Human Fatigue

Evolution, Health and Performance

Francesco Marino

• Publisher: Routledge
• Year: 2019
• ISBN:
  9781317380115
• Edition: 1
• Topic: Medicine
• Subtopic: Physiology

Fatigue is a condition spanning the breadth of human functioning in health and disease and is a central concern in sport and exercise. Even so we are yet to fully understand its causes. One reason for this lack of understanding is that we seldom consider fatigue from an evolutionary perspective - as an adaptation that provided reproductive success.

This ground-breaking book outlines the evidence that fatigue is a result of adaptations distinctive to humans. It argues that humans developed adaptations which led to enhanced fatigue resistance compared with other mammals and discusses the implications in the context of exercise, health and performance. Highly illustrated throughout, it covers topics such as defining and measuring fatigue, the emotional aspect of fatigue, how thermoregulation affects the human capacity to resist fatigue, and fatigue in disease.

Human Fatigue is essential reading for all exercise scientists as well as graduate and undergraduate students in the broad field of physiology and exercise physiology.

Chapter 1

Evolution and natural selection in human performance, health and disease

How extremely stupid not to have thought of that.

– T.H. Huxley’s reaction after reading Darwin’s The Origin of Species

Introduction

Any typical textbook dealing with the broad topic of exercise physiology or exercise science will include cursory information on human evolution, if at all. This is perhaps not surprising given that evolutionary theory is generally accepted and that scientists have a working knowledge of the topic. On the surface this is likely to be true, for the basic assumption is that humans, like all other living organisms, evolved over a long period of time – evolutionary time. On this point, we take this to signify millions of years of tinkering that eventually spawned the modern form of Homo sapiens. One of the problems with this line of thought is that the evolution of Homo sapiens was directional and that we are the outcome. That is, evolution was linear and the only possible result is what we have today. This view is so far from reality and the facts that we have typically become passive in our understanding of evolutionary theory and how it can be applied to understand and solve our modern-day problems. In fact, very few of us would consider evolutionary theory as a means of understanding our biology as applied to human performance and health.

In order to address this fundamental understanding of human physiology as it pertains to performance, health and disease, this chapter introduces some of the most fundamental concepts of evolutionary theory before venturing into the complexities of form and function. This chapter considers the concepts first introduced by Darwin and Huxley, along with the classic studies confirming that evolution is something more than just a theory, but a process which is alive and dynamic. To begin our quest in understanding the place of evolution in biology and in human performance we need only reflect upon the reasoning given by eminent scholars in the field.

In his highly cited 1973 paper titled “Nothing in biology makes sense except in the light of evolution,” Theodosius Dobzhansky (1973) outlines the arguments and counter-arguments that are typically used in the debate as to whether evolution is a verifiable theory. Although this paper is usually invoked as a way of asserting that the cornerstone of biology is in fact evolutionary theory, it is worth reiterating the basis of Dobzhansky’s argument. In his description of life’s remarkable diversity, Dobzhansky also notes that the unity of life is in fact no less remarkable. That is, from the most seemingly rudimentary life, such as a virus, to the most complex organism, the biochemistry is not only similar but also simple. This is not to suggest that DNA and RNA are uncomplicated; on the contrary, it is the universality of the biochemistry that is indicative that all life is intimately connected by preserving the most basic primordial features. The classic theories of Oparin and then Haldane (Miller 1953; Miller et al. 1997) provide the basis from which life likely arose, whereby the Earth’s atmosphere, composed largely of nitrogen, ammonia, methane and helium, combined in a primordial soup to form the building blocks of life, the amino acids. The way in which amino acids constitute sequences for given kinds of proteins and that vary within and between species, along with single amino acids arising by genetic mutations, Dobzhansky submits “that all these remarkable findings make sense in the light of evolution; they are nonsense otherwise” (p. 128). However, the essence of Dobzhanky’s reasoning is as follows:

There is, of course, nothing conscious or intentional in the action of natural selection. A biologic species does not say to itself, “Let me try tomorrow (or a million years from now) to grow in a different soil, or use a different food, or subsist on a different body part of a different crab.” Only a human being could make such conscious decisions. This is why the species Homo Sapiens is the apex of evolution. Natural selection is at one and the same time a blind and creative process. Only a creative but blind process could produce, on the one hand, the tremendous biologic success that is the human species and, on the other, forms of adaptedness as narrow and as constraining as those of the over specialised fungus, beetle, and flies.

(p. 127)

Although there are numerous texts outlining the basic tenets of evolution, the purpose of this section is to provide what might be regarded as the fundamental aspects of evolutionary theory thought to have a direct relationship to understanding human performance and our ability to live healthy lives and avoid disease. To this end, there are two terms used in the foregoing passage taken from Dobzhansky’s paper that are fundamental to understanding evolutionary theory. These are natural selection and adaptedness. In order to apply the principles of both natural selection and adaptedness to any aspect of biology, especially when attempting to understand human health and disease from an evolutionary perspective, we must first consider the context in which they were originally used by Darwin in The Origin of Species (Duzdevich 2014). In articulating what Darwin considered to be natural selection, he noted that very slight variations regardless of their causation would in some way benefit the individuals of a particular species in their relationship with other living things and/or their physical environment.

One critical aspect of this would be that these benefits, however small, would be passed on to the offspring. This transmission from parent to offspring will provide a greater chance for reproductive success and, therefore, the preservation of the species. Although the process of natural selection would seem straightforward, Darwin also recognised that this was dependent on the key observation that only a small number of individuals that are born can actually survive. This simply means that every living thing is always striving to increase its numbers. But why? The answer to this is not apparently obvious. Huxley (2010) in his classic work notes that there is a tendency for all organisms to increase in geometrical ratio. That is, in the early stages of existence the offspring are always more numerous than their parents, even though the numbers of a given species tend to remain approximately constant. Thus, if more young are produced than can survive, the only conclusion that can be drawn is that there must be competition for survival (Huxley 2010). It is this competition or struggle for existence that will assist in the accumulation of variations as being either favourable and therefore passed on or unfavourable and not passed on because of the failure to reproduce. Natural selection is concerned only with improving the chances of reproducing.

The term adaptation or adaptedness implies that there is a shaping of a particular feature or features of an organism in order that there is a better fit with its physical environment. However, for adaptation to take place the process relies on the multitude of individual differences which appear in a population. In essence, all species, including humans, are variable. That is to say that individual members of a particular group will vary in a number of their characteristics. Some members of a group will be taller than others, more or less agile, more or less muscular and more or less resistant to disease. In fact, this kind of variation is seen at all levels of the organism – from molecular to gross anatomical features. Let us now look at two typical examples which help illustrate the importance of adaptation. The most widely cited example of rapid adaptation is that of the peppered moth during the Industrial Revolution in England (Cook & Turner 2008). There are two types of peppered moths: the light-bodied and the dark-bodied. Before the Industrial Revolution the dark variety was thought to be rare. However, the light-bodied variety was much more numerous as it was able to blend in with the light-coloured lichens on trees, whereas the black-bodied variety would be easily picked off by birds due to their higher visibility. Within decades of the Industrial Revolution the trees between London and Manchester became darkened as a consequence of soot deposits from coal burning in addition to the lichens dying on the trees as a consequence of the increased sulphur dioxide emissions. The darkened trees now effectively provided less camouflage for the light-coloured moths, whereas the number of dark-coloured moths being picked off by birds was dramatically reduced. The outcome was a rapid increase in the number of dark-coloured moths. To confirm that the population of dark-coloured moths was indeed due to natural selection and rapid adaptation to the environment, it is believed that a series of experiments (Kettlewell 1955, 1959) in which the filming of the predation of birds on moths in both polluted and unpolluted woodlands is an instance where Darwinian evolution was captured in action (Majerus 2008).

The observations on the peppered moth provide some compelling evidence at the macro level for evolution by natural selection. Even more compelling are the experiments conducted on the bacterium E. coli (Lenski & Travisano 1994). These researchers took the opportunity to put to the test evolution by natural selection in fast-forward motion since these bacteria reproduce asexually, so that cloning can create a huge population of identical individuals in a very short time. After taking 12 separate populations of identical bacteria and propagating them in replicate environments for 10,000 generations, they were able to report on two properties of the evolving bacterial population: cell size and mean fitness. These two properties were studied because size is a trait which influences the functional properties of an organism, whereas fitness is the trait which is utilised by the organism to compete for resources. The interesting aspect of this experiment was that the researchers employed natural selection by imposing an environment on the bacterial populations rather than artificial selection. In this way they were able to ascertain whether there were any heritable properties that would enhance reproductive success in that particular environment. A key aspect of this experiment was the chosen environment in which the bacteria were placed. The environment was essentially a glucose broth which was calculated to support a given number of cells. Each day the bacteria were observed to grow until the glucose was depleted, at which time some of the bacteria were transferred to new replicate environments. This continued for 1,500 days until such time that the original bacteria reached 10,000 generations in 12 different populations. The first interesting result is that cell size in the bacterial population increased rapidly for approximately the first 2,000 generations when introduced to the new environment. However, when the environment remained unchanged for several thousand generations, increases in cell size were less than negligible. As suggested by the authors, the trajectories appear to be very similar but reached different plateaus for cell size, leading them to conclude that the populations diverged from the common ancestor and from one another in at least the size of the cells.

To measure the fitness of the bacteria the researchers then resurrected the original (ancestral) frozen bacteria and placed them in direct competition with the more recent generation. The questions they were asking were whether the evolutionary trajectories of fitness were similar to that observed for the cell size or fitness improved at a constant rate throughout the experiment. Beyond the striking similarity between cell size and relative fitness is that adaptation to the environment in all 12 populations was rapid when introduced to the new environment compared to when the environment was constant over several thousand generations. Taken together, the increase in cell size and the change in relative fitness show that significant variation arose in the early stages of the experiment and then persisted until the end of the experiment at 10,000 generations. The fact the mean relative fitness was initially equal to zero in all populations is indicative that mutations arose independently in each population, even though the environments in which the bacteria were placed were identical.

These two examples of the adaptedness of the peppered moth and E. coli highlight the three conditions required for evolution by natural selection to occur: (1) there is variation among individuals within a population, (2) the variation is heritable, and (3) the variation is related to the success of individuals in competing for resources.

The preceding discussion considered only the overarching understanding of evolution by natural selection. It did not, for example, consider the workings of genetics in the grand scheme of evolution. The purpose of the preceding discussion was merely to outline the key determinants of evolution and provide the reader with the conceptual framework which will be used throughout the book.

Evolution, disease and human fatigue

In the preceding section, we considered two examples of evolution by natural selection: the peppered moths and the bacterium E. coli. One might well ask, what have the peppered moths and E. coli to do with evolution by natural selection with respect to human performance, health and disease? The simple answer is that whatever mechanisms were at play in the adaptedness of the peppered moths and the bacteria to their environment would surely also have been at play when we consider our evolutionary path, our resistance to disease and our ability for physical and mental performance in a wide range of human endeavours. It is a safe assumption within the scientific community that there is no dispute that natural selection occurs. However, its significance in explaining human performance in a wide range of activities and situations must be demonstrated by way of evidence. This section will explore factors thought to have led to the unique human traits that distinguish us from our ancestors, both living and past.

When we consider impressive human feats such as the breaking of the four-minute mile, the 100m sprint in under ten seconds, edging ever closer to the two-hour marathon and even the impressive swimming records, one can’t help but be amazed at the resilience and beauty of the human in action. We would be forgiven if we thought, just for a moment, that our biology was indeed perfect. In reality, when we consider the uniqueness of humans, we cannot help but also notice that many of our ailments can arguably be related to our imperfect biology. Thus, there is an imperative to understand how and why our biology is the way it is and what limitations it imposes when we interact with our environment. As we have already seen, an organism’s survival is dependent to a large degree on its adaptability and whether those advantageous traits are passed on to the next generation. This is the case for humans as well. Therefore, it would be more than helpful to understand how humans evolved and which adaptations are useful and which are not. If we understand this aspect of human evolution, we might then be in a position to not only understand but also predict why we get sick and why our performance is perhaps limited in certain circumstances as well as truly appreciating the wondrous feats we are so accustomed to seeing. By way of illustration, there are some modern-day chronic diseases which we can use to illustrate this point.

There will be little argument that obesity is a prevalent disease for which we seemingly have had no answer, at least within the last 20 years. The latest obesity update from the Organisation for Economic Co-operation and Development (OECD; www.oecd.org/health/obesity-update.htm) indicates that at least 18% of the adult population in OECD countries is obese, constituting about 1%–3% of healthcare expenditure in most countries but up to 5%–10% in the United States. The knock-on effect of the disease is likely to result in the appearance of other chronic diseases, such as Type 2 diabetes mellitus. Although the OECD update and others (Gard 2011) also report the rate of rise in obesity in the last five years has slowed, there is little to indicate that we have dealt with the problem effectively. There is little doubt that significant gains can be made by understanding the physiology on a systemic and molecular level of such a disease, yet little or no attention has been paid to truly understanding why we have been powerless to curb the incidence of the disease. Is there an alternative approach that can be used to solve this problem? How can evolutionary theory by natural selection help us to understand (1) why obesity has increased so rapidly in a relatively short period of human history, and (2) how we can use our knowledge of natural selection to guide us in finding the solution for such a modern-day disease? This, of course, is not to assert that obesity never existed in the past, when it clearly did. Rather, the issue is why this disease has appeared so suddenly and taken hold so rapidly when in 1980 fewer than one in ten people were obese, whereas this is now up to six in ten people in some countries. At the very least this gives us a clue that the disease is almost entirely preventable.

So, the big question it seems is, why does it occur? Why are humans susceptible to this disease and why are some of us more likely than others to fall into its grasp? These questions alone are compelling enough to make us think deeply about our roots and understand the factors of our past that have made us so susceptible to the disease. Perhaps a better question to ponder is, what parts of our biology actually haven’t changed since humans appeared, and are these the responsible villains?

Adaptations and health

Returning to the phenomenon of adaptedness, it is perhaps within this construct that the answer to our crisis of chronic ill health lies. In considering this, the most obvious question to ask is, what adaptations took place over millions of years that were useful over a long period of our existence in extremely different environments and provided for reproductive success, but perhaps are now conspiring to cause ...