Human Evolution Led to an Extreme Thirst for Water
Experts have now long been predicting that the next wars will be fought on water. Dramatic as it may sound, access to potable water is a real challenge in many parts of the world – the 3L&3S has featured several pieces on the subject. This research article in the Scientific American by human biologist Asher Rosinger shows this isn’t a new problem. Rosinger traces back the human body’s ever growing thirst for water to our evolution – “Without enough water, our physical and cognitive functions decline. Without any, we die within a matter of days. In this way, humans are more dependent on water than many other mammals are. Recent research has illuminated the origins of our water needs—and how we adapted to quench that thirst. It turns out that much as food has shaped human evolution, so, too, has water.”
“…To understand how water has influenced the course of human evolution, we need to page back to a pivotal chapter of our prehistory. Between around three million and two million years ago, the climate in Africa, where hominins (members of the human family) first evolved, became drier. During this interval, the early hominin genus Australopithecus gave way to our own genus, Homo. In the course of this transition, body proportions changed: whereas australopithecines were short and stocky, Homo had a taller, slimmer build with more surface area. These changes reduced our ancestors’ exposure to solar radiation while allowing for greater exposure to wind, which increased their ability to dissipate heat, making them more water-efficient.
Other key adaptations accompanied this shift in body plan. As climate change replaced forests with grasslands, and early hominins became more proficient at traveling on two legs in open environments, they lost their body hair and developed more sweat glands. These adaptations increased our ancestors’ ability to unload excess heat and thus maintain a safe body temperature while moving…”
While sweat glands help maintain the body temperature, they also compromise on our ability to retain water especially in dry arid conditions which makes us more vulnerable to dehydration than other mammals.
“Mammals vary in the size and shape of their kidneys and thus the extent to which they can concentrate urine and thereby conserve body water. The desert pocket mouse, for example, can live without water for months, in part because of the extreme extent to which its kidneys can concentrate urine. Humans can do this to a degree. When we lose copious amounts of water from sweating, a complex network of hormones and neural circuitry directs our kidneys to conserve water by concentrating urine. But our limited ability to do so means we cannot go without freshwater for nearly so long as the pocket mouse.
Neither can we preload our bodies with water. The desert camel can drink and store enough water to draw on for weeks. But if humans drink too much fluid, our urine output quickly increases. Our gut size and the rate at which our stomach empties limit how fast we can rehydrate. Worse, if we drink too much water too fast, we can throw off our electrolyte balance and develop hyponatremia—abnormally low levels of sodium in the blood—which is just as deadly if not more so than dehydration.
Even under favorable conditions, with food and water readily available, people generally do not recover all of their water losses from heavy exercise for at least 24 hours. And so we must be careful to strike a balance in how we lose and replenish the water in our bodies.”
However, nature helps us find alternative sources of water. Whilst it is recommended that we drink 2-3ltrs of water everyday, not all need to come by drinking. A lot of food especially fruits and vegetables which are high on water content can help rehydrate us. “Our dietary flexibility is perhaps our best defense against dehydration. As I learned the hard way on that sweltering night, the amount of water present in food contributes to total water intake. In the U.S., around 20 percent of the water people ingest comes from food, yet my work among Tsimane’ found that foods, including fruits, contribute up to 50 percent of their total water intake. Adults in Japan, who typically drink less water than adults in the U.S., also get around half their water from the foods they eat. Other populations employ different dietary strategies to meet their water needs. Daasanach pastoralists in northern Kenya consume a great deal of milk, which is 87 percent water. They also chew on water-laden roots.”