than those of other primates and that the different regions or lobes of human and Neanderthal brains are highly coordinated during growth, whereas this condition is only met by immature individuals across other primate species. This may have played an important role in delivering our high intelligence and capacity for abstract thought. It also suggests that Neanderthals may have been more like us than we used to think.
Our big brains, are the result of a “Peter Pan syndrome”, they never really grow up. Concepts underpinned by a “Peter Pan syndrome” gained interest within the field of evolutionary biology some half a century ago, when great scholars such as Stephen Gould, Ashley Montagu, and Lewis Wolpert championed the view that the human brain is, in fact, a juvenilized, always eager to learn version of the ordinary ape brain.
Juvenilisation refers to the retention of immature traits through to sexual maturity or beyond, and is universally recognized as a potent evolutionary force. In the context of human evolution, although strongly criticized in the past, and variously dubbed a myth or a bankrupt concept, the Peter Pan syndrome is still widely acknowledged, although it remains extremely difficult to prove.
The finding also lends support to the famous, yet hard to prove theory of the great British archaeologist Steven Mithen, who posited that humans possess a unique degree of cognitive fluidity, with a singular capacity to mix different domains of thought. “The human mind is especially creative, able to mix abstract thoughts generated in different domains of intelligence into new combinations which provide new and often unanticipated affordance and goals. We suggest that the high coordination between different brain areas identified in our study may be the mechanism underpinning Mithen’s cognitive fluidity hypothesis” commented Pasquale Raia, who was involved in the research.
Finding how much the ‘perpetual youth’ parabola made us the Faust of the animal kingdom, presents an exciting and promising future research agenda.
For completeness, there is one more interesting review of the article at https://theconversation.com/human-and-neanderthal-brains-have-a-surprising-youthful-quality-in-common-new-research-finds-191594
And last but not least ….
Surtil : 