Evolution and Human Health

HEALTH AND DISEASE

For any living organism, health can be defined as follows:

Health is that physiological and behavioural state most likely to ensure survival and successful reproduction. 

In the case of animals, health is thus consistent with optimal performance, in terms of procuring food and water, avoiding predators, mating, giving birth and, in many species, successfully raising young. However, health is a relative concept, in so far as the state of an organism can be anywhere on a continuum from optimum health at one extreme to near death at the other.

The health needs of animals, including Homo sapiens, are determined by their evolutionary background.  This is because, through the processes of evolution, species have become well adapted in their innate biological characteristics to the conditions prevailing in the environment in which they are evolving.  It follows that these conditions are capable of satisfying their health needs.

If an organism is exposed to conditions of life that differ significantly from those that prevail in its natural environment – that is, the environment in which it evolved − it is likely to be less well adapted to the new and different environment and is likely to show signs of maladjustment. It will be less healthy than in its natural environment.  This fundamental evolutionary health principle applies to all plants and animals.

Thus, in the case of plants, all species are biologically adapted to the conditions prevailing in the environment in which they evolved.  That is, they are adapted to certain kinds of soil (e.g. soil depth, chemical constitution and water content), a certain intensity and quality of solar radiation, certain atmospheric conditions and a certain temperature range.  If they are exposed to conditions which differ significantly from those to which they are adapted, they will not grow well or will die.

The evolutionary health principle is taken for granted by those responsible for the health of animals in zoos.  Zoo keepers try to arrange that creatures in captivity receive the same kind of food that they normally eat in the wild and, if possible, to ensure that they are exposed to temperatures similar to those of their natural habitat. If, like hippopotamuses, they naturally spend most of their time in water, they will be provided with water to wallow in.  If the animals in the wild live in trees, then they will be provided with branches to climb.

Clearly, the natural environment does not satisfy the health needs of all creatures all the time.  Every animal eventually dies.  But in animal populations in their natural habitats most of the individuals are in a state of good health most of the time. This applies to all species, including our own.

HUMANS

As in the case of all other animals living in their natural environments, most of the time most of the members of hunter-gatherer communities were in a state of good health.  Indeed, they had to be in order to survive and successfully reproduce under the demanding conditions of their lifestyle and habitat. 

Most people: 

  • Would have been well nourished.  There is no diet better for any animal than that which is typical of its natural lifestyle and environment.  Undernutrition, malnutrition and obesity were rare in normal circumstances, although in periods of unusual drought people’s health would have deteriorated.
  • Would not, before contact with people from urban societies, have suffered from such infectious diseases as influenza, the common cold, measles, cholera, typhoid and plague.  There were simply not enough humans living together to support these pathogenic microbes.
  • Would not have suffered from such organic disorders as appendicitis, duodenal ulcer, diverticular disease of the colon and cardiovascular disease.  It is known that blood pressure tends to remain more or less constant in adults in primeval societies after the age of about twenty years, rather than increasing steadily after this age as is frequently the case in modern communities. 

On the other hand, the primeval life style was characterised by some built-in hazards which are absent from modern society.  There was a considerable risk of serious injury acquired during hunting, and severe wounds often became infected leading to gangrene or septicaemia.  Any incapacitation due to injury or ill health was of much greater survival disadvantage in the hunter-gatherer setting than under the protective conditions of modern civilisation. People did not have the benefit of the artificial antidotal measures like antibiotics, chemotherapeutic agents and surgery that are available today.  The average age in primeval populations was probably around 25 years. 

Numerous definitions of health have been proposed for humankind.  Here we adopt a biological definition similar to that given above for all living organisms:  health in humans is that physical and mental state that would have been likely to ensure survival and successful reproduction. Another appropriate definition is:  health is that state of body and mind conducive to, and associated with, full enjoyment of life.

Of greater practical interest than the definition of human health is the identification of the health needs of our species.  There are various approaches to this issue, ranging from one’s own personal experience to the application of knowledge from medical research.  Here we adopt a biological approach based on the evolutionary health principle, which recognises that the conditions to which humankind had become genetically adapted through evolution satisfied the survival and reproductive needs of our ancestors for many thousands of generations, and that significant deviations from these conditions are likely to be associated with signs of maladjustment or ill health.[i]  The evolutionary health principle clearly applies to a wide range of physical aspects of life conditions in humans.  There is no diet better for humankind than that which was typical for hunter-gatherers.  It is also clear that the principle is applicable to some aspects of behaviour.  Marked deviations from natural sleeping patterns cause maladjustment, and health is likely to be impaired if patterns of physical exercise deviate markedly from those of humans in the natural habitat. 

There are good reasons for supposing that the evolutionary health principle also applies to psychosocial and relatively intangible aspects of life experience.  For example, the conditions of life of hunter-gatherers are usually characterised by incentives and opportunities for creative behaviour, a sense of personal involvement in daily activities and plenty of convivial social interaction.  Most of us would agree that such conditions are all likely to promote health and wellbeing in our own society.

Taking our knowledge of the conditions of life of hunter-gatherers as a starting point, we can put together a working list of physical and psychosocial conditions likely to promote health and wellbeing in our species (Box 1).  They are referred to as universal health needs because they apply to all members of the human species, wherever or whenever they may be living. 

Not every item on this psychosocial list is absolutely essential for health. Lack of satisfaction of one psychosocial health need may be offset to some extent by the satisfaction of others.  On the other hand, every item on the list will, if satisfied, make a positive contribution to health and wellbeing.

Most of the items on the list of postulated psychosocial health needs, like creative behaviour and sense of personal involvement, cannot be defined and measured as easily as the physical health needs; but this does not mean they are less important.

Unfortunately, conventional measures of social wellbeing in our society today do not take the less tangible psychosocial aspects of life experience into account; nor do they feature on the platforms of the major political parties. However, it is crucial that deliberate effort be made to take these psychosocial factors into consideration in any assessment of current human life conditions or in planning for the future.

Brief comment is called for on the concept of stressors and meliors.  Stressors are experiences which cause anxiety and distress, and they are a normal aspect of life.  If they are short-lived and not too severe they can be seen as contributing positively to the quality of life and wellbeing; but if they are excessive and if they persist they can interfere seriously with both mental and physical health.  Equally important are experiences which have the opposite effect to stressors and which give rise to a sense of enjoyment.  Such experiences have been called meliors.  Common meliors include the experience of creativity, fun, aesthetic enjoyment, and conviviality. 

Every person can be considered at any given time to be at some point on a hypothetical continuum between a state of distress and a state of enjoyment.  Their position on this continuum is largely a function of the balance between meliors and stressors in their recent experience.  

The cultural environment has an immense influence both on the levels of meliors and stressors in an individual’s daily experience as well as on the nature of the factors that cause them.  Culture also affects the energy and pollution costs of attempts to avoid stressors or to experience meliors.

Universal health needs of the human species

Physical needs

Clean air (not contaminated with hydrocarbons, sulphur oxides, lead etc.)

A natural diet (that is, calorie intake neither less than nor in excess of metabolic requirements; foods providing the full range of the nutritional requirements of the human organism, as provided, for example, by a diverse range of different foods of plant origin and a little cooked lean meat; a diet which is balanced in the sense that it does not contain an excess of any particular kind of chemical constituent or class of food; foods with a physical consistency of that of natural foods and containing fibre; foodstuffs devoid of potentially noxious contaminants or additives)

Clean water (free of contamination with chemicals or pathogenic micro-organisms)

Absence of harmful levels of electromagnetic radiation (e.g. alpha, beta, gamma, ultraviolet and x-rays)

Minimal contact with parasites and pathogens

Protection from extremes of climate (temperature, wetness)

Noise levels within the natural range

Levels of sensory stimulation which are neither much lower, nor much higher, than those of the natural habitat

A pattern of physical exercise which involves short periods of vigorous muscular work and longer periods of medium (and varied) muscular work and stretching; but also frequent periods of rest

Psychosocial needs

An emotional support network, providing a framework for care-giving and care-receiving behaviour, and for exchange of information on matters of mutual interest and concern

The experience of conviviality

Opportunities and incentives for co-operative small-group interaction

Opportunities and incentives for creative behaviour and for practising manual skills

Variety in daily experience

Contact with the natural environment

An environment and lifestyle conducive to a sense of personal involvement, purpose, belonging, responsibility, challenge, comradeship and love.


[i]  This does not mean that evolutionary change in the human species has come to a halt.  There has been a relaxation of some selection pressures that were powerful in the hunter-gatherer environment and in the long term this will result in genetic changes in human populations (J. M. Rendel 1970. The time scale of genetic change. In S. Boyden (Ed.) The impact of civilisation on the biology of man. Canberra: Australian National University Press).  There have also been some new selection pressures associated with the advent of farming that have produced changes in some populations.  A well known example of this is emergence and  spread in European populations of lactase production into adulthood in response to the availability of bovine milk as a food source. For discussion of this change and for other examples see G. Cochran and H. Harpending (2009). The 10,000 year explosion: how civilisation accelerated human evolution. New York: Basic Books.

2 thoughts on “Evolution and Human Health

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s