The ancestors of humans shared many similar qualities with the current human. The body shape and function is relatively similar to today’s human. Every trait and part of the human body has evolved in some manner from our ancestors over the course of millions of years. The brain has become far more efficient and complex just as the hand and fingers have, that is to say the nervous system and skeletal system has improved tremendously. Similarly, the hair and eye of the human has changed drastically from that of its ancestors.
Hair, a filament that the skin grows, has undergone a large reduction since the early times of primates in Africa. Additionally, humans feature multiple different types of hair; color, texture and consistency make every human’s hair unique. Similarly, the functionality of the eye has improved extensively, and, it also takes different forms like hair does. Many hypotheses aim to identify exactly why hair and the eye evolved in the way they did.
Scientists postulate the evolution of hair and the eye was a mixture of both natural selection and sexual selection over millions of years.
Early hominids feature full coats of body hair. Chimpanzees and bonobos have dark, frizzy hair that covers all parts but their face. Orangutans have reddish brown hair that covers every part of their body except a few areas on their face. The body hair helps to retain heat in times of cold in a process called thermoregulation. It also protects from ultraviolet exposure, and from trauma such as scrapes from sharp branches or damage from animals and insects.
Over a very long period humans have differentiated from our hominid ancestors. Now humans present a much different application of hair. Most humans do have body hair, but it is significantly different than that of the other hominids. Although it looks like the other hominids have significantly more body hair than humans, it is just the size of the apes’ hair that makes the difference noticeable, not the quantity. Human hair is much smaller. This reduction in size allows for humans to regulate their body temperature more effectively in hot climates (Yesudian 2011). Hominids began to develop other uses for hair as well. Eyebrows, along with eyelashes, ear, and nasal hair, serve primarily as protective measures. Many believe part of the evolution and selection for eyebrows in addition to protecting the eyes from sweat had to do with the ability for communication to improve through the use of eyebrows in emotional expression (Yesudian 2011). Nasal hair developed to protect the nasal canal in a number of different ways and still today is greatly effective. The nasal hairs protect airborne pathogens from entering the nose and traveling towards the lungs. As humans traveled north from Africa, the air became increasingly cool and dry. Nasal hair provides extra humidity to the larynx and lungs upon breathing in dry air to help prevent irritation. Tiny fibres such as these illustrate the functional yet intricate hair in humans.
A subtraction of body hair on humans was naturally selected for. As humans populated the hot climate of Africa, balancing the regulation of their temperature was difficult. With extreme sunlight and humidity without many external temperature controlling agents, humans were forced to evolve. Most importantly, hunting in those conditions was difficult. Humans who had smaller body hairs and more sweat glands became naturally selected for. A reduction in body hair in exchange for an increase in sweat glands allowed for efficient regulation of temperature and longer periods of hunting in the heat, raising their chance of finding prey, thus boosting their rate of survival. Early humans were not the first to develop sweat glands, as they are found in other apes as the main players in the thermoregulatory process, but humans are significantly better at this regulatory process (Kamberov, et al. 2018). Although there is limited evidence to measure this, it is widely believed the density of human sweat glands are the highest out of any ape in history. Thus, humans were able to effectively use sweat to evaporate additional heat off of their bodies. It could be reasoned that other primates never saw a decrease in body hair and development of more sweat glands because they did not require arduous hunting for their dietary needs.
Hominids, who populated Africa first, developed dark skin and dark hair to protect themselves. Higher levels of melanin are found in dark skin and dark hair which helped to prevent from excessive ultraviolet exposure. Darker skin and hair, therefore, was naturally selected for. However, as humans started to migrate out of Africa and further from the equator, there was no need for this type of skin and hair. In fact, darker skin and hair in an area where the sun was weaker made the process of obtaining Vitamin D more difficult. Naturally, whiter skin and hair became more prevalent in the areas further from the equator, as it allowed for quicker Vitamin D absorption. Furthermore, as time goes on and more and more people populate land away from the equator, more mixing of genes occurs, causing varying hair and skin color phenotypes (Balter 2017).
Natural selection certainly affected the evolution of human hair, but most of its variance seen today is due to sexual selection. The benefits of humans having less body hair than other primates and the superior ability to sweat to regulate temperature are legitimate. However, variation in hair and eye color seems to suggest sexual selection is its catalyzer. Northern Europe has a distinctly diverse set of hair and eye colors, and scientists believe extreme hunting conditions on the tundra for males lowered the operational sex ratio(). In the tundra, men hunted over large distances and committed huge amounts of time to capturing and killing large prey. As this task was very difficult and food was scarce, many men failed in providing enough food and died at early ages, creating an unequal ratio of man to woman ratio for mating. The division of labor also changed during this time. Instead of women’s past duties of gathering foods, they spent much more time now preparing the meat that their husband has seized and making temporary shelter. Little foods were unavailable in the cold climate of the tundras. In turn the rate of polygamy decreases as men do not have disposable time to allocate to care for and mate with multiple women. With this unequal ratio of men to women, men had more women to choose from as their mate. Exotic physical characteristics in women yielded a sexual advantage in being chosen as men were drawn to that rare appearance. Lighter skin, hair and brightly colored eyes are the main physical traits that men began to prefer (Hoffecker 2002). Studies support the hypothesis that “rare color advantage” is still very prominent in humans. The less abundant a certain hair color is in a gene pool makes that hair color more attractive to suitors (Thelen 1983: 164).
The most striking characteristic of human hair versus its ancestors is its length. Head hair of human ancestors existed but not to the degree and length it does now. It is estimated humans began developing yard long after their migration out of Africa. There are four primary evolutionary hypotheses that receive a degree of scientific inquiry and support. The first is known as the Aquatic Ape Hypothesis but has little credibility in the scientific world as there is a lack of evidence towards it. The hypothesis states the development of long hair started in women because of its effectiveness in keeping children close in water settings. It presupposes an aquatic phase of ape, where apes spent much of their time in water. With mother’s long hair, children could use the long hair of the mother, an aid which floats on the surface of the water, to hold onto to stay close and afloat. The second hypothesis suggests long head hair showcased good health in ancestral humans, therefore identifying good mates. This logic is flawed because as humans later populated Africa, their hair was not long but rather short and frizzy. Too much is unexplained to recognize this as a viable hypothesis.
Moreover, the Relaxation Selection Hypothesis figures long head hair developed merely because it could. Short hair helped “dissipate body heat and [kept] the brain from overheating” (Frost 2015: 276). Once humans migrated from Africa to colder areas the need for short hair no longer existed. This weakening of selection against long hair creates an ability for human hair to begin to vary in length and texture. Similarly, this hypothesis has contradicting evidence. Humans migrated into “northern Eurasia some 40,000 years ago” (Frost 2015: 276), but this alteration must have taken longer. It is possible for humans to experience rapid change in their code in such a short period of time, but there must be an extremely strong selection for or against a particular gene for this to occur, and hair length does not offer evidence as to why it meets such criteria. The last hypothesis which Charles Darwin writes about is a sexual selection for long head hair in women. Darwin supports his claim that long head hair is valued higher than short head hair much more often as is demonstrated “the works of almost every poet” (Darwin 1936 : 906). He also goes on to admit that every race of human has their own individual preference on beauty which weakens this argument of sexual selection because the less amount of males preferring long head hair in women decreases the rate in which its selection appears in succeeding generations.
Humans have evolved to display numerous different shades of dark, light, red, and even no hair. Scientists purport the ancestors of those with red hair migrated to the Steppes of Central Asia from the Middle East because of the opportunity for herding the land provided. Red hair came about from these people as a mutation of the MC1R gene an estimated 100,000 or so years ago. In search of metal, these people migrate to the Balkans and Western Europe during the Bronze Age. Because of the striking appearance of red hair, people assumed those with red hair were evil, or god like (Johnson 1976). Furthermore, red hair is one of many noticeable genetic mutations to hair. Humans have developed hair diseases such as alopecia where they do not grow hair, or a more common hair loss called pattern baldness. The genetics and evolution of hair from hominids to humans is highly complex which is the case with the evolution of the human eye.
The human eye is of irreducible complexity, a term coined by Michael Behe that means any single part of the eye is as important as the next, which is to say without any single one part of the eye, it would not function as a whole. Because of its unexplainable complexity, many believe an intelligent, godlike being was the ultimate creator of the eye rather than evolution. Nevertheless, using evolutionary theory, the eye must have been evolving for an incredibly long time to get to where it is now, at a state of irreducible complexity. In fact, scientists predict the eye to have been around for over 550 million years. 96% of animals have eyes, proving the importance of sensing light in some way (Jones 2014). The first eye was nowhere near as effective as current eyes. It likely could have detected light to determine behavior based off of day and night cycles. Sight is an invaluable means of survival. One who can see its prey or predator, or a shadow of it, has a significant advantage. The difference between merely detecting light and having the ability to decipher prey from predator is a monumentally important skill for survival. Unlike the evolution of hair, the eye is not nearly as simple, nor does it present as much evidence to follow its logical, evolutionary steps.
The evolutionary steps of the eye from scientific understanding are not perfectly minute, sequential steps. However, they do make sense. At some point before 550 million years ago, a spot on an organism was light sensitive and in some way gave it an advantage for survival. Over time, this patch was randomly deepened into a pit-like structure that allowed for sharper vision. Then, this pit became increasingly narrow to create a tiny hole for light to seep through. Each and every small change in this structure must have “conferred a survival advantage” (Nilsson 2001). The next step was the transformation of this sensitive light patch to a retina of some sort, and the development of a lens. It is possible that the lense developed the convex shape due to a double layered tissue that consisted of liquid (Nilsson 2001). As the eye improved for animals, their other abilities did too. Animals developed the ability to use muscles to control the eye as opposed to it moving involuntarily. Over long periods of time, the lenses improve to allow photoreceptors to organize light to make sense out of it. Different species developed different shapes of eyes to best suit their survival needs.
Today it seems almost every person has a different eye color than the next person. Humans had brown eyes originally until a genetic mutation in the OCA2 gene that reduced the production of melanin. A reduction of melanin in the iris diluted the brown coloration to blue. Green eyes, however, are somewhere between a lot and little amount of melanin in the iris. The first human to have the mutation of blue eyes started the process that demonstrates how exactly evolution works. A mutation occurs, nature proves it is advantageous, then it is passed on. In this case, blue eyes correlated with lighter toned individuals who thrived in colder climates where the amount of Vitamin D from the sun was slim. Additionally, as explained earlier, sexual selection rapidly increased the number of people blue eyes over generations. Eye coloration is neither a positive or negative mutation so it does not have any measurable performance differences, thus proving sexual selection was its biggest influencer (Eiberg, et al. 2008).
The physical appearance of hominids to early humans and to modern humans is very noticeably different. Human migration out of Africa coupled with faulty genetics left the future of homo sapien appearance in the hands of nature. With all the research that has been done, it is still impossible to take the theories explaining why hair and the eye evolved in the way they did simply because of how long ago this evolution took place. Even the father of evolution, Charles Darwin, admittedly did not have a satisfying explanation to the evolution of the eye. What he did get right, was the influence sexual selection has on evolution. Still, many questions exist regarding the evolution of hair and the eye. If hair dissipated for heat regulation, then will humans get continuously less hairy because of the prevalence of effective clothing? Will we lose sweat glands as cooling systems improve? Although the answers will never arise, scientists will continue to work strenuously to learn as much as possible. The only way to effect the future of humanity is by first learning how humans function over time.