Ever since our species discovered how to make and control fire, it has sought to control more and more of its environment. The invention of clothes allows us to travel in all weathers; the domestication of animals means we no longer have to worry about finding prey to hunt; agriculture has done away with the need to roam widely to seek vegetable food; air-conditioning and central heating keep our homes at the same, comfortable temperature all the year round.
This growing isolation of mankind from the vagaries of climate and environment has stimulated some scientists to ask : Are we still evolving? In the process of evolution, genetic variation in a population is maintained by mutation (new mistakes when genes copy themselves) and new combinations of genes which arise through mating. Members of the population which are best adapted to their environment survive to pass their genes to the next generation.
So, is this process happening in the modern world? Is natural selection still at work?
Our best instincts are to try and keep everyone alive, and we are becoming better at doing so. In the Middle ages a European baby would have had a 50% chance of surviving to reproduce. Now the figure is more like 99%. In the developed world, premature babies which would have died a century ago, now survive to adulthood. Complex births no longer kill both mother and child. Genetically based conditions such as haemophilia and diabetes are no longer fatal. In this sense (and only in this modern western society) evolution through natural selection appears to have stopped. Genes which would previously have died out, now live on in our populations and we are becoming specialised to survive only in our technological society. Some people call this "reverse evolution"
A trend in the opposite direction is that we are capable of tinkering directly with the gene pool. Through in vitro fertilisation and embryo screening we can select for certain characteristics in our children and ensure they do not have known genetic defects. Some people dream of thus selecting clever, beautiful and healthy offspring. (This seems a rather chilling prospect somehow).
It is evident that we have evolved since we left Africa some 60,000 years ago. The most obvious adaptation has been our skin colour. The dark skin of Africans was selected to protect it against the damaging effects of ultra-violet light. The pale skin of Nordic peoples selected to enable the synthesis of Vitamin D under conditions of low sunlight. When we move into environments to which we are not adapted, we pay the price. Take for example the growing skin cancer problem in the white population of Australia, and Rickets disease (caused by lack of Vitamin D) among people who have moved to the UK from the Indian sub-continent. In each case, a genetic trait that had survival value in one environment has proved to be a liability in another.
An interesting example of a gene out of context, is the presence of "Sickle Cell Anaemia" in African Americans. This genetic disease is characterised by abnormally shaped red blood cells and leads to a reduced life expectancy. About one third of Sub-Saharan Africans also carry this gene. Why should such a genetic "defect" persist in these human populations? Surely natural selection would get rid of it?. The answer is that in malarious regions of Africa the sickle cell gene provides a survival advantage. If you are unlucky enough to have two sickle cell genes (one from your mother, the other from your father) you will suffer this debilitating disease. But if you have one sickle cell gene and one normal one, you will have fewer symptoms and, as a bonus, you will have a natural immunity to malaria. So, in places where malaria is prevalent, the positives for this gene outweigh the negatives. In the US, it confers no survival value, but the lives of sufferers are prolonged through medical treatment. Since carriers of the sickling gene survive to reproduce, the characteristic is passed on and is found in about one in 12 African Americans.
Evolution can also be promoted by cultural development. For example, our ability to drink milk as adults is not a trait inherited from our ape-like ancestors, but is something new. Before pastoralism, the gene to digest lactose, the main sugar found in milk, was switched off after weaning, because it was no longer used. However when people domesticated cattle, they acquired cow's milk as an additional source of food. The ability to continue to digest lactose then became of huge survival advantage. Some people had a genetic mutation which left the milk-digesting gene active, and in cattle owning societies such people had ten times as many descendants. Almost all Dutch people and 99 percent of Swedes are lactose tolerant, and this is attributed to a mutation which arose the Funnel Beaker pastoral culture which flourished 5,000 to 6,000 years ago in north-central Europe. In addition, three newer mutations for lactose tolerance have occurred independently in African pastoral communities as recently as 2700 years ago. The speed with which these mutations have spread through human populations shows how great a survival advantage it is to be able to drink milk.
There is other evidence for the influence of culture on evolution. Researchers Gregory Cochran and Henry Harpending have put forward highly controversial ideas about its effect on inherited intelligence. They claim that the intelligence of the Ashkenazi Jews has increased over the past 1000 years. They point out that because this group of people was not allowed to follow common trades they were forced to make a living from intellectual pursuits such as finance. Smart members of this community were the most successful and had more children thus passing on their genes. It is interesting to note that Ashkenazi Jews also have high rates of some fatal genetic diseases, several of which are neurological. Cochran and Harpending assert that one copy of the "faulty" gene makes Jews smarter, but two copies give rise to the serious neurological diseases. They liken the situation to that of Sickle Cell Anaemia in Sub-Saharan Africans.
With mankind's a population of billions and given a changing environment, it seems most probable that we are still evolving. However it is debatable whether we are evolving to become more intelligent (though some scientists believe we are). It is important to remember that evolution results only in change. There need not be any biological "progress". In fact species can devolve and become simpler, less complex.
Mankind is, however, undergoing cultural change which does seem to be progressing. Our brains may not be evolving physically, but with the development of language we have learned to share ideas; with the invention of writing, to remember these shared ideas through generations; and we have created machines which do calculations in seconds which a person could not complete in a life time and which open new doors of understanding.
We have evolved to differ genetically one from the other, with different abilities, disease resistance and adaptations to the natural world. However every one of us alive today is an evolutionary success story. We are each the product of an unbroken line of evolution which started when life began billions of years ago, and together we comprise the only intelligent species on this planet: Homo sapiens. It is unlikely that we shall ever evolve into new species, since speciation depends upon populations being reproductively isolated from each other. This is something which will not happen in our planetary global village. However, there is one possibility. Perhaps one day man will colonise distant planets and diversify first into new varieties, and then new species. Homo stellaris?
Christine Betterton Jones - BSc. (Zoology), PhD (Parasitology)
text revised by Mary Sears
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