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Human Cranial Evolution is the study of how the human cranium, or skull, has changed since the existence of the human being through evolutionary processes of natural selection, and neutral evolution, to stress the two main forces.
The human cranium is composed of distinct bones that encompass the brain. Often in evolutionary contexts it is studied with regard to its three distinct regions: the cranial base, the cranial vault, and the face.
In terms of human evolution, the cranium is studied extensively because, if used with caution, some cranial traits can lend insight into developing the human phylogenetic tree (or evolutionary history of life).
Studies have shown that the evolution of the cranium is attributable to the mechanism of natural selection, where as other studies have shown that it is due to neutral evolution. Natural Selection, as first proposed by Charles Darwin in his famous publication, “On the Origin of Species,” is the mechanism by which evolution occurs between generations because some individuals are more evolutionarily fit than are others in a population. These fit individuals have better reproductive success, produce more offspring, and pass their traits to successive generations more than less fit individuals. Certain features of the cranium may be selected for in certain environments, and through this selection the human cranium evolves.
Neutral theory, on the other hand, is a mechanism of evolution that does not depend upon selection, but on random, or stochastic evolutionary events. Selectively neutral genes, those that are neither helpful nor harmful to an organism, either increase or decrease in frequency in a population. This means that by pure chance, characteristics of the human cranium that are correlated with neutral genes will either increase or decrease in frequency. And in this light, the cranium also evolves. Interestingly, the evolution of the human cranium has been explained using these two completely opposing mechanisms of evolution. Specific examples of both will be outlined below.
Natural Selection of the Nasal Index
As one requirement of natural selection stands, in order for evolution to occur, traits must both vary in a population, and be heritable. Some external facial traits, such as the nasal index, have substantial degrees of heritability. This heritability suggests that these facial shapes could be controlled by selection. For example, nasal height and width are “among-region” traits for which differences are correlated to differences in climate. Morphological differences in nasal index strongly correlate with differences of minimum annual temperatures between populations. Individuals who live in extremely cold environments, like Siberia, vary in fitness according to their nasal indexes.
Based upon “thermoregulatory breathing hypotheses,” it is thought that narrower, longer noses will be selected for, so that individuals can breathe adequately, and efficiently. In other words, differences in both nasal height, NLH, and nasal breadth are correlated to differences in climate across global populations.
Individuals in extremely cold climates, therefore, were found to have longer, narrower noses than did those of warmer climates, who had shorter, broader noses. The nasal index serves as one example of natural selection acting on cranial morphology. However, this may not be exclusive to the nasal index. Cranial traits in general may be subject to selection that dictates regionally specific morphologies.
Neutral Evolution of the Temporal Bone
Different morphological cranial differences in populations are proportional to genetic distances. In this case, neutral genetics are responsible for cranial evolution. In comparison to other “equivalent cranial units,” the temporal bone is the most neutrally controlled bone of the human cranium. The temporal bone is a part of the cranial base, or basicranium, a structure that is believe to have been preserved over human history.
When a Mantel test was used to compare landmark cranial distances and genetic distances, the temporal bone was the one of at least five cranial bones that was most significantly correlated to genetics. This means that shape changes correlated to genetic changes, and therefore, genetics can sufficiently explain shape change (as compared to before, when selection explained shape change). In this respect, we can suspect that climatic and selective forces do not lead to temporal bone change because as it is so strongly, genetically controlled, it is less susceptible to other evolutionary forces. Also, because these genes are neutral in terms of fitness, selection would be unable to act on such traits, because variation in the traits does not correlate with variation in reproductive success and/or survival.
Selection vs. Neutrality
As there exists specific evidence for natural selection and neutral evolution in terms of the human cranium, it is important to consider both mechanisms of evolution. When studying human history, and building phylogenies, however, one must choose traits carefully. Genetically controlled traits serve as accurate characters for phylogenetic trees because they are limited to variations of base pairs; unlike traits that are selected for or against, neutral genes are not subject to the many contributing factors of natural selection that can create confusion.
That being said, neutral cranial traits are useful for understanding population histories. Likewise, selection-derived cranial traits are useful for understanding external traits of the human face, to name one example. It is most important to recognize the convoluted nature, and mechanism of human cranial evolution, and that cranial morphology is sometimes the result of several evolutionary forces in action.
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