Cranial capacity is seen as an important factor in separating modern from archaic fossils in order to find a better understanding on the origins of the human species (Lieberman et al., 2002). Two hominid species were examined in this report, Homo sapiens and Homo erectus. Provided with 30 Homo erectus specimens with information on their cranial capacities and locale, Graph 1 was plotted indicating the relationship between age of the specimen and its cranial capacity. It is clear from this graph that, by the line of best fit, there is a visible decrease in cranial capacity as the specimens age geologically from newest to oldest. This indicates a pattern among these Homo erectus specimens of an increase in cranial capacity as the hominid fossils become more recent. As a result, it can be stated that there may have been motivation to evolve in order to increase brain size and capacity.
Graph 1: Cranial capacity estimates for Homo erectus specimens. Specimens are ranked in order from most recent geologically (specimen number 1) to oldest (specimen number 30)
In order to examine and compare data pertaining to Homo sapiens one must first distinguish between archaic and modern specimens. Lieberman and colleagues (2002) stated that anatomically modern Homo sapiens are typically characterised due to Day and Stringer’s cranial characteristics aiming to separate them from their archaic counterparts. There is however difficulty in providing an exact description for anatomically modern Homo sapiens as many modern skulls are missing some of these characteristics, and it is therefore best to refer to Homo sapiens as a diverse species consisting of both modern and archaic specimens (Lieberman et al., 2002).
According to a paper written by Leigh (1992), Homo erectus and Homo sapiens show an increase in cranial capacity over time and it must be noted that the patterns in which the crania of these hominid species have developed are to act as the distinguishing factor between the two hominid groups. This indicates that the variation in cranial capacity in these two hominid species follow the same developmental pattern with regard to an increase in size and therefore the graphical representation of Homo sapiens is likely to follow that observed in graph 1.
Morphological patterns of the development of Homo sapiens indicate that there is a characterisation due to smaller brow ridges, the development of chins and the overall cranial appearance that is similar to modern human skulls (Pearson, 2008). This is compared to the characterisation of Homo erectus where there is an overall increase in brain size, a rounded occipital bone and a vertically oriented frontal bone as opposed to a slanted frontal bone in Homo sapiens (Pearson, 2008). It is therefore clear that Homo erectus and Homo sapiens follow different developmental patterns in the evolution of their individual cranial features contributing to cranial capacity, while still following an overall pattern of increased cranial capacity over time based on morphology.
There are many other important variables that contribute to cranial capacity variation. One main variable is that of geographical location of the specimens. Geography encapsulates climatic conditions and differences in stature and weight of hominids. The stature and weight of individuals has an impact on the proportional relation of body mass to brain size. The Homo erectus specimens that were provided are all found in temperate or tropical geographical regions where the climate is warmer and there is an abundance of different faunal and floral species. This may result in patterns associated with cranial development or the overall development and growth of specimens due to the abundance of resources available to provide sufficient nutrition.
In order to test the hypothesis that climate and geographical location affect cranial capacity, Graph 2 was plotted. From this graph it is clearly seen that there is a pattern associated with geographical location and cranial capacity. Although there is a visible decrease in cranial capacities as the specimens age geologically from newest to oldest, there is a clear separation of the lines of best fit for each geographical region. The Indonesian specimens and East African specimens are all found in regions where the climate is seen to be more tropical, while most of the Chinese specimens are found in regions where the climate is more temperate. It should be noted that the age of the specimens is not provided and if it were, it may have an impact on the graph as the lines of best fit for the specimens from similar climates may overlap.
Graph 2: Cranial capacity estimates for 3 specific Homo erectus specimens. The green data represents the Indonesian specimens, the blue data represents the Chinese specimens, and the yellow data represents the East African specimens. Specimens are ranked in order from most recent geologically (specimen number 1) to oldest (specimen number 30)
In 1984, Beals et al. suggested that the bioclimatic model has an impact on cranial morphologies, body size and therefore cranial capacity. It was hypothesized by Beals and colleagues (1984) that due to movement into colder geographical areas, head shape and therefore cranial capacity of hominids were altered. Upon further investigation it was concluded that temperature and climate have a larger effect on the crania of hominids than on the rest of the body and as a result race does not have as great an effect on cranial capacity as that of climate.
It can therefore be stated that, due to the evidence provided in this report, Homo erectus and Homo sapiens follow the same pattern for cranial variation with respect to the gradual increase in cranial capacity over time. The manner in which an increased cranial capacity was obtained in each case however differed as the morphologies of the crania of Homo erectus and Homo sapiens differ.