Human Evolution on Trial - The Human Star
During the 2000 Olympics I heard a TV announcer say something to the effect “it’s amazing how Africans dominate middle distance running.” My immediate reaction was they were dominating sprint and long distance running as well. People of West African origin, especially Afro-Americans with possibly some European genes, dominated sprint. East Africans dominated long distance. Several African groups had dominated middle distance running, especially those across the north from Morocco to Northeast Africa. This indicates that Africa contains different regional genetic groups. In fact a great deal of evidence shows there is more human genetic variation in Africa than there is in the rest of the world. This fact is used as evidence in support of the idea humans originated in Africa. Human genes are not distributed evenly around the rest of the world either though. We could say there are several regional kinds of human.
Although populations from opposite extremities of Europe and Asia look different it’s possible human movement across these continents at various times has obscured many earlier regional differences. We actually know several movements from history.
The boundaries are impossible to define. But blond-haired, pale-skinned people with blue eyes and relatively prominent noses were originally concentrated in the northwest of Europe. People with flatter faces, narrow eyes, straight black hair and so-called yellow skin are found mainly in the Far East, especially the Northeast. In the extreme Southeast we find Australian Aborigine and Melanesian people. Inspection reveals that all these extremes merge gradually into Asia, a series of what are called clines. For example people with mousy coloured, red or brown hair border the blond haired population. In biology the word cline is used when there is a gradual change in the appearance of a species across its geographic range.
Geography
I became aware of just how important geography was on human movements through my interest in music, especially the blues. I noticed that, with a couple of minor exceptions, the regions of the United States of America where the early blues singers had come from were all less than 200 metres above sea level. This was obviously not because the air was too thin for blues singing above this fairly low altitude. Rather it defined the area where cotton, tobacco and other crops requiring a large amount of cheap labour were grown.
The first Africans who went to America came mainly from the Gambia and Senegal Rivers at the very western tip of Africa. The blues seems to derive most of its African musical influence from these first arrivals even though many more slaves actually came from further south, even from as far south as Angola. It appears the first arrivals dominated the developing Afro-American culture. Some words from languages of this region have even entered English.
The blues were usually sung in English in fact and so we can safely say there was a European element in it as well. And Rock ’n’ Roll is just one more time that African music has influenced the white peoples’ music in the region. In spite of division between these two groups the people and cultures have mixed. There has even been quite a bit of gene flow between the races. African Americans for example usually have between one twenty-fifth to one third European ancestry (Olson 2002 and Jobling et al 2004).
This sort of population movement and mixing has happened almost continuously. In fact we can trace the pattern right back to, and even beyond, the origin of our species. But first we need to realise that the various technological, cultural and genetic expansions of humans around the world have been like ripples or waves in a funny shaped pond. Barriers to the expansions such as seas, deserts, tropical rain forest and mountains have continually deflected the ripples or even altered them and sent them back.
Africa, for example, is almost entirely surrounded by sea. Many ripples have been confined to Africa. Except for during some ice ages and more recently with the development of boats (and more recently still aircraft) the exit and entrance has been confined to the Sinai area for virtually all our history, our evolution.
Deserts and forests expand and contract with changing climate. They too have often influenced human ability to move freely around the world. For example within Africa the Sahara Desert has periodically been a barrier. Evidence shows it has been much moister at various times though. In fact it was “semi-arid grassland” again until 5500 years ago (Jobling et al 2004).
The banks of the Nile have also occasionally provided a route north through the region. Deserts have also formed at times in Arabia, India and Central Asia (not to mention Australia and the Americas). The tropical rainforest and swamp of the combined Ganges and Brahmaputra River deltas seems to have kept Southeast Asian and Indian populations fairly separate in the past, at least until boats were invented, although it is today one of the most densely populated regions on earth.
The development of glaciers during cold periods makes mountains even more effective barriers but they have always been there during human expansion. Outside Africa the Taurus Mountains of Turkey and the Zagros Mountains of Iran form the first mountain barrier to human expansion. Beyond them the Himalayas, Pamirs and Hindu Kush mountain ranges of Central Asia have always interrupted human movements. In fact the presumably ancient remnant language Burushaski survives in an isolated valley in this mountain complex.
Although the Black and Caspian Seas have periodically been joined to each other north of the Caucasus Mountains the mountains themselves seem to have been virtually impassable throughout human history. In Europe the Carpathian Mountains seem always to have been a barrier to free movement and the Ural Mountains and the swamps to their east became significant towards the end of the last ice age (Clark 1969), as did the mountains of Northeast Siberia and Alaska. The Chin Ling Mountains served for a long time to separate the people of North China from the Hoabinhian people of South China (Bellwood 1978).
A Map
I will now present a map of the world showing these tropical rainforest, desert and mountain barriers. But seeing I’m from New Zealand I’ll put New Zealand at the top left so we can examine the rest of the world more easily. This also puts at the top the part of Africa where humans seem to have first developed. Human movement into the Americas is comparatively recent and so I’ll hide most of that double continent. I have drawn ovals around each region where the human populations are most different. The horizontal lines represent degrees in latitude from the equator. The dotted lines show shorelines at times of low sea level.
If you look at the map you will see the distribution of humans round the earth can be represented diagrammatically by a five-pointed star. The extremes of human appearance are found at the points, with the rest of the population grading from the middle and from neighbouring points on the star. I’ll call the five points “East Asia”, pre-European “Australia”, “South and East Africa”, “West Africa” and “Northwest Europe”. The gap between Southeast Asia and Australia is called Wallace’s Line.
Although I have used Australia to demonstrate the type found at one point historically humans didn’t begin to move into Australia until about 50,000 years ago. But people of a similar physical type must have been present in mainland Southeast Asia from at least that long ago. A migration of people south from the East Asian point, perhaps as recently as six thousand years ago, has gradually diluted earlier human genes in that region from.
Here is the diagram of the human star
There are also little sub-points between the main points of the human star. They represent populations that are a combination of people from the middle of the star and the points on either side of them. Apart from the Indian subcontinent most have been occupied only since the end of the most recent ice age, about 10,000 to 12,000 years ago.
For any species the individuals at the geographical extremities of its distribution will be the most different. This has been recognised for a long time (Mayr and Diamond 2001). From the middle genes or ripples can flow in many directions but at the points gene flow reaches a dead end (if it reaches the point at all). On the other hand while ripples can flow from a point into the middle of the star or to a neighbouring point they are less likely to reach a more distant point.
I have divided Africa into two main points because most scientists would agree that, until relatively recently, the population of the two regions was quite different. The many African languages from the two points have actually been grouped into just four families. Within the point I have called South and East Africa the Khoisan language family (from the south) and Nilo-Saharan (from the northeast) may be distantly related to each other. There is evidence an expansion of Nilo-Saharan languages south at some time has isolated some northern Khoisan languages though.
The word Khoisan is commonly used to combine two Southern African groups: Hottentots (Khoi) and Bushmen (San). Anyway Khoisans and some Ethiopians are genetically closer to each other than they are to West Africans (Cavalli-Sforza et al 1994). In the last few thousand years people speaking Bantu, from the third African language family (Niger-Kordofanian), have migrated from the West African point through the middle of the South and East African point.
They have interbred with, or replaced the original Khoisan people (Jobling et al 2004). This has interrupted what had probably earlier been a cline from Ethiopians to Khoisans. Many tribes in the region show by their appearance, though, they are a genetic mix of types from the two neighbouring points of the star, West Africa and South and East Africa, and elements of the Khoisan languages survive in some Bantu languages, for example “clicks”. The Bantu people proper have arrived in the southeastern tip of Africa only in the last three to five hundred years. The fourth African language family, Hamitic, is spread across North Africa and into Ethiopia.
Before we leave Africa and look in turn at the two bottom, or northern, points of the star we’ll look briefly at the little subpoints.
The Pygmies were probably the first people into the rainforest of the Central West African sub-point and they probably evolved in it. Cavalli-Sforza (1995) starts his book “The Great Human Diasporas” with an account of the Pygmy people. He states the Pygmy populations do not share a common language. Their languages have been adopted from other people they have come in contact with. The sub-point has more recently been slowly colonised by people from the neighbouring points of the star, especially by West Africans.
I have made India a sub-point, mainly to give myself five points on the star but in fact it does show characteristics of a sub-point. Entrance and exit seem to have been difficult and a volcanic eruption in Indonesia around 70,000 years ago may have emptied it. People there have probably resembled a mix of East African and Australian Aborigine types for much of human history but very few early human remains have been found. The modern people of India are the result of mixing of any original population with more recent movements, from both the west and the east, of Dravidian-speaking people and East Asians. The expansion of Indo-European people, originally from the middle of the star, has even more recently had a huge influence on Northern India (Mallory 1989).
The people in the sub-point of Island Southeast Asia and the Pacific are a sort of hybrid between the Australian Aborigines and the East Asians. I’ll have to leave the American and Mediterranean Islands sub-points for another day.
The White Man
Now for the remaining two points on the star, Northwest Europe and East Asia.
One of the things that encouraged me to take on this project was that during a discussion on environmental effects on human populations one of my brothers wondered where white skin and blue eyes came from.
This brings us to the question of “race.” It is interesting that any study that ranks human races along a scale of superiority or advancement always surprisingly finds the race of the person making the study is the most superior or advanced. Throughout history most of us have probably believed our own tribe, culture and mythconceptions are superior to others. They are the ones we know best.
More than 2300 years ago Aristotle claimed that some people were naturally inferior and should be ruled by superiors but the idea whole races can be inferior may be a fairly recent development. In fact it has been suggested the idea evolved in order to justify the use of slavery in the intensive farming systems developed in the Americas in the last 400 years (Davidson 1974). The idea of white supremacy was further refined, especially from the 1880s to 1920s, to justify European imperialism.
History shows we humans are capable of justifying anything if we use a little creative thinking. There is certainly no doubt the slave trade disrupted sub-Saharan Africa to an incredible extent. Any excuse that slavery was long established in that continent is irrelevant. Slavery and the use of convict labour had been common everywhere, but huge numbers (perhaps more than ten million, a large proportion of the population) were taken from Africa. Many more than that number were killed during the required raids. The continent has still not recovered from the disruption caused by this economic exploitation.
Humans in different regions have certainly had different selection pressures applied to them by the environment. People who have lived for many generations under tropical sun have developed very dark skin probably as protection from the sun’s ultra-violet radiation (see for example Stringer and McKie 1996) but paler skinned people have survived well enough away from the tropics.
Some scientists suggest the different regional human varieties or races could have developed as a result of “sexual selection”. But it is difficult to believe ancient humans behaved any differently to the way many sexually active humans behave these days. Any preference individual humans may have to mate with blond people for example is unlikely to be instinctive. It is much more likely to be due to cultural factors. I suggest sexual selection is unlikely to be the cause of human variation.
The white colour in humans is often said to have developed in order for the human body to make vitamin D under conditions of short daylight hours (again see for example Stringer and McKie 1996). It is possible to get enough vitamin D from our diet though (Jobling et al 2004). Besides, humans didn’t get anywhere near the Arctic Circle (66° north), where vitamin D production would be a problem, until more recently than 30,000 years ago. By then they were fully clothed. Even some supporters of Intelligent Design would probably accept that the development of white skin probably happened before clothes were invented.
It is likely that in prehistoric times humans were able to reach at least latitude 50° north during periods of warmer climate. For example in Europe even Neanderthals were able to exist to about latitude 50° north for much of their existence. This was both by genetic adaptation to the cold and because ocean currents have usually warmed Western Europe a little compared to continental areas further east. But it is also almost certain that during colder periods Central Asian populations especially have been pushed south beyond 40° north. This has separated the Northwest European and East Asian populations at times. It is my bet natural selection in each region led to two different adaptations to an environment with winter snow.
It might be significant that until recently so-called white people were confined to the area where the natural cover is northern deciduous forest. White people actually become brown in summer through the process of tanning. This colour change is reasonably common in many creatures that live in regions of winter snow. Ermine, arctic foxes, varying hares and two or three species of grouse, or ptarmigan, are animals I immediately think of. For any other creature that had white hair, blue eyes and changed from white to brown with the seasons we would have no hesitation in pronouncing it was an adaptation to winter snow. Do we see here the assumption humans somehow obey a different set of biological rules? Of course in other animals the colour change involves a change of hair, fur or feathers but we know nature is remarkably clever at achieving the same result by different methods.
A Cline
Luigi Luca Cavalli-Sforza (1994 and 1995) has done a huge amount of work mapping the frequency of various human blood protein markers and enzymes. His map of the most important genetic landscape of Europe, the first principal component, accounts for nearly 30% of the genetic variation in that point of the human star (map 2).
We could say the map is a bit like a weather map. The darkest shading of the opposite types of crosshatching are the equivalent of pressure systems. They show the greatest concentrations of two different and opposite genetic combinations within the European population. The bold dashed line can be thought of as being a sort of boundary between the two genetic extremes although in fact they merge gradually. People at the opposite ends of this European cline tend to have different skin and hair colouring but the map is actually based on difference in blood proteins and enzymes not skin colour.
Cavalli-Sforza interprets this map as reflecting the spread of farming into Europe from the Middle East. Some people feel it also reflects the expansion of genes from Northwest Europe (Jones 2001). This has happened with Vikings, Germans and Celts within historic times and there were almost certainly prehistoric movements. In any case the map demonstrates the Northwest European point is genetically different from the middle of the star and there is a gradual merging of the genes, or a cline, across Europe.
Maps of other parts of the world given in “The History and Geography of Human Genes” (Cavalli-Sforza et al 1994) show similar genetic gradients or clines in other points of the star. In fact the phenomenon is found throughout nature. But of course different species may each have a star of different size and shape, especially a different number of points. This will depend on their individual geographic distributions.
Many genetic ripples have left their mark around the earth and can be traced. For humans we can also trace technological and cultural ripples. We can use all this to explain the origin and evolution of our species. In fact all species.
Altitude
But we’ll return to our human star. The people of East Asia developed a different response to a cold environment. They have very little facial hair compared to other human populations. This could be an adaptation to a very cold environment. If you have seen photographs of polar explorers or mountaineers you will have seen the buildup of ice in their beards from their breath. It would not have been an advantage to have a permanent ice-block around your face all winter. East Asians show several adaptations to a high light intensity environment and so it seems fairly likely that they originally developed in a reasonably treeless region at a fairly high altitude.
For example their yellowish skin contains only a small amount of melanin (the stuff that makes skin black or brown) but the outer layer of their skin is packed with keratin (Bellwood 1978). This reflects sunlight very efficiently and resists the penetration of ultraviolet light. But East Asians’ most obvious feature is what is called the Mongolian eye-fold. Anyone who has been on open snow in full sunlight without sunglasses will know what a great advantage this development would have been. Glare is also a problem in desert. The Khoisan of the Kalahari Desert for example also have narrow eyes, but no eye-fold. Cavalli-Sforza (1995) describes them as having “almost Oriental features”. East Central Asia is fairly dry and summer desert conditions may have reinforced this characteristic in East Asians.
So how long ago did these sorts of differences in human populations develop?
The breeding of domestic animals shows selection by humans can change such things as colour, shape and size quite quickly. Change can also be rapid in nature. Some bird species that can have been isolated on Long Island in Northern Melanesia for only 400 years have already started to look noticeably different to the populations that are presumably their ancestors (Mayr and Diamond 2001). Perhaps the changes in the human species (in zoological terms Homo sapiens, “man the wise”) have all happened quite recently.
In fact some people claim that all this change in human populations has happened only in the last six to ten thousand years. Even that the different kinds of human descend from just three or four pairs of human survivors on an ark from some indeterminate time in the past. Perhaps the much longer time since modern humans are believed to have come out of Africa is sufficient. I suggest that population movement has been too rapid for that sort of change to have developed even in that time.
The evidence indicates the Tasmanian Aborigines were as dark as (and possibly darker than) mainland Australians though they existed for a very long time in regions of winter snow beyond latitude 40° south. It was obviously not long enough or perhaps far enough south to develop whiteness. On the other hand natives of the Andes are exposed to some of the highest ultraviolet radiation in the world but haven’t been exposed to it long enough to develop black skin (Stringer and McKie 1996).
Is it possible the development of a white skin or an eye-fold extends as far back as 200,000 years ago, the time of Neanderthal / Archaic Homo sapiens? Or even further back to the days of Homo erectus? Of course many people believe the evidence shows these species have made no contribution to modern human genes. However the wave theory of genetic, cultural and technological evolution allows us to see how they may have.
References
Bellwood, Peter (1978) Man’s Conquest of the Pacific. Collins, Auckland.
Cavalli-Sforza, Luigi Luca, Menozzi, Paolo and Piazzi, Alberto (1994) The History and Geography of Human Genes. Princeton University Press, New Jersey.
Cavalli-Sforza, Luigi Luca and Cavalli-Sforza, Francesco (1995) The Great Human Diasporas. Addison- Wesley
Clark, Grahame (1969) World Prehistory. Cambridge University Press, UK.
Davidson, Basil (1974) Africa in History. Paladin, UK.
Jobling et al (2004) Human Evolutionary Genetics. Garland Science, New York.
Jones, Martin (2001) The Molecule Hunt. The Penguin Press, London.
Mallory, J. P. (1989) In Search of the Indo-Europeans. Thames and Hudson, New York.
Mayr, Ernst and Diamond, Jared (2001) The Birds of Northern Melanesia. Oxford University Press, New York.
Olson, Steve (2002) Mapping Human History. Houghton Mifflin Company, New York.
Stringer, Christopher and McKie, Robin (1996) African Exodus. Random House, UK.
