Thursday 17 April 2014

White skin - Neanderthal inheritance

White skin - Neanderthal inheritance

My younger son Daniel pointed out to me an error in my last post, the topic of which was a recent increase of the frequency of rickets in children in the UK. As would be predictable from previous experience, the incidence was higher in children of South Asian and Black ethnicity than in white children.

I mentioned in the Post that the dark skin people living in tropical parts of the world is an evolutionary adaptation to the high intensity of the sun. Daniel pointed out that strictly speaking this is not correct, as Homo sapiens originated in Africa and the vast majority of the world population is dark-skinned, living in tropical or semi-tropical zones.

It is the white-skinned population of the world that is a small minority, differing from the majority, and it is this group that is an evolutionary variant, and adapted to living only in the northern temperate area of north-west Europe. What was the origin of this small group?

Neanderthal (Neander tal) is a valley close to Dusseldorf in Germany. It is indicated by a sign on the autobahn viaduct that crosses the valley. Skeletal remains of a humanoid species were first discovered in the valley in the 18th century and have since been discovered in other parts of Europe. The humanoid became know as Neanderthal Man, scientifically Homo sapiens neanderthalensis.

Recent genetic studies have shown that the white-skinned variant of Homo sapiens has in the genome some DNA derived from the now extinct species Neanderthal Man. In other words at some time in the distant past there was a small amount of interbreeding between the new Homo sapiens spreading out of Africa and the older Neanderthals living in Europe, about 60,000 years ago. Little genetic advantage was passed on to Homo sapiens but one is a genetic factor for fair skin, an advantage when living in northern Europe. Another genetic factor is fair straight hair.

Darwin introduced the idea of survival of the fittest, indicating that if a new gene gives an advantage, then those with that gene will thrive. In north-west Europe a pale skin gave an advantage in the form of enhanced vitamin D synthesis.

And so, thanks to the Neanderthal humanoid, white-skinned people are genetically adapted to living very distant from the equator. Dark skin is the global norm of pure Homo sapiens. The Neanderthals became extinct as Homo sapiens proved to be superior, but their genetic influence lives on in white-skinned people.


Wednesday 2 April 2014

Rickets - a recent increase in children

Rickets is a disease of children. Formation of bone is impaired so that the bones become soft. When the child starts to walk the bones of the legs bend, giving rise to “bow-legs”. In severe cases the pelvis will contract and if in a girl it will lead to subsequent difficulty with labour. 

It is due to deficiency of vitamin D, which is essential for the process of “ossification”, the incorporation of calcium into bone. Rickets was very common in industrial cities in the 19th century. A hundred years ago it was recognised as being due to lack of sunlight, which acts on the skin to produce vitamin D. 

It was also discovered that vitamin D was present in fish oils, and this led to an opportunity to treat or prevent rickets despite lack of sunlight penetration through heavily polluted atmosphere. Cod liver oil became a standard medication for children in northern Europe, but this became out of fashion in the latter half of the 20th century when clean air legislation and holidays in the sun became effective.

Rickets became a disease of history, but in the 1960s there were medical case reports of rickets in children of immigrants from South Asia into the northern parts of the UK. This became a more widespread problem as the result of increased immigration into the UK. In the USA rickets was identified in Black children.

Dark pigmented skin is an evolutionary adaptation to tropical and sub-tropical regions where sunlight intensity at ground level would cause severe burn in non-pigmented white skin. The pigmented skin is less efficient in the synthesis of vitamin D but this is not important when sun intensity is high.  It is of great importance when the individuals move to live in north-west Europe. The northern parts of the UK are further north than anywhere in China, and sun intensity is relatively low, even in the summer.

South Asian people are adapted culturally to living in a hot hot sunny climate by dress that covers virtually all of the body. This is an important additional factor leading to vitamin D deficiency when resident in north-west Europe.  

The trickle of case reports has now reached a significant number, and among white children in addition. A study from Oxford has shown a significant increase in the incidence of rickets during the past 15 years. Incidence means the number of new cases each year. The numbers are small, one increasing to three per 100,000 children aged less than 15 years. The study includes both the incidence within Oxford and also within England.

Increase in rickets in recent years

The incidence is lower in Oxford than in England overall, probably the result of a lower immigrant population. Ethnicity is important in the development of rickets, but it has been occuring in white children in recent years.

Overall an important part of the increase in the incidence of rickets is an increase in the proportion of the ethnic minority and especially the South Asian of the UK population. During the past few years, the birth rate of the indigenous population has decreased by 2%, whereas that of the ethnic minorities has increased 19%. 

 Proportion is the the proportion of children who have rickets

But the incidence of rickets has increased in white children, and this requires an explanation. There is no suggestion of a decrease in the intensity of the sun and so we must for behavioural factors.