How Has Evolution Shaped the Differences in Cranium Size between Humans and Neanderthals?
Although Neanderthals and humans share many similarities overall, it is the morphological differences in their crania that baffles many scientists and even causes uncertainty in the two groups taxonomic relationship. Evolution, specifically geographic location and subsequent diet, visual system, and behaviour, have contributed to the distinct differences in cranium size between these two groups. The differences regard the shape of the neurocranium and the degree of mid-facial projection, including Neanderthals possessing larger brow ridges and overall skull size, as well as rounded braincases on the posterior of the cranium (Weaver et al. 2007) (Haviland and Crawford 2009). Although Neanderthal crania were larger, it is important to note that their skulls were proportional to their overall skeletal figure, as Neanderthals possessed bigger bodies than humans.
One contributing factor to this overall dissimilarity in proportion is the geographic location of the groups. Neanderthals inhabited areas of higher latitude than humans and therefore endured colder climates (Haviland and Crawford 2009). This difference in conditions created pressures that selected for body shapes that made thermoregulation easier (Weaver and Steudel-Numbers 2005). To cope with warm weather, humans have slender bodies with lengthy limbs, while Neanderthals posses short limbs and wide bodies to preserve heat in cold climates (Weaver and Steudel-Numbers 2005). The genetic drift that occurs as a result of high latitude geographic isolation between Neanderthals and humans also contributes to their cranium and overall body proportion dissimilarities (Weaver et al. 2007). Along with greater body proportions, the inhabitation of areas with higher latitude resulted in a diet that “consisted primarily of large, terrestrial herbivores” (Hockett 2012). There is an impact on cognition, physical growth, and development, whenever micronutrients are under, or over consumed (Hockett 2012), therefore it is probable that high levels of protein consumption associated with an animal-based diet would have an effect on Neanderthal cranial size and/or shape. Studies by Marlijan Noback and Katerina Harvati, “show significant correlations between global cranial shape and diet” (2015). This relationship between subsistence and cranial shape is most strongly correlated when regarding plant-based diets compared to animal-based diets (Noback and Harvati 2015). Specifically, general cranial size increases with high consumption of animal-based foods, such as the subsistence of Neanderthals, and decreases with consumption of plant based foods, seen in the more varied human diet (Noback and Harvati 2015). It is in this way that diet has contributed to the differences in cranium size observed between humans and Neanderthals.
The relation of human and Neanderthal visual systems is also a factor taken into consideration when analyzing cranium sizes. Neanderthals had larger eyes than modern humans, they would have also had larger visual cortices (Pearce et al 2013). Eye size is related to the primary visual cortex (V1) (de Sousa and Proulx 2014). A larger V1 can process more information efficiently due to more neurons present (de Sousa and Proulx 2014). This clearly demonstrates that eye sizes in each species reflects on the volume of the visual cortices of Neanderthal and modern human craniums, as well as predicted cognitive abilities. It is emphasized that identical brain volume does not completely mean identical organization. This is important when reflecting on Neanderthal and human visual systems. Neanderthals had larger visual systems than modern humans. Modern human endocranial volumes range from 1090-1880cm3, while Neanderthals’ range from 1172-1740cm3 (de Sousa and Proulx 2014). Independent t-tests confirmed that Neanderthals had significantly larger orbits than modern humans, implying that Neanderthals also have significantly larger eyeballs and visual cortices (Pearce et al 2013). The cortex size may also be important for making predictions about perceptual experience (de Sousa and Proulx 2014). Eiluned Pearce and his team (2013) found that Neanderthal brains had larger amounts of neural tissue associated with the brains of modern humans. By analyzing the relation of the optical system, brain structure and cognitive function are interrelated. This is important when investigating the evolution of humans and their cranium size.
Figure 1. The primary visual cortex (V1), as related to eye size.
With the visual system prioritized in Neanderthals, their frontal lobes were smaller in order to compensate. Humans however, had a larger frontal lobe allowing for them to develop many new mental abilities, particularly egocentric and allocentric way finding abilities as well as spatial awareness (Burke 2009), which would increase mobility and territory ranges for the humans. Additionally, “when AMHs developed the ability to maintain spatially extensive social networks and adopted a more mobile lifestyle (i.e., during the course of the MSA) they created conditions under which specific spatial abilities were selected for, thus placing new demands upon the neural substrate” (Burke 2009) allowing for these skills, and new ones, to become commonplace.
References
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