A new study suggests that humans evolved to work with less water than the closest primates.
Durham, North Carolina – When you think about what separates humans from chimpanzees and other apes, you might think of our large brains, or the fact that we move on two legs rather than on all fours. But we have another distinguishing feature: water efficiency.
This is the new study conducted, for the first time, that accurately measures the amount of water that humans lose and replace each day compared to our closest living relatives.
Our bodies are constantly losing water: When we sweat, go to the bathroom, even when we breathe. This water needs to be replenished to keep the volume of blood and other body fluids within normal ranges.
However, the research was published March 5 in the journal Current biology It shows that the human body uses 30% to 50% less water daily than our closest animal relatives. In other words, humans evolved among primates to be the low-flow model.
Lead author Hermann Pontzer, associate professor of evolutionary anthropology at Duke University, said an ancient shift in our bodies’ ability to conserve water had enabled our ancestors of hunters and gatherers to venture away from streams and watering holes in search of food.
“Even just being able to survive a little longer without water would have been a huge advantage as early humans began earning a living in dry savannahs,” Bunzer said.
The study compared the average water turnover of 309 people with a range of lifestyles, from farmers and hunters to office workers, with 72 monkeys living in zoos and reserves.
To keep fluid balance within a healthy range, the human body or any other animal is somewhat like a bathtub: “The incoming water must equal the exit of the water,” Bonzer said.
You lose water through sweating, for example, and your body’s thirst signals are triggered, telling us to drink. Drink more water than your body needs, and your kidneys get rid of the extra fluid.
For each individual in the study, the researchers calculated the amount of water consumed through food and drink on the one hand, and water lost through sweat, urine and the digestive system on the other hand.
When they combined all the inputs and outputs, they found that the average person treats about three liters, or 12 cups of water, every day. Chimpanzees or gorillas that live in a zoo pass twice that amount.
Pontzer says the researchers were surprised by the results because humans, among primates, have an amazing ability to sweat. For every square inch of skin, Bonzer said, “humans have 10 times as many sweat glands as chimpanzees.” This makes it possible for a person to sweat more than a half gallon during an hour-long workout – the equivalent of two Big Gulps from 7-Eleven.
Add to this the fact that great apes – chimpanzees, bonobos, gorillas and orangutans – lead a lazy life. “Most monkeys spend 10 to 12 hours a day resting or feeding, then sleep for 10 hours. They only move around a few hours a day,” Bonzer said.
But the researchers controlled for differences in climate, body size, and factors such as activity level and calories burned daily. So they concluded that providing water to humans was a real thing, and not just a function of where individuals lived or how physically active they were.
The results indicate that something changed over the course of human evolution that reduced the amount of water the body uses each day to stay healthy.
Back then, as now, we’re probably still alive just a few days without drinking, Pontzer said. “You might not break that environmental leash, but at least you’ll get longer if you can stay longer without water.”
The next step, Pontzer says, is to determine how this physiological change occurs.
One hypothesis, suggested by the data, is that our body’s thirst response has been reset so that, in general, we crave less water per calorie than our monkey relatives. Even when babies are babies, long before we eat our first solid food, the ratio of water to calories in human breast milk is 25% less than that of other great apes.
Another possibility lurks before our faces: Fossil evidence indicates that about 1.6 million years ago, with the onset of Homo erectus, humans began to develop a more prominent nose. Our cousins of gorillas and chimpanzees have very flat noses.
The nasal passages help conserve water by cooling and condensing the water vapor from the exhaled air, turning it back into a liquid inside our nose where it can be reabsorbed.
Having a more prominent nose may have helped early humans retain more moisture with each breath.
“There is still a mystery to be solved, but it is clear that humans conserve water,” Bonzer said. “Knowing how to do that is exactly where we’re going next, and that would be really fun.”
This research was supported by the US National Science Foundation (BCS-0643122, BCS-1317170, BCS-1440867, BCS-1440841, BCS-1440671), United States Agency for International Development (APS-497-11-000001), National Institutes of Health ( R01DK080763), John Templeton Foundation, LSB Leakey Foundation, Wenner-Gren Foundation (Gr. 8670), University of Arizona, Duke University, and Hunter College.
Quote: “The Evolution of Water Conservation in Humans”, Hermann Pontzer, Mary H. Brown, Brian M. Wood, David A. Reichlin, Odax. ZP Mabulla, Jacob A. Harris, Holly Dunsworth, Brian Hare, Kara Walker, Amy Luke, Lara R. Dugas, Dale Schoeller, Jacob Plange-Rhule, Pascal Bovet, Terrence E. Forrester, Melissa Emery Thompson, Robert W. Shumaker, Jessica M Rothman, Erin Vogel, Francesca Solisteo, Shaheen Alawi, Didik Prasetyo, Samuel S. Urlacher, and Stephen R Ross. Current biology, March 5, 2021. DOI: 10.1016 / j.cub.2021.02.045
Robin Ann Smith
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