We are obsessed with water. Just take a look around you: bottled water is the new craze. Our culture has seen hydration move from a biological necessity driven by internal biological signals to a fashion trend. Everyone is drinking. Why?
But where’s the proof?
There isn’t any. In fact, dehydration is a normal physical state. Drinking to excess is far more dangerous to your health than delaying drinking and waiting until your thirst tells you that you need to drink.
A short history lesson on how the human body evolved on the hot and arid African savannah will help in understanding this.
BORN TO RUN
Our species, Homo sapiens , split from the last common ancestor shared with the great African apes about seven million years ago. Indeed, Charles Darwin, unquestionably the most important biologist of all time, predicted that man must have evolved in Africa, since our closest mammalian relatives –the chimpanzees, bonobos and gorillas –are found only on the African continent. What he did not describe were the specific activities that made humans become human. Were it not for these activities, humans would have evolved along the branch that led to chimpanzees.
There are two crucial physiological differences that decided our differing evolutionary trajectories over the past seven million years:
1. Human’s ability to run long distances at a moderate pace.
2. Human’s ability to maintain a safe body temperature when exercising in extreme dry heat.
Over the past decade, evidence has accumulated that supports a theory that might perhaps have astonished even Darwin –that nature has equipped humans with all the tools they need to be the best hot-weather distance runners of all species on the planet.
To understand this process, we need to begin at the ‘starting line’ of the race that produced humans.
Evolutionary edge no. 1 – bipedalism
Roughly seven million years ago, the common ancestor of humans and chimpanzees stood upright. We know this thanks to the work of anthropologist Tim White and his team, who in 1994 uncovered the remarkable skeleton of Ardipithecus ramidus in the Awash region of Ethiopia.
Unexpectedly, more than four million years ago Ardipithecus walked on two legs, but was still an adept tree climber. This establishes that the most likely last common ancestor of chimpanzees and humans walked upright, which in turn means that chimpanzees moved from walking upright, or bipedalism, to what they do now –knuckle-walking –whereas humans became increasingly competent walkers and ultimately the world’s most effective endurance runners.
As humans we differentiate ourselves by standing upright. At first it was thought that we had evolved to stand on two legs simply as a means to free our hands, which would then be used to fashion stone tools or weapons, or carry food or our offspring. This is an obvious benefit. But as the climate of eastern and southern Africa changed three to four million years ago, our ancestors moved from the security of the forests to the baking heat of the open savannah. This move produced two more reasons that favoured bipedalism.
First, bipedalism reduces the surface area exposed to the direct heat of the sun. It also raises the body into a cooler microclimate above the level of the surrounding vegetation, where there is an increased airflow. The cooler air drifting over our skin is heated up, thereby removing heat from our bodies in a process known as convection. So, to keep cooler in the hot sun of the African savannah, we needed to stand upright.
Second, bipedalism allowed us to become more effective runners, which, as Professor Raymond Dart noted in 1959, was of great evolutionary value in the environment of ‘a more open veld country where competition was keener between swiftness and stealth, and where adroitness of thinking and movement played a preponderating role in the preservation of the species’. Ardipithecus walked upright, but would have been a poor runner because his lower limbs, pelvis and muscles were not yet adapted for walking and running. Four million years later, Homo sapiens , the direct descendant of Ardipithecus , is an extraordinarily graceful runner, as is visible whenever we watch the languid African distance runners.
Evolutionary edge no. 2 – sweat
The second important evolutionary development was that, of all the creatures on the earth, we became the animal with the most prodigious sweating capacity. Sweating is the most effective mechanism to remove heat from the body, especially the heat generated during exercise.
When you begin to exercise, the blood flow to your muscles increases. As the blood passes through your muscles, it is heated up. This heat is then distributed throughout your body, but especially to your skin. Your hotter skin then offloads its heat by convection, and by sweating, which is the process of evaporating water (as steam) secreted from the sweat glands. Relatively few mammals sweat. Baboons, horses, donkeys, camels and certain large antelope, such as the eland and oryx, do sweat, but none more so than humans.
This occurs because humans have a greater concentration of sweat glands in the skin than any other mammal. There has to be a reason for our large number of sweat glands. Ardipithecus ramidus would not have had as many sweat glands; chimpanzees, in fact, do not sweat, so they are unable to survive in climates in which the air temperature exceeds 38 °C. Some crucial event therefore occurred after the appearance of Ardipithecus that produced this extraordinary sweating capacity in Homo sapiens. Logic says that it had to be the need to lose great amounts of body heat. Simply living in hot conditions would not be sufficient to drive this development –if it were, chimpanzees would have the same capacity to sweat as modern humans have.
The key is that when mammals exercise, they generate heat in proportion to their body mass and the speed at which they are travelling.
Thus, the moment humans begin to exercise, they produce much more heat than they do when they are at rest. The faster they run, the more they need to sweat if they are to maintain safe body temperatures and not fall victim to heatstroke.
Humans can survive in relatively hot environments without the need to sweat as much as they can, but once they start to exercise more vigorously, especially in the heat, they need to sweat more, in proportion to how fast they run.
The converse of this is that, because humans evolved this magnificent sweating capacity, they are able to survive in resting conditions at temperatures that appear to be impossible to survive.
Smaller mammals generate less heat and therefore need to lose less water by either panting or sweating to maintain a safe body temperature. Look at the elite distance runners of today, most of whom are physically small, weighing about fifty to fifty-five kilograms –not much bigger than Ardipithecus. Being smaller is definitely a major advantage during exercise in the heat.
The potential downside of sweating is that it promotes the loss of fluid and electrolytes, especially sodium chloride (salt). But the benefits of sweating in allowing humans to exercise safely, even in extreme dry heat, far outweigh any risks associated with these losses. In fact, we evolved an almost fail-safe mechanism to ensure that we do not endanger ourselves during exercise. We become thirsty and develop sensations of fatigue, including the feeling of unbearable hotness, when running in the heat and direct sunlight. These sensations cause us to slow down and ultimately to stop exercising, although our body temperatures are still within the safe range and all our other bodily systems are still functioning appropriately.
So although humans need both salt and water on a daily basis to survive, since both are lost in sweat, we do not need to replace those losses the instant they develop during exercise. Our complex battery of mechanisms allows us to replace them after exercise, when we have the opportunity to eat and drink without restraint.
Evolutionary edge no. 3 – no fur
The third advantage humans have over other mammals is an absence of fur. This allows an increased capacity to lose heat from the skin by either convection or sweating.
The disadvantage of having no fur is that it reduces our ability to reflect the sun’s rays. As a result, humans absorb more radiant heat from the sun and the environment than, for example, antelope with light-coloured reflective fur, like the camel or oryx. As mentioned, however, standing upright reduces the area of our bodies exposed to this radiant heat load.
But the ability to sweat more efficiently and to lower the skin temperature more effectively during exercise in the heat outweighs any small disadvantage posed by the absence of the furry coating of our ancestors.
Evolutionary edge no. 4 – a brain that protects us from heat injury
The unmatched ability of humans to live in so many diverse environmental conditions –from the heat of the desert to the cold of the Arctic –is due to many factors, the most important of which is that we can maintain an almost constant internal environment regardless of what is happening on the outside of our bodies. A large component of this ability is that we modify our behaviour when we are exposed to taxing environmental conditions –we are less active in the heat than in the cold, and we also wear less clothing. The modification of our behaviour to regulate our body temperatures is termed ‘behavioural thermoregulation’. Naturally, it is the brain that ensures that these behaviours are appropriate for the environmental conditions, and it is the brain that protects us from damage.
During exercise in the heat, the brain ensures that we only ever exercise at an intensity and for a duration that won’t allow our body temperature to rise to a dangerous level. It does this by calculating exactly the acceptable rate of heat production by the muscles. The brain then chooses the appropriate sweat rate that will keep the body cooled during the exercise.
If these systems should make an error and allow the body temperature to rise to 42 °C or higher, the condition of ‘hyperthermic paralysis’ develops. With this condition, the brain simply refuses to direct the legs to continue exercising; the athlete feels as if he or she is paralysed and is unable to continue.
The remarkable success of the sports-drink and bottled-water industries over the past twenty years has been based largely on their ability to market the opposite theory –specifically that humans are very poorly adapted for exercise in the heat and that they lack biological controls to ensure that they do not overtax themselves during exercise. Instead, unless they force themselves to drink ‘as much as tolerable’ (a phrase used in a 1996 volume of Medicine & Science in Sports & Exercise, the official journal of the American College of Sports Medicine), they will become ‘dangerously dehydrated’ and at risk of dying from heatstroke every time they exercise. Had this been the case, Homo sapiens would never have evolved from Ardipithecus ramidus.
HUMANS ARE HUMANS BECAUSE WE ARE DESIGNED TO RUN IN THE HEAT
It is necessary to understand that it is not alien for humans to run long distances in extreme heat. If we had not developed as hot-weather runners, we would not be human.
Part of the proof of this, as pointed out by D.M. Bramble and D.E. Lieberman, is an obvious indicator of our humanness –our legs. Relative to our body weight, humans have the longest legs of any species. Our legs are also relatively thin, reducing the energy cost of using them. In addition, we have ‘springs’ in our legs –long, spring-like tendons attached to short muscles –that allow energy to be alternatively stored and released with each stride as we run.
Running does indeed expose the joints to loading stresses when landing, but this is reduced by increased joint surfaces. Humans have substantially larger joint surface areas than chimpanzees, for instance.
We also have the ability to keep our centre of mass stable by rotating the upper body while stabilising the head and neck when both feet are off the ground. This occurs in running, but not in walking. Thus the fact that humans are able to rotate their upper bodies (to hit a golf, tennis or cricket ball, for example) while our nearest living relatives –chimpanzees and gorillas –cannot, surely indicates that the development of this ability became a crucial difference that drove our subsequent evolution. The only factor that could have compelled this was the need for evolving humans to become expert runners.
We are obviously less stable when running, so our body requires additional muscular stabilisation, including well-developed back muscles, the action of the uniquely human gluteus maximus muscle (the buttock muscle), and the necessary brain controls to maintain balance and prevent falling when running.
We also need to be able to maintain balance when one of our legs is off the ground while running. Again, evolutionary pressures ensured the optimum solution. When running, we instinctively swing the opposite arm independently of the pectoral girdle (the two shoulder blades and collarbones) while keeping the head still.
We have enlarged sensory organs in the ear to improve the sensitivity of the reflexes that control the rapid pitching movements that we encounter when we run. These forces do not develop when we walk, again confirming that it was the specific need to run, not to walk, that drove that change. Without these nervous controls, humans would be unable to perform acrobatic feats in gymnastics, surfing or other similar activities.
Another important part of our physiology is that we drink frequently in small amounts, and, as discussed earlier, can delay this need to drink until after the exercise bout ends. Indeed, humans are able to delay the onset of thirst. We drink the most fluid when we eat, especially at the evening meal.
A smaller stomach and intestine than most mammals means that humans cannot drink large volumes of fluid quickly. A camel can drink 100 litres of fluid in ten minutes and a donkey can consume ten to twelve litres in five minutes. Humans, by contrast, can usually drink about one litre in ten minutes.
So the evidence becomes increasingly clear: humans evolved to be extremely adept long-distance runners with an unmatched ability to regulate their body temperatures when exercising in the heat. And our brains developed the ability to delay the need to drink –a crucial adaptation if we were to chase after our potential meals in the midday heat when there was little water available and no time to stop the hunt to search for fluid. Any time spent searching for water would mean the difference between a successful and a failed hunt.
If you remain sceptical, let me take you on a typical hunt with a !Xo San hunter, drawn from the research of Dr Louis Liebenberg, who began studying the !Xo San hunters in 1985. What follows is based on two research papers by Dr Liebenberg, ‘Persistence hunting by modern hunter-gatherers’ (2006) and ‘The relevance of persistence hunting to human evolution’ (2008).
The hunters wait until mid-morning, when the temperature exceeds 40 °C and when a human’s superior ability to maintain a safe body temperature while jogging for four or more hours in hot conditions gives our species the one unique biological advantage over the antelope. The hunters follow an antelope, never allowing it to stop for sufficiently long to cool its body temperature by panting. Eventually, an animal suffering from hyperthermic paralysis becomes too hot to continue. Only when it is in that condition of complete paralysis are the small, unarmed humans able to throttle the exhausted animal to death without the risk of being impaled on its horns. (It was only about 400 000 years ago that humans first developed spears –until then, they killed large antelope with their bare hands.)
In his study of just under a dozen hunts, Dr Liebenberg found that most hunts lasted from two to almost seven hours in temperatures ranging from 32 °C to 42 °C, covering distances from seventeen to thirty-five kilometres at running speeds ranging from four to ten kilometres per hour.
In the film The Great Dance , produced by the Cape Town brothers Craig and Damon Foster, one of the last surviving !Xo San hunters, Karoha Langwane, ran in the Kalahari Desert for six hours in 40 °C to 46 °C heat with no cloud cover but in low humidity. He ran barefoot and covered approximately thirty kilometres. During the hunt, Langwane drank a total of about one litre of fluid.
Langwane did not die from either dehydration or heatstroke, nor did he describe any significant symptoms other than thirst.
Yet at least three female marathon runners in the United States, running in cool conditions with free access to all the fluid each would ever need, died from a disease –exercise-associated hyponatraemia (EAH), which I was the first to describe in 1981. Their deaths, we subsequently proved, were caused because they had drunk too much.
The lesson is a simple one: humans have the greatest capacity of all mammals to run in extreme dry heat without drinking much. Since evolution designed our bodies to function in this way, all we have to do is listen to our bodies rather than to those who wish to sell us a product on the premise that humans are fragile beings. We are not. If we were, we would not be here in the first place.
Considering all the evolutionary evidence indicating how perfectly designed the human body is to push itself to the limit of human endurance in hot-weather conditions, imagine the challenge faced by the manufacturers of sports drinks and bottled water, who must convince athletes that they need to drink regularly –even during exercise lasting only a few minutes –if they are to avoid severe injury, or possibly death. Once the evolutionary drives that made humans human are understood, it becomes more difficult to fall for the marketing deceptions that have been promoted by these industries for the past twenty years.
Challenging Beliefs: Memoirs of a Career
Dr. Timothy Noakes