Why do Europeans, North Asians and Native Americans have pale skin?

Skin colour is determined by the amount and type of the pigment melanin within it. This can be generated in small amounts by sun exposure but is nearly exclusively genetically influenced and therefore ancestry is the major contributor to the shade of an individual’s skin. Melanin in the skin absorbs and disperses 99.9% of UV in sunlight as heat. Without it, the UV radiation would generate direct and indirect damage to the DNA of living cells and the malfunction of their gene product, potentially damaging the health of the organism, namely through sunburn and skin cancer.

It is useful to consider the evolution of human pigmentation into the pale skinned populations of Europe, Asia and Native America as occurring in two stages: the first being from the pale pigments of apes and primates into the dark skinned early African humans and then paler again as humans migrated north, away from Africa. The early hairless humans evolved around 4.5 to 2 million years ago in the rain forests of Africa. They first became hairless to combat extreme heat through sweating and sweat evaporation. They moved out onto the East African savannah where they were exposed to much more sun and so more to the risks of UV exposure. This appears to be the first time a major selection pressure was imposed of the skin colour of the human race, although many different theories have been suggested as to the key selection pressure, initial thoughts were varied. One such was the risks of skin cancer and its development causing a selection pressure. However most skin cancers do not develop until later life (after many years of exposure) certainly after the age of child bearing and so would not reduce or remove the low melanin genes from the gene pool. Another suggestion was that sun burn to the nipple area prevented sufficient breast feeding for the young of low melanin individuals and thus their survival was less likely than high melanin individuals. However the greatest and most accepted selection pressure that resulted in the darkening skin of early humans is to do with sunlight’s affect on Folate levels. Very little sun exposure in pale skinned individuals can reduce Folate levels in the skin significantly. Folate has a large role in the development of foetal nervous system and bone marrow, and in sperm and red blood cell production. Pale skin, high sun exposure and therefore low Folate levels, decreases the fertility of males and developing embryos are less likely to reach full development; a huge selection pressure. The advantage of being dark skinned in Africa evolved quickly in early humans; and the trait is still seen today in African ancestries.

However, as well as being dangerous to our health, sunlight is also necessary for the production of Vitamin D in our skin. Vitamin D plays a vital role in the regulation and control of calcium and phosphate uptake and bone growth. Human skin cells have the ability to produce Vitamin D endogenously using energy from sunlight. In the diet of the early hunter-gather humans in Africa there was no need to allow sunlight into skin for the biosynthesis of vitamin D, as there was enough consumed from fish caught to maintain life processes. In fact it has been hypothesised that settlements in equatorial areas such as Africa could not have established without man’s ability to fish, and so maintain exogenous supplies of Vitamin D as skin evolved darker. This was also the reason early European settlers were able to survive until human skin pigmentation return to a paler shade around 6,000 to 12,000 years ago with dark skin, when man migrated north 40,000 years ago. It was only at this time, when farming arose as the main source of food, that levels of vitamin D consumed in the human diet decreased significantly. This presented a trade-off between reducing melanin in the skin allowing more sunlight and UV into the skin cells to produce Vitamin D, with the problems this presented in fertility and foetal development; a balancing act if you like. In Europe, where the average exposure to sun was much less than Africa, the paler skinned individuals of the population allowed more sunlight to penetrate the skin and therefore could internally produce more Vitamin D. This made them healthier, with stronger bones, and gave them a survival advantage over darker skinned individuals. The balance only swung in favour of paler skin here because the sunlight intensity was less than in more southerly areas and the ability to produce Vitamin D greatly outweighed the negative effects of reducing melanin in the skin. In fact, the further north the population was, the less sun exposure they experienced and the further the balance swung. In the far north there were nearly negligible risks of reduced Folate due to the lack of sunlight, and the biggest selection pressure was on those who could extract the most energy, from the little sunlight there was, for Vitamin D production; i.e. the paler in the population. This is reflected in modern European ancestry patterns of skin pigmentation. Celtic and Scandinavian population are very pale with low melanin as their environments had low sun exposure most of the time; whereas the lower European populations, such as the Spanish and Italians, where sun exposure is greater, have darker skin with more melanin enabling them to produce Vitamin D but also giving them greater protection against reduced Folate.

Northern Asians and Native Americans reside in a similar, or the same latitude, as Europeans (around 47O N) and their origins from the common early ancestors in Africa mean that the explanation for the pale colour of their skin is almost identical: the balance between using UV for Vitamin D production against the risks of reducing Folate. However, although pale compared to early humans and more southerly populations, Northern Asians and Native Americans are still darker than the people of Europe. One suggested reason for this has been that Asians and Native Americans consume more fish in their diets than Europeans. This is certainly the believed wisdom for the explanation of the dark skin of the Inuit population, who live at more northerly latitudes than almost all Europeans and yet have darker skin. The high fish diet appears to provide enough Vitamin D to compensate for the lesser UV sunlight without the need to evolve lighter skin. However, many Northern Asians and Native Americans settle in areas that are land locked and have very little fish in the diet, and vice versa many of the palest of the European population, for example Swedes and Danes, have very high fish consumption in their diet, yet have still evolved pale skin.

One suggested reason I found to be plausible is based on the central premise of sexual selection in mates. Research into mating preferences and perceptions of attractiveness pre-industrially has shown a “cross-cultural preference for lighter (skinned) women” (as concluded by van den Berge & Frost, 1986). Where man was capable of farming in the warmer climate of Sub-Saharan Africa, women were largely involved in the cultivation and production of food. This left men with more time and more capability to seek and provide for multiple wives, leading to a highly polygamous culture. This left few unmated individuals and very little preferential choice in mates which ultimately caused the unusual and unnecessarily dark pigment of Sub-Saharan Africans. When humans moved north into colder climes, the land supported less life and hunter-gatherers had to go further and in harsher conditions to get food. This meant that men had to work a lot harder to provide for multiple wives and many children, and for this reason European men became less-polygamous. The increased risks of travelling further in harsh conditions and the increased risk of running out of food also meant the male mortality rate was higher. These factors together increased the number of unmated men and women in the population and hence increased the choice available for preferential mating, which as previously discussed, was for paler women. In the majority of Europe, especially Scandinavia, Britain and North East Europe, the polar caps stretched further south 6000 years ago than today and the conditions for those living here were significantly harsher. This meant that particularly here, the choice in mating was much greater and the population became noticeably paler than in comparable latitudes. This is still evident in the extremely pale population of Celtic and Scandinavian ancestry.

The question still remains though; if the conditions are no longer this harsh in modern Europe, and probably haven’t been for a good few thousand years, why have these populations remained so pale, especially with such an obvious selection pressure as the risks to health of UV damage, such as sun burn and skin cancer?

Epithelioma (a type of skin cancer) prevalence correlates very well with latitude of settlement and therefore UV exposure, and also correlates well with areas of the body which are most exposed, for example the face and hands. The darker a person’s skin is naturally, the less chance they have that UV exposure will cause damage resulting in these epitheliomas. The reason this has not been experienced as a selection pressure may be the point I have already touched on: the average age of cancer onset is post child bearing age and so any pale skinned individuals that die due to UV induced cancers are likely to have already passed on their genetic material. In addition humans are unique in their ability to adapt not just by genetic mutation, but by cultural and social evolution, and it is my view that this is relevant to the pigmentation of these populations. For example humans clothe themselves and so reduce sun exposure to the skin by this method, removing the need to genetically evolve to reduce UV exposure. I feel this may contribute to a widely backed idea that populations reach an adaptive plateau, where they become well adapted enough to their habitat and environment that little further evolution occurs. For example, the early Scandinavians faced extreme challenges in their environment and so adapted very well to the harsh conditions, such as they’re paler skin. When the environment became less harsh they possessed characteristics that posed no disadvantage, and together with the human ability to culturally adapt to any further small changes in conditions, the requirements for them to evolve further were absent.

In conclusion, the evolution of pale skin in European, Northern Asian and Native Americans as humans migrated out of Africa was a result primarily of an evolutionary trade-off between the need for Vitamin D and prevention of the loss of Folate, and secondarily, the sexual preference and ability to choose mates that were of pale skin.

Why do Europeans, North Asians and Native Americans have pale skin?

There is significant scientific evidence to suggest that Europeans, North Asians and North Americans are all descended from Africa. The Out Of Africa model suggests that homo sapiens arose in Africa and then migrated to other areas of the world to replace other hominid species such as homo erectus, and the Multiregional Continuity model suggests that hominid species that left Africa then evolved into homo sapiens all around the world. Whichever view is taken, it remains that homo sapiens descended from groups that lived in Africa. These groups almost certainly had dark skin to protect themselves from the strong tropical sun in Africa, and so there poses a question, why now do Europeans, North Asians and North Americans have pale skin, a clear differential from the dark skin of their ancestors.

There is one main hypothesis as to why this differentiation occurs. The basis of this answer lies within vitamin D. Vitamin D is needed for effective bone mineralisation and when it is deficient, bone softening diseases such as rickets and osteomalacia in adults occur, and possibly osteoporosis. These diseases cause the legs to be deformed, making walking harder. The thoracic cage also becomes deformed, causing more effort to be exerted in order to breathe. However, the most important problem with this disease in terms of evolution is pelvic deformations, where the pelvic bones collapse together, so childbirth and therefore reproduction is prevented from occurring. In countries more northern than Africa, the protective dark skin useful in Africa will not let enough of the weaker sun in, and so deficiencies could occur. The gene for skin colour has been identified in Europeans and Africans as SLC24A5, with one allele being present in 98% Europeans and the other allele being present in most Africans and East Asians. The gene for skin colour in North Asians is still being investigated.

Recent evidence has uncovered that in fact human skin stayed dark for 30 000 years after the migration out of Africa, when it was originally thought light skin occurred when they first moved. Therefore, this theory about vitamin D is flawed. There are theories as to why there was such a long gap. One of them is that at first humans were hunter-gatherers, herders and fishers, and so their diet included enough vitamin D not to cause any problems. However, at around the same time as the skin colour change occurred, farming became a new way of life and expanded, which did not provide enough vitamin D in the diet. Farming allowed for much larger population sizes to live together, and because mutations are rare, larger populations give a higher chance of a mutation occurring, and that that mutation is actually useful. A mutation is also more likely to occur in one of these populations because they are living in a challenging environment (not enough UV) that causes selection pressures. These new challenging environments and a growing population cause accelerated human evolution, and in this example it would be a lightening of skin colour. Another factor in this could be cultural for example heavier clothing being worn, and so decreased the area available for absorption of UV.

There are other theories however, as many people think that pale skin is not an advantage in most environments in the higher latitudes, due to the vulnerability to sunburn and malignant melanomas, and so do not think that pale skin evolved due to natural selection. The two presented here are sexual selection and parental selection.

Another theory about how lighter skin has come about is that there has been a common selective force that acted on three characteristics – skin, hair and eye colour - in the northern latitudes. This force has been suggested to be sexual selection, and could have acted on an existing sexual dimorphism of men generally being browner and women being fairer. The selection is seen in all upper classes around the world of having a preference for fairer skinned females to marry. Because of this, upper class men may choose lighter females and so over generations the class lightens. Some believe that this sexual selection for light skin counteracts natural selection for dark skin. The difference in skin colour geographically may be due to a balance between natural and sexual selection, so suggesting that it is a combination of both factors. This balance differs according to the latitude, so at low latitudes dark skin is prevalent as natural selection overrides the male preference for light skin. But at higher latitudes where natural selection ceases to act, sexual selection becomes more important, and causes light-skinned populations with the females lighter than the males.

Another factor that is argued by Judith Harris is parental selection, mainly maternal selection. In many cultures a form of birth control was infanticide, where the child was killed after birth. The most recent example of a culture in which this takes place is the !Kung. This way of living gave mothers the power to exert an influence on evolution by deciding to keep or abandon a new born. A decision may be made before birth to abandon the child due to bad timing as the previous child may not have been weaned, or food was scarce, or there was no partner to provide for the baby. However, once the baby is born, this decision may be overturned depending on the reaction to the newborn itself. The newborn may have been sickly, weak, the wrong sex, or have a congenital malformation. However the decision may go the other way if the baby is especially beautiful, conforming to the mother’s standards of beauty. Although this decision may only be made in a small percentage, this all adds up and Harris argues that it is big enough to exert an influence on the characteristics of our species. Harris proposes that hairlessness and pale skin colour seen in northern countries is partly the result of parental selection. She accepts that sexual selection also played a part but that it was parental selection that speeded the evolutionary process up, producing very noticeable changes in a relatively short space of time. She combines the concept of hairlessness and pale skinned because a hairless pale human would be very exposed to the dangers of the sun, and so she believes there must be some other reason why so much of the world’s population is not adapted as best as it can to the environment. The concept of beauty that may sway a mother to keep her baby is of a pale skinned baby. This may be linked to sexual selection, as parents would want a pale skinned daughter so that she can have more choice of husband, and so passing on desirable characteristics that the parents will have as grandchildren.

It appears that there is a difference in opinion regarding how pale skin came about. Unfortunately, evidence to prove either theory is hard if not impossible to come by, and so there is a possibility we will never know. However, in my opinion, I believe that it could be a combination of all three, but with the main influencing factor being natural selection. This is because i don’t believe that such a huge change could come across just through sexual or parental selection alone. Along with this, there is not concrete evidence to support the sexual selection claim and one study has shown that there is no evidence for a correlation between increasing distance from the equator and increased sexual dimorphism.


References:
Skin colour evolution in Europeans and social skin colour vs disease in Puerto Ricans, Dienekes Anthropology Blog
Sexual selection as a cause of human skin colour variation: Darwin's hypothesis revisited by Aoki K, Pubmed.
Human skin colour dimorphism, Kelly and Madrigal, Department of Anthropology, University of South Florida
Judith Harris, Parental Selection: A Third Selection Process in the Evolution of Human Hairlessness and Skin Color

Why do Europeans, North Asians and Native Americans have pale skins?

The first hominid species, Homo habilis, is believed to have evolved around 2 million years ago in East Africa, The modern human, Homo sapiens, is believed to have evolved from an earlier hominid species, Homo erectus, and quickly colonized much of the African continent. As Homo sapiens emerged, groups of individuals spread slowly around 60,000 years ago to other geographically close by regions through what can be described as the Out of Africa hypothesis. Different tribes adapted to their new habitats through evolutionary changes. Most notable appearance-wise is skin colour, which is at its darkest in individuals living at the equator and palest in those living nearest to the poles. Even if humans did evolve, as suggested by Milford Wolpoff and his associates in their Multiregional hypothesis, separately and independently, the same evolutionary mutations would hold true as to the reasoning behind varying skin colouration. Pigmentation arises because of melanin, a pigment that comes in two types: pheomelanin (red, and found predominantly in freckles and reddened areas such as the lips) and eumelanin (very dark brown). Both the amount and type are determined by four to six genes which operate under incomplete dominance. One copy of each of those genes is inherited from each parent. Each gene comes in several alleles, resulting in the great variety of different skin tones. So what purpose does this pigmentation serve?

Unlike us, many of our hairier mammalian cousins do not have such a pigmented skin; by having a hairy coat, they are protected from UV radiation-induced damage as the hairs themselves absorb or reflect most-short wavelength solar radiation. Non-human mammals that are active in hotter, sunnier climates tend to be sparsely coated in order to facilitate passive heat loss (and sweating is limited to certain areas). Unsurprisingly, they tend to show increased melanin concentrations in the skin along their backs where they are exposed to more sunlight. Our primate ancestors would also have had thick hair and hence pale skin.

Early hominids in Africa were active hunters and gatherers and as such, it was an evolutionary advantage to have less hair and more exocrine glands for increased sweat production, allowing early man to pursue prey and be active in the sun for longer without overheating. Having virtually naked skin, it was imperative that hair loss had to be coupled with increasing melanin production in order to protect underlying skin cells from the effects of over-exposure to UV radiation (which can cause mutation in underlying cells and hence lead to melanomas and cancer) by absorbing and dispersing it as infrared radiation (felt as heat).

A widely accepted hypothesis regarding variations in pigmentation is that of Vitamin D Biosynthesis. It is known that sunlight, particularly ultraviolet rays, stimulate the production of vitamin D, which is partly responsible for calcium uptake in bones. A deficiency of vitamin D can lead to once-common conditions such as rickets, in which bones do not form properly and become misshapen. Heavily pigmented skin in areas of high sunlight exposure not only provides excellent protection from UV radiation, but can also limit vitamin D biosynthesis and hence reduce the risk of developing toxicity. Too much vitamin D can lead to hypercalcaemia followed by nausea, vomiting, weakness, polyuria and polydipsia.

UV exposure decreases the further south or north one travels from the equator due to sunlight having to travel through more ozone and atmospheric gases (because of the tilt of the Earth's axis) and hence, an individual with heavily pigmented skin would struggle to produce enough vitamin D. Such conditions would favour slightly paler skin as adequate protection from sunlight is achieved, whilst maintaining favourable levels of vitamin D₃ synthesis. The first European settlers would have been quite dark in complexion, and with a considerably lower exposure level of UV radiation, vitamin D synthesis was impeded. An American study carried out by the the United States Department of Agriculture found “87% of African Americans to be Vitamin D deficient”. To address this issue, some countries (particularly Canada and the USA) have programmes to ensure fortification of milk with vitamin D.

Also contributing to evolutionary adaptations of skin colour is the folate hypothesis. This suggests that folate (or folic acid), which is vital for the synthesis, repair and expression of DNA. It is also essential in red blood cell and sperm production (and as such its presence is quite necessary for human reproduction) Darker skin helps protect folate from UV-induced photolysis and hence helps maintain adequate stores of it. Darker skin however would be of no greater advantage in less sun-intense climates because vitamin D production would be more of a priority and hence, paler skin prevailed.

Interestingly, it has been observed that females tend to exhibit lighter skin pigmentation than males in all populations, and that sexual selection is partly responsible for a reduction in pigmentation in expanding populations. This hypothesis is based on observations that the attraction of human females and human males is partially due to their lighter pigmentation. Thus, lighter-coloured females are perceived to be more feminine than their darker counterparts and therefore would be preferred partners. This idea was further advanced in that paler females would have higher vitamin D production and would be able to meet the calcium demands of pregnancy and lactation. Due to maternal chromosomes, further generations would in theory become gradually paler. This can still be seen today in many Asian countries, particularly in the Far East where it is more desirable for women to be paler. This tends to be for cultural reasons, rather than to bring about any advantageous characteristics in children; for example, in China, having a tan is linked to historically working in fields and being a peasant- having a lighter complexion reflects a higher social status rather than any indication of undesirable genetics and many women practice diligent sun avoidance and take up chemical skin-lightening .

Early migrants to Europe would still have had fairly dark skin, and nowhere near as light as today's modern northern Europeans. However as mentioned, there would have been less exposure to the sun and this would have led to even lower production rates of vitamin D and lighter skin would've proven more favourable. Given that increased vitamin D would give individuals stronger skeletons as well as serving a function in pregnancy, it would have allowed modern man to quickly establish stable and expanding colonies. A superior intellect combined with technologically greater tools and weaponry would have given Homo sapiens a considerable advantage over already present populations of Homo neanderthalis. These factors may have led to the retreat of many neanderthal tribes to the southern tip of Europe (and their eventual extinction). Homo sapiens would prove the victorious conqueror in Europe.

Tribes still exist and have done so (although in considerably smaller numbers ) in the colder regions of Northern Asia, for example in Siberia, for thousands of years after travelling (according to the Out of Africa hypothesis) north east from the middle and near East. Like Europeans, they are also of pale skin because of reduced sunlight exposure. However, populations living nearer to the Siberian Arctic, as well as many other Inuit tribes may be of a darker, more South-East Asian complexion as they gain enough vitamin D from their fishy diet and biosynthesis becomes less important. More melanin in the skin may also serve to prevent over production (even though sunlight is of a lower intensity).

Native Americans share certain phenotype characteristics with the indigenous peoples of North Asia, because it is believed the same people traversed huge ice plains thousands of years ago and colonised North America and consequently South America. Interestingly, many native American tribe populations cannot grow or grow very little facial hair, although whether this provides an evolutionary advantage remains to be seen- it may be through sexual selection, females preferred males with less facial hair and so such genes were gradually removed from the population.

To conclude, inhabitants of Europe, North America and North Asia are genetically paler than their African ancestors who spread and conquered new lands, as an evolutionary response to differing amounts of sunlight (or more precisely UV radiation). Melanin serves to protect underlying cells from the harmful effects of UV. Dark skin provides the best protection from UV-induced cancers and folate photolysis, but leads to reduced vitamin D production. The opposite is true in pale skin. In real world terms, there is essentially a trade off between each factor and an optimum level of melanin concentration is achieved based on basically how strong the sun is. Given today's chemical and technolgical advances, although we can choose to make our skin lighter or darker to appease a personal preference or reflect our socioeconomic status, we were all born a certain colour because of the actions of our ancestors and how they evolved to enable them to prevail wherever they chose to settle.

References: The Evolution of Human Skin and Skin Color - Nina G Jablonski

How can we explain the course of the recurrent laryngeal nerve?

First of all it is essential that the actual use and course taken by this nerve is outlined.

The recurrent laryngeal nerve is just one of many branches of the vagus nerve, the 10^th cranial nerve. This particular branch is important, as it is the sole supplier of motor and sensory innervation to the larynx, commonly referred to as the voice box. The co-ordination of these muscles allows many animals to produce a wide range of sounds. This ability is indispensable in some species, such as humans who use their voice box to produce sounds in the form of words in order to communicate. Communication is an essential asset. Those animals that in the past had this ability had a distinct advantage over those who could not communicate. It provides protect when hunted and stealth when hunting.

In humans and mammals, the recurrent laryngeal nerves take a rather absurd route. As I have already stated, the target of the nerves is the larynx which is located anteriorly in the neck. The nerves originate in the brainstem, specifically the medulla oblongata, which is a portion of the central nervous system between the brain and the spinal cord. The nerves leave the medulla whilst still in the neurocranium (skull), but quickly exit via the right and left jugular foramen. After leaving the skull, the nerves descend down the neck, one on each side, and into the thorax, before looping back on themselves and into the neck once again before reaching their final destination. The route is slightly different on both sides of the body. On the right side, the nerve loops under the right subclavian artery before its ascent to the larynx whereas the left recurrent laryngeal nerve loops beneath the aorta.

Although this route seems to be strange in humans, the extremity of this illogical route is epitomised in a giraffe. The indirect route means that at least six extra feet of nerve is required when compared to a direct alternative. Personally, this seems tremendously inappropriate, as it is an extreme waste of resources, and also poses a larger potential for damage especially for the left recurrent laryngeal nerve, as not only can problems in the neck cause nerve damage but also problems in the left side of the thorax. For example, a tumour in the left lung could cause compression on the left recurrent laryngeal nerve leading to paralysis of the muscles on the left side of the larynx, possibly resulting in a decrease in the ability to communicate and the subsequent loss of an evolutionary advantage.

Consequently, I believe that it is fair to state that this situation lacks an intelligent design, as there is obviously a much more ideal design that would not waste resources or pose an increased risk of problems occurring. If a mere mortal such as myself can easily outline the flaws of this situation, it is acceptable to assume that no omniscient, creator would have designed the recurrent laryngeal nerve as it stands today. Therefore, I shall now look to see if Charles Darwin’s work can help answer my query and provide the reasoning beyond this strange phenomenon.

The presence of this nerve can be traced back down the branching tree of life to fish-like vertebrates, which appear to share the presence of a similar nerve. However, the nerve in fish does not have the same route, and therefore does not share the same name but is still the fourth branch of the vagus nerve.

Instead, the nerve in fish-like vertebrates travels straight to its target, which is the 6^th gill as it appears to follow the route of the 6^th arterial arch. This same general principle, of the nerve following the arterial branch, is also shown in mammals and humans.

During evolution, some fish-like vertebrates appear to have migrated onto land and evolved into land creatures. This was only recently proven by evidence in the fossil record. Before the discovery of a certain fossil, it was only assumed that some of the water living animals evolved into land living creatures, but since the first report in 2006 of the Tiktaalik roseae, evolutionists believe that they have found the missing link; the bridge between aquatic life in the oceans to life on land. These fish-like vertebrates are our ancestors.

This statement is further confirmed by the fact that humans and mammals follow a similar developmental principle as their fish-like ancestors, as during the embryological development of land animals gill like remnants can be seen. However, during the evolutionary step from fish to land animals, the gills of the fish disappeared in the adult form of land animals.

The pharyngeal arches, located on an embryos neck appear extremely like the gills of a fish. However, their function is no longer to supply oxygen by becoming gills as they did in previous aquatic species, but they are now responsible for giving rise to the development of internal bodily structures.

As already stated, we have a particular interest in the 6^th arch. In both fish and humans, this arch gives rise to the 6^th arterial arch. However, the location which this arch migrates to is different. In fish, the arch runs next to the 6^th gill and therefore this means the following nerve has a direct route to its target.

However, in humans, the 6^th arterial arch gives rise to the right and left pulmonary arteries and the ductus arteriosus which are vessels located around the heart. Hence the nerve follows this path and ends up travelling from the brainstem into the thorax. However, the 6^th pharyngeal arch also contributes to the formation of the larynx, including its cartilaginous skeleton and its intrinsic musculature. Therefore, the nerve then takes a detour from its position in the thorax, after the development of the arterial system is complete, back up to its target in the neck to help in the formation of the larynx.

The answer to why this happens in this particular order is all hidden in the past. Once a species develops, the developmental process cannot be undone as there are essential elements that must be maintained in order for an organism to survive i.e. the development of the arterial system. This is often portrayed in words as ‘the knot evolution cannot untangle.’ The need for some developmental elements to be maintained, places evolutionary constraints upon future species that are to evolve. This is evoked strongly in this example.

The constraint of the development of the arterial system means that the fourth branch of the vagus nerve has to follow the migration of the 6^th arterial arch first as this is a fundamental step in the development of an organism. This cornerstone means that any evolutionary advances have to occur after this step has taken place. Therefore, when the larynx evolved, the laryngeal nerve had to evolve too, but in accordance with the constraints put upon it. It therefore had to travel from the thorax to the larynx, and as a result became known as the recurrent laryngeal nerve due to its reoccurring route through the neck.

I believe I have revealed that this nerves route is an example of unintelligent design and that its route can be explained clearly and precisely through the work of Charles Darwin and the process of evolution, that can only move forwards, resulting in some weird and absurd but often wonderful creations.

Sources:

The Rough Guide to Evolution – Mark Pollen

http://www.sciencedaily.com/releases/2008/10/081015144123.htm

Evolution – Mark Ridley

Evolution - Monroe W. Strickberger

Clinically Orientated Anatomy – Keith Moore and Arthur Daley

Tay-Sachs disease in Ashkenazi Jews: Origins and Preventions

Tay-Sachs disease is an autosomal recessive disease which is fatal in its most common form, known as infantile Tay-Sachs. It is known to have a high prevalence in Ashkenazi Jews. In this essay I am going to explain how the high prevalence of the mutation could have originated in the Jewish population and how the disease can be prevented.

Approximately 1 in 25 Ashkenazi Jews are carriers of Tay-Sachs disease which makes this population particularly susceptible. Only 1 in 250 people are carriers in the general population. Although Ashkenazi Jews are more susceptible to many other diseases Tay-Sachs is a particularly terrible disease as it leaves a child blind, deaf and unable to swallow. Babies with Tay-Sachs are born without Hexosaminidase A which would otherwise clear out unwanted fatty proteins which interfere with baby’s sight, hearing, movement and mental development.

It is not known for definite as to why this population has a high carrier rate but there are some very strong explanations. The first theory I found involves the infectious disease tuberculosis. From some evidence it appears that Tay-Sachs provides some protection against TB, and it is this that may explain why Ashkenazi Jews have a high carrier rate. When World War 2 broke out TB spread through the eastern European Jewish settlements. This was particularly dangerous because these Jews were prevented from moving out of the ghettos that they were forced to live in by the Nazis. However, healthy parents who had children with Tay-Sachs disease did not contact TB, even though they were sometimes repeatedly exposed. The parents must have been carriers of Tay-Sachs in order to have children with the disease, since it is an autosomal recessive infliction. Therefore being a carrier of Tay-Sachs seems to have provided some sort of protection against TB, although why it provides protection is not so clear and is still not fully known.

To relate this idea to evolution, the Ashkenazi Jews at the time were exposed to a serious epidemic of TB in their area, so many people who were not carriers would have contacted TB and died meaning they would not raise offspring in the future. On the other hand individuals with the allele for Tay-Sachs would not have contacted TB and passed on their genes to their offspring. It is an example of survival of the fittest (natural selection) so even though Tay-Sachs is a dangerous disease, at the time of world war 2 many people without the allele for the disease were wiped out by TB making the allele more prevalent in the population.

Genetic drift is another theory as to why Ashkenazi Jews have a higher carrier frequency of Tay-Sachs. Genetic drift is the random fluctuations of frequencies of the appearance of genes in an isolated population. It relies on the fact that alleles of offspring are a random sample of those of their parents, and one allele can become more prevalent in a population by chance. As I have already described the Ashkenazi Jews were isolated in ghettos by the Nazis which is why they were susceptible to genetic drift. The effect of genetic drift is much larger in smaller populations which is why it doesn’t usually play a large role in allele frequency for the general population. Using this theory, it is possible the Tay-Sachs is prevalent in Ashkenazi Jews just through a random process. The fact that there was a small population involved meant that it was easier for one allele to become more prevalent than another allele. Genetic drift would have also played a role in making the prevalence of the Tay-Sachs allele high in the population even if the Ashkenazi Jews were not forced to stay in ghettoes. This is because the population of Ashkenazi Jews was small in the first place, and a small population would allow genetic drift to be more effective. Natural selection would not have caused the allele to become prevalent without the effect of TB because natural selection only allows alleles giving an advantage to be passed on to offspring, which is why genetic drift is a strong theory.

The last theory is the founder effect which occurs when a population is started from a very small number of individuals. It believed the Ashkenazi Jew population was started from only a few thousand individuals who lived in Eastern Europe, and millions of people can now trace their ancestry to this population. Therefore even if a small number of people in the founding population are carriers of the mutation, the defect would become amplified over time and so when the population reaches high numbers, there will be a large number of people with the defect. This theory would need to rely on the chance presence of certain mutations in the founders of today’s Ashkenazi Jewish population.

At the moment there is no cure for Tay-Sachs disease. We can only try and prevent more cases from occurring, and this is done through screening for potential carriers of the disease. This is especially important if the future parents have both high risk ethnic origins such as both being Ashkenazi Jews. The potential parents both have genetic tests done to see if they are carriers or not, so they can be warned about the dangers of starting a family. Genetic tests of the amniotic fluid in the womb can be done to diagnose Tay-Sachs, so the parents can chose whether to terminate the pregnancy. These tests used to take a long time to show results so they were not so effective in the past. However now they can be done with a simple blood test with results being shown in under 48 hours. The blood samples can be analyzed by enzyme assay or DNA studies. Some Jewish communities have organised pre-marital counselling to help reduce the occurrence of the disease. These methods can reduce the amount of children being born with Tay-Sachs.

In conclusion I cannot decide which theory is the most likely to have caused the high prevalence of Tay-Sachs in Ashkenazi Jews as they are all valid explanations. Even though genetic drift and the founder effect are two different theories, both may have contributed to the high prevalence while the TB epidemic the Jews faced only increased the proportion of carriers in the population. So it is likely that all three theories played a part in keeping the mutation high in Ashkenazi Jews. Regarding the future of the population, it is vital to advertise genetic testing more to the populations at risk as the methods used seem to have been successful in reducing the numbers of children with Tay-Sachs.

Human Genetics: Concepts and Applications (Second edition, 1997) pp. 247-248 by Ricki Lewis

http://www.nhs.uk/Conditions/Tay-Sachs-disease/Pages/Prevention.aspx

http://www.genome.gov/10001220

http://judaism.about.com/od/health/a/geneticdisorder.htm

                Darwin and slavery

               Sam Pearson

“I thank God, I shall never again visit a slave country. To this day, if I hear a distant scream, it recalls with painful vividness my feelings, when passing a house near Pernamabuco, I heard the most pitiful moans, and could not but suspect that some poor slave was being tortured, yet knew that I was as powerless as a child even to remonstrate”.

 

This is a direct quotation from Darwin outlining one of the times he experienced the maltreatment of slaves during his travels in south America as a small child. He writes later in life how he lived across the street from a women in Rio de janeiro who regularly tortured her slaves and these experiences seem to have shaped the views that Darwin would have on the matter for the rest of his life.

 

Indeed Darwin came from a family who had worked towards the abolition of slavery as both his grandfathers where keen abolitionists. His dads father Jos Wedgewood is indeed said to be the person who created the famous picture of a slave on his knees asking his master if he were not his brother. His grandfather even sold the wedgewood showroom in London and put the proceeds into the antislave movement in particular the work of Thomas Clarkson who traveled around collecting statistics on slavery. With such strong family beliefs on slaver it seems impossible that the topic would not affect Darwins life.

 

It has been written before that Darwin created his theory of common descent from black africans as support to his cause of abolishing slavery. It is true that there are many instances before Darwin published this theory of him stating his hatred of slavery and it seems plausible that he would have doctored his results in order prove that we shared a common ancestory and to prove to the ruling classes that they were no better than the slaves that they emprisoned. However if you look at the evidence that Darwin collected on the matter his quote in the descent of man “it is … probable that our early progenitors lived in the African continent” seems far less biased and far more scientifically viable indeed even to this day the case that we all descended from Africans gets stronger and stronger. Indeed the place were the human race evolved is believed to have been found and human fossils 40,000 years older than anywhere else in the world have been found in Africa.

 

So how did Darwin’s work undermine the slave trade? It is important to note that at the time of Darwin’s writing the british empire had abolished slavery and policed its waters in an effort to reduce the trade of other countries. Darwin’s main targets were therefore the slave traders of north and south America. At the time many plantation and factory owners in America saw those of black descent as a completely different species to that of the European descended white man. They therefore saw no moral problem with enslaving this so called inferior race. Darwin’s work showed that in fact white man had descended from black Africans. This undermined the ideals of the enslavers who proposed that all none Caucasian people had descended separately and therefore could be treated differently and although it may not have directly affected the slave trade it certainly would have altered the beliefs of those who believed in Darwin’s work. It also provided scientific evidence to aid the antislave movement. Although Darwin omitted human evolution from the origin of species saying “the subject was so surrounded with prejudices” it was well known amongst the science community that human evolution was at its heart and indeed Darwin published his views on human descent in later works.

 

Indeed Darwin devoted 35 pages in the descent of man to ‘on the races of man’ a chapter where he outlines that the differences between different races are actually very small in the grand scheme of nature and that the physical differences are not caused by natural selection ie there are not different evolutionary pathways for all races. Instead Darwin wrote that these differences are actually caused by sexual selection whereby males compete for females and certain characteristics are desireable in different subgroups. This theory therefore proposed that the black population was equal to the white population and that the minor differences between them were definitely not due to differences in evolution.

 

At the time Darwin’s name was slightly tainted by subtheories of his work namely social Darwinism as proposed by Herbert spencer. Darwin had stated that sympathetic behaviour towards other humans was an important part of natural selection but he rejected the work of Spencer.

 

On the beagles return voyage three uneducated islanders traveled with Darwin and the crew. Although the cultural differences between the two groups where enormous Darwin noticed that they shared many mannerisms and that there were indeed many similarities between the two groups. It has been suggested that this was one of the triggers for Darwin’s brainwave that all humans descended from a common ancestor.

 

In conclusion Darwin’s family background and upbringing were extremely important in his beliefs on the atrocities of slavery and without these his groundbreaking work on mans common ancestor that helped undermine the racist slave traders may never have come to the fore and one of the great scientific works may never have come into being.

 

Sources:

 

Darwins twin track, Adrian Desmond

 

Massachusetts Darwin 2009 bicentennial project, Johnathan King

 

Darwin’s Evolution Theory, Inspired By Abolition of Slavery? David Fierce

Purpose of this blog: June 2009

This blog was created by Professor Mark Pallen for the purpose of allowing medical students to blog on selected topics during a one-week student-selected activity. For the June 2009 SSA, students who choose to blog, should post their work (1000-2000 words) by 7pm on Wednesday 17th June 2009.