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Are Humans Still Evolving?
Member Name: crispy
Date: 07/09/01, updated on 07/09/01 (1609 review reads)
This is an essay I wrote in the third year of my biology degree. I hope no-one minds if I post it verbatim. Dooyoo admin people - does self-plagiarism count?
Are Humans Still Evolving?
While the cultural evolution of man is undoubtedly continuing at an accelerating pace, scientific opinion differs over whether or not Homo sapiens is still evolving biologically. There are several explanations as to why we may not still be evolving in theory, but on looking at evidence from several specific examples it must be concluded that the biological evolution of mankind is most definitely continuing, and may even, as for cultural evolution, be accelerating.
The descent of man from Australopithecus afarensis, through Homo habilis and Homo erectus to Homo sapiens, has been a greatly studied subject amongst evolutionary biologists. What, however, of the continuing evolution of mankind since the dawn of H. sapiens? Have we reached our evolutionary end-point, or are we still being shaped and moulded by the powers of natural selection?
The evolution of Homo sapiens can be broadly divided into two main categories – those of cultural and biological evolution. I shall cover cultural evolution first, as there is far less controversy in this subject.
Cultural evolution can be defined as the passing on of knowledge from generation to generation. It is unique to our species, and has arisen either as a result of, or more likely in conjunction with, the increasing complexity of the human brain and the development of language.
The process of cultural evolution is, as for all evolutionary processes, a continuous one, however it may be divided up into three main stages so far(1). The initial stage is that of the nomadic hunter-gatherers on the African grasslands up to 2 million years ago, who made tools, organised communal activities, and divided labour
amongst individuals. The second stage was the development of agriculture, which occurred in Africa, Eurasia and the Americas between 10,000 and 15,000 years ago. Agriculture obviously demanded permanent settlements, and this in turn led to the formation of the first cities. The third major stage in cultural evolution was the Industrial Revolution, which began in Europe in the 18th century. We may have already reached a fourth stage, however it is hard to identify such a development until its effects have time to make themselves plain. Contenders for the third major development in cultural evolution could include space flight, genetic engineering or the Digital Revolution.
Since the Industrial Revolution the development of new technology has escalated exponentially. For example, within a single generation, we have progressed from the first flight of the Wright brothers to Neil Armstrong landing on the moon1. Our rapid and accelerating cultural evolution is indisputable, and is neatly summed up by Jared Diamond (1989)(2), who wrote: ‘After the Great Leap Forward (approximately 35,000 years ago), cultural development no longer depended on genetic change. Despite negligible changes in our anatomy, there has been far more cultural evolution in the past 35,000 years than in the millions of years before.’
This point is reiterated by Richard Klein (1992)(3), who wrote: ‘A major transformation in human behaviour occurred about 40,000 years ago. Prior to this time, human form and human behaviour evolved hand-in-hand, very slowly over long time intervals. Afterward, evolution of the human form all but ceased, while the evolution of human behaviour, or, perhaps more precisely, the evolution of culture, accelerated dramatically.’
Klein’s statement above leads on to the second main category of human evolution – that of biological evolution. This is where scientific
opinion has disagreed enormously, over whether natural selection is continuing to drive human biological evolution, or whether our species has instead ceased to evolve biologically, and is poised to degenerate.
Arguments against continued biological evolution
Ernst Mayr (1963)(4) wrote: ‘To be sure, there may have been an improvement of the brain (over the last 100,000 years) without an enlargement of cranial capacity but there is no real evidence of this. Something must have happened to weaken the selective pressure drastically. We cannot escape the conclusion that man’s evolution towards manness suddenly came to a halt… The social structure of contemporary society no longer awards superiority with reproductive success.’
Mayr’s point was seconded by Steve Jones5 in 1991, who wrote: ‘Most babies now survive until they themselves have babies, so that existence is less of a struggle than it was. Natural selection involves inherited differences in the chance of surviving that struggle, and as most of us do survive nowadays until we have passed on our genes, the strength of selection has decreased… It may even be that we are near the end of our evolutionary road, that we have got as close to utopia as we ever will.’
These arguments against the continuing biological evolution of our species are based around the postulated lack of selection pressures in the modern world. This is certainly true to an extent – after all, as Jones(5) says, most individuals survive to adulthood, find a partner and have children. Certainly, when compared to other mammal species such as deer or gorillas, where one dominant male within a social group has exclusive mating rights with all the females present in the group, humans show greatly reduced competition for mates.
It can also be argued that we have developed culturally to the point where, rather than
adapt biologically over many generations to suit our environment, it is far quicker and easier for us to modify our environments to suit ourselves, thus negating the influence of the environment on our evolution. We construct shelters to protect us from the elements, and wear clothes to keep us warm when we venture outdoors. Christopher Wills (1998)(6) writes: ‘Our cultural prowess has helped us to modify virtually every part of the planet’s surface and has enabled a substantial fraction of us to live long, disease-free, enriched, and stimulating lives. As a result, we have become very well adapted to our cultural milieu – but remarkably ill adapted to the natural world.’
Modern medicine is also credited with removing many of the selection pressures we would have faced in the past. Infectious diseases have undoubtedly been powerful sources of selection throughout our evolution, and it must therefore be highly relevant that over the last few centuries, and particularly in the last hundred years, mortality from these diseases has plummeted in the developed world. Americans, for example, now live nearly twice as long on average as their great-grandparents did(6). In all but the least developed areas of the world, any healthy child born now has an excellent chance of growing to adulthood and having children of its own. It does not seem unreasonable to assume that our culture, and especially our advanced technology, is shielding us from many of the problems that our ancestors faced before us.
Arguments for continued biological evolution
While the above reasoning does provide a perfectly plausible theory as to why we may no longer be evolving, there is little hard evidence to prove that we have so much as slowed down, let alone stopped. It is far more likely that the nature of the selection acting upon us has shifted to something less obvious. This possibility is raised by Ro
ger Lewin (1991)(7): ‘Why would social complexity have taken 90,000 years to manifest itself after the origin of anatomically modern humans? One possibility, of course, is that a subtle intellectual evolutionary change may have occurred rather recently in human history, one that does not manifest itself physically.’
David Hamburg (1975)(8) had actually taken this idea a step further, pointing out that cultural change is in fact constantly redefining the boundaries, and that the selection pressures that result from cultural change are themselves changing: ‘The poignant dilemma is that ways of fostering survival, self-esteem, close human relationships, and meaningful group membership for hundreds, thousands or even millions of years now often turn out to be ineffective or even dangerous in the new world which man has suddenly created. Some of the old ways are still useful, others are not. They will have to be sorted out, and sorted out soon.’ He suggests that essentially selection pressures have shifted from those that test the resources of our bodies to those that test the resources of our minds.
This does not mean, however, that no physical evolution is taking place. Many examples can be drawn from different areas of the world showing continuing physical evolution. A lot of these involve the adaptation of populations to extremes of climate that not even our modern technology can completely shield us from.
The native inhabitants of Tierra del Fuego are a case in point(9). Tierra del Fuego is a group of mountainous, partially glaciated, islands located off the southern tip of South America. One of the most thorough surveys of the islands was carried out by Robert FitzRoy, captain of the HMS Beagle, accompanied by Charles Darwin. The islands were found at the time to be inhabited by three separate tribes descended from people who had probably colonised from the north an estimated 8000 years ago. The climate on the i
slands is harsh, and even in summer is cold, with frequent gales, heavy rain and snow. However, the Fuegans showed remarkable tolerance to this climate, and were tall, strong and apparently healthy, despite their scanty clothing and crude shelters. They were clearly much better adapted to withstanding prolonged, severe cold than the visiting Europeans, however were unfortunately far less adapted to resisting disease. Argentine miners in the 1880s introduced many infectious diseases such as measles, tuberculosis and smallpox, and by the end of the 19th century all three tribes were decimated, and two have since become extinct.
The Tibetans inhabit one of the most hostile environments on the Earth, the Himalayas(6). They show a range of adaptations to living at an altitude of around 4000 metres, where the air contains about a third as much oxygen as at sea level. In most people, low levels of oxygen cause the pulmonary arteries supplying blood to the lungs to constrict, which is a reversal of the reflex that occurs at birth when oxygen rushes into the lungs and the arteries expand. This does not happen in Tibetans – the reflex is lost soon after birth. Tibetan women also have unusually large uterine arteries, so high rates of blood flow can be retained even under hypoxic conditions. Interestingly, the Tibetans actually show a slightly reduced number of red blood cells, and slightly less haemoglobin, than in people living at sea level. This deficiency is made up for by increased blood flow, and also conveys the benefits of protecting against high-altitude pulmonary edema and dehydration. The bodies of Tibetans have also been proven to be far more sensitive to changing levels of oxygen than the rest of us.
The above examples indicate continuing evolution amongst specific isolated groups, but what about the general population? Christopher Wills (1998)(6) suggests that human evolution is not just continuing, but is in fact accelerating. The incr
easing ease of global transport and migration is resulting in the bringing together of gene pools that had previously been isolated since the first radiation out of Africa. This was predicted by Charles Galton Darwin (a grandson of Charles Darwin himself) in his 1952 book The Next Million Years, in which he said that in the future people would move from the developing world into the first world, as a result of the population vacuum caused by the demographic transition there. This is happening even now in Europe, with migration from the less developed world increasing rapidly. For example, almost ten percent of the population of Germany is now foreign-born. This has important implications for the evolution of the species – the average heterozygosity of the population will increase as isolated islands of genetic homogeneity that have built up over tens or hundreds of thousands of years disappear. Such a large-scale change in the gene pool must have a dramatic effect on our evolution.
Wills(6) also criticises the view that modern medicine is preventing our continued evolution. He states that four factors still remain that will lead to further disease-driven evolution. The first is the continuing global ecological change we are causing. Ecological changes in central Africa have resulted in the emergence of the AIDS virus, and more such epidemiological emergencies are likely to occur. Recent examples include newly resurgent dengue haemorrhagic fever and yellow fever. Secondly, most of the disease-caused global mortality is due to common respiratory and diarrhoeal illnesses that mostly affect the very young and the very old. These are most prevalent in the developing world, and will not be eradicated for some time. Thirdly, the indiscriminate use of antibiotics and other such drugs is producing new strains of resistant pathogens that some people will be more resistant to than others, a resistance that will be increasingly valuable as the immunity of the pa
thogens to pharmaceuticals increases, which will in turn alter our gene pool. Fourthly, pathogens are being found to play a larger role in disease than we had previously imagined, even in the developed world. Various micro organisms that were previously unknown, or assumed to be benign due to their prevalence, are now suspected to contribute to diseases previously attributed to other factors such as poor diet, unhealthy living conditions, or psychological factors. A recent example is that of Heliobacter pylori, which is now known to be responsible for many stomach and duodenal ulcers. It has also recently been discovered that heart attacks are influenced by bacterial and viral infections.
Other plagues, not caused by disease organisms, are also becoming more important in modern society, such as addictions to tobacco, alcohol and drugs and violence. Wills(6) says that ‘Death by gunshot or a drug overdose can be just as effective selective agents as death by infectious disease.’
Future human evolution
In the face of the evidence for the continuing evolution of our species cited above, it is difficult to maintain the idea that human evolution has stopped. However, if we are indeed still evolving, then in what direction is evolution taking us?
I consider that, in the developed world at least, the future evolution of the species will increasingly involve the rapidly developing field of biotechnology. One of the most recent areas of research has been that of genetic engineering, a process still in its infancy, whereby the DNA of an organism can be altered or added to. Currently this process is still largely theoretical, and can only be used to correct a single deficient gene at a time, however it is this idea of actually altering one’s DNA that most people are either excited or worried about(10).
The potential for this technique is immense, though, especially once the Human Genome Pr
oject, a worldwide effort to determine the nucleotide sequence of the human genome, is completed. This is expected to occur within the next 13 years, but the estimates are becoming ever shorter. The process in theory will allow mankind to edit its own DNA as much as it wants, such as including genes for disease resistance, improved general health, or increased longevity. It would also allow the possibility of people tailoring their children’s genetic makeup, choosing exactly what sort of child they want from gender down to even small details like eye colour or individual personality traits. Were this idea to become a reality on a large scale, I believe the effects on our evolution would be considerable. Natural chance combinations of genes that lead to new, potentially advantageous characteristics occurring would effectively be eliminated, hence seriously hampering if not actually preventing any form of evolution from taking place. The whole evolutionary principle of genes being inherited from one’s parents would also become invalid, as the offspring will not be limited to just the genetic material provided by their parents, but will also have genes inserted from a stock ‘library’ of genetic material. We would be turning our back on the natural process of evolution, and taking the future development of our species entirely in our own hands(10-12).
They key to this, however, is how widespread the technique becomes. Wills(6) writes: ‘The human gene pool is a very big place. With 5.5 billion people and 200,000 genes per person, the genes that are changed by deliberate intervention will simply be a drop in the bucket. Genetic changes driven by natural selection will continue to dwarf the efforts of even the most indefatigable gene therapists, at least for the foreseeable future.’ He goes on to say: ‘Just to remind you how inextricably natural and artificial selection are intermixed: the people who succumb to the blandi
shments of this dazzling array of genetic “fixes” will not be a random sample from our gene pool. It seems likely that one result will be selection for genes that contribute to caution, and against genes that contribute to reckless behaviour!’
Wills(6) also looks even further to the future, and considers the effect of colonising other planets, or even galaxies or universes, on the evolution of our species. Who knows what selection pressures our descendants may face on other worlds? What is certain though, is that it will mean a return to geographical isolation and fragmentation of our population, but of a more extreme nature than previously encountered by any species on our planet. The consequences for evolution are mind-boggling, but one thing is almost certain; we will never stop.
While it is impossible to contradict our rapid and continuous cultural evolution, arguments can be put forward that suggest reasons why we may no longer be evolving biologically. However, when considered in more detail, many of these arguments prove not to hold. Human evolution is continuing, and if anything, as Christopher Wills argues, is actually accelerating.
1 Neil A. Campbell, Biology 4th Edition, Benjamin/Cummings, 1996
2 Jared Diamond, Discover 10 (1989): 50-60
3 Richard G. Klein, Evolutionary Anthropology 1 (1992): 5-14
4 Ernst Mayr, Animal Species And Evolution, Belknap Press of Harvard University Press, 1963
5 J. S. Jones, New York Times (22 September 1991)
6 Christopher Wills, Children Of Prometheus: The Accelerating Pace Of Human Evolution, Perseus Books, 1998
7 Roger Lewin, Human Evolution: An Illustrated Introduction, Blackwell Scientific Publications, 1993
8 David A. Hamburg, in The Quest For Man, edited by V. Goodall, Praeger, 1975
9 Peter Skelton, Evolution: A Biological And Palaeontological Approach, Addison-Wesley, 1993
Shifting towards human evolution - changing the world
http://homepage.esoterica.pt/~jpnitya/earth/sh ift.htm (now defunct)
11 Eugenics - controlled evolution of mankind
http://homepage.esoterica.pt/~jpnitya/science/ eugenic.htm (now defunct)
12 Genetic Engineering - Defining our children's traits
http://homepage.esoterica.pt/~jpnitya/science/ germ.htm (now defunct)