Introduction and background
Pearce and Dunbar’s 2011 publication aims to establish a direct, clear, traceable relationship between human orbital size and latitude at which these individuals lived. Their argument is primarily based on the assumption that eye size changes as a direct response to the available photopic light in an effort to increase visual acuity in low-light conditions. The second anatomical correlation they make is that an increase in the size of a human orbital is clearly indicative of an increase in the actual volume of the eyeball it encases. Furthermore, they propose that this direct scaling extends to the brain as a bigger eye would feed more information to the brain, thereby creating a need for a larger visual cortex. Thus, if their theories hold through, orbital size, eye volume, and brain size would all be clearly and inextricably linked to latitudes via their corresponding light levels.
Their first assumption regarding eye size in response to available photopic light falls in line with the widely accepted theory in evolutionary literature. While it is largely accepted that eye size corresponds to an animal’s main hours of activity, that is, whether it is nocturnal or diurnal, Howland et al. (2004) established that the connections between eye size and behaviors like nocturnality and predation had not yet been tested or explored and it remained so until 2006 when Kirk examined and compared eye volumes and orbital sizes among nocturnal and diurnal primates and determined that although relative orbital aperture size does vary, relative eye size does not. Rather than being dependent on light availability, eye size appears to be more closely tied to the necessary activity needed to maintain diet. Nocturnal predators that relied on visual cues, were indeed found to have larger eyes than their diurnal counterparts, but this was not true for nocturnal frugivores whose relative eye size was found to be the same as diurnal frugivores. Furthermore, Kirk (2006) argues that orbital size is not entirely reliant on eye volume and that the size of the cornea and the orientation of the orbit are important factors in orbit size that are often ignored. Because his research determines orbit size and activity pattern correlations to be weak, he advises that orbital aperture measurements should be used with caution when reconstructing fossil habitats or patterns of activity.
The human diet which is often thought of to have been revolutionized by our ability to outlast other animals during persistence hunting, is also proposed as a mechanism for the selection of the eye (Heard-Booth and Kirk 2012). Leuckart’s Law which states that larger eyes are selected for in animals capable of traveling at great speeds in order to increase visual acuity to maintain awareness of their surroundings, had been based on the behaviors of birds. Heard-Booth and Kirk’s (2012) study established that the relationship also held true when examining mammalian eyes. However, according to their data, when using the eye’s axial diameter for measurement and established maximum speeds during endurance running and sprinting, humans would be expected to have an eye of approximately 19.89 mm. However, the average human eye is around 23.3 mm which is around the size of the eye of the Canis lupus (gray wolf) or Erythrocebus patas (patas monkey) which are capable of traveling 64 km/hr and 55 km/hr respectively, compared to the maximum human sprint 36.7 km/hr. Thus, the size and scaling of the human eye according to expected velocities has been largely problematic in the general mammal trends.
Meyer et al. (2014) furthermore demonstrated a weak but present correlation between latitude and raw orbit size when compared by cross-sectional area. In addition, they criticized Pearce and Dunbar for their use of the area of the foramen magnum as a control for body size when previous literature has found this relationship to yield inadequate measurements (Martin 1980). However, Meyer et al. used this measurement for consistency to yield more comparable results. Interestingly, their data lead them to conclude that latitude and orbit size are actually negatively correlated.
It is also important to mention that larger eyes in themselves do not directly lead to increased visual acuity. Eyes adapted with larger corneas and lenses are more adequately equipped to really improve vision (Kirk 2006).
Research Question and Hypothesis
Due to the ambiguity of these theories about the evolution of the eye as well as the under sampling of populations in the southern latitudes, this project aims to study populations native to the region of South Africa in hopes of clarifying these relationships. I hypothesize that the relationship between latitude and orbital size and eye volume will not be direct and traceable because of the wide array of time periods in which the people are most active. Because of this, I also do not expect to find a smaller average cranial capacity when compared to other more extreme populations as there is no need for additional visual input processing centers. I also believe that due to the energetic expenses surrounding the development and maintenance of a larger brain, available photopic light in itself would not be a strong enough factor for a significant increase in brain size, especially in the harsh environments that are typical of latitudinal extremes as human eyes are already generally very well adapted to be sensitive to light in low-light conditions.
Methods
This project has obtained permission to examine the skulls of native South African populations in the collections at the Iziko Museums in Cape Town. To examine cranial vault capacity for comparison, I will be funneling millet seed and measuring its volume with a large graduated cylinder. To avoid factoring in the volumes of other cavities, I will be using cotton balls to block smaller apertures and plastic wrap to bridge any missing skull bone. For consistency with previous experiments, body size will be controlled for using the cross-sectional area of the foramen magnum. In addition to these measurements, I will use a light meter to track available light during hours of peak activity in populations around South Africa.
Budget and Resources
This project will require a flight to South Africa and subsequent trips around the country to examine the times and types of peak activity of different native peoples especially those that partake in certain kinds of traditional hunting activities. This will attempt to help me rebuild the type of environment and sustenance activities that older humans may have encountered that could have influenced some regional variations in eye size and cranial capacity. By measuring light levels, I hope to better inform the arguments surrounding the evolutionary response to low-light environments.
Simple equipment will be used to conduct these measurements as there is the issue of transporting it to South Africa and then transporting it while collecting data in the field as our short expedition cannot rely on our obtaining these tools in the country in a limited time span. Millet seed might have to be transported as well for this reason. We also had to purchase a graduated cylinder, cotton balls, and plastic wrap for this project as well as smaller tools like a brush to gently clean millet seed from our specimens.
Overall, transportation and lodging is expected to account for the majority of the financial resources used in this 20-day trip.
Statement of Intellectual Merit
This project aims to complete and expand upon the existing literature on the effect of increasing latitude and its associated decrease in photopic light on the size of modern eyes and the visual cortex needed for visual processing. In doing so, it hopes to clarify the conditions under which the eye is susceptible to change. This will aid other paleoanthropologists in reconstructing ancient habitats and patterns of behavior as our current understanding of behaviors and observed morphologies may be incorrect.
Statement of Significance and Broader Impact
The broader implications of this project are significant in terms of the relatively recent use of electricity to allow for activity outside of Homo sapiens’ typical time range of activity. If our eyes are in fact extremely sensitive to light levels, our increased reliance on technology emitting artificial light may have undiscovered deleterious consequences for an individual during the span of a lifetime. It may also be slowly changing the eye structure for future generations as well. Already there have been studies on the effect of the “blue light” in lit screens on the circadian rhythms of its user. It is important to understand our past in order to prepare for and perhaps solve problems we might encounter in the future.