Humans have evolved big, energy-hungry brains that require us to consume many more calories than our closest animal relatives. The same, however, does not appear to hold for our water intake.
Compared to apes, a surprising new study has found our bodies churn through far fewer fluids on a daily basis.
Researchers found that, on average, humans processed 3 litres, or about 12 cups, of water a day. Chimpanzees, bonobos, and gorillas living in a zoo, on the other hand, go through nearly twice that much.
The results were somewhat unexpected. Since humans have 10 times as many sweat glands as chimps, and are, on the whole, much more active than apes, you'd expect us to lose more water every day, not less.
Yet even when accounting for outside temperatures, body size, and activity levels, humans still required less water to maintain a healthy equilibrium.
"Compared to other apes, humans in this study had substantially lower water turnover and consumed less water per unit of metabolised food energy," the authors write.
This suggests early hominins somehow evolved a way or ways to conserve their bodily fluids, allowing them to travel from the rainforest to more arid regions. Exactly how that was achieved remains unclear.
"Even just being able to go a little bit longer without water would have been a big advantage as early humans started making a living in dry, savannah landscapes," explains the study's lead author and evolutionary anthropologist Herman Pontzer from Duke University.
In the study, researchers tracked the daily water turnover of 72 apes, in both zoos and rainforest sanctuaries, using doubly labelled water containing deuterium and oxygen-18 as trackers. This was able to tell researchers how much water was gained through food and drink and lost through sweat, urine, and the GI tract.
The results were then compared to 309 modern humans who drank the same doubly labelled water. These humans came from a range of lifestyles, including farmers, hunter-gatherers, and sedentary office workers.
Even among a small sample of adults in rural Ecuador, who drink a remarkable amount of water for cultural reasons (over 9 litres a day for men and nearly 5 litres a day for women), the overall water to energy ratio still matched humans elsewhere, roughly 1.5 millilitres for every calorie consumed.
In fact, it's worth noting that this same ratio is apparent in human breast milk. The breast milk of apes, on the other hand, has a ratio of water to energy that is 25 percent lower.
Such findings suggest the human body's thirst response has somehow been 're-tuned' over time, meaning we might crave less water per calorie than our ape cousins.
In the rainforest, apes get most of their water from plant food, which means they can go days or weeks without directly drinking at all. Humans, however, can only survive about three days without water, possibly because our food is not nearly so wet.
This inevitably requires us to drink fluids more frequently than apes, which means we can't stray too far from our ties to lakes and streams (or running water).
Pontzer refers to this as an 'ecological leash', and he argues natural selection has given humans a longer lead so we can travel further without water, allowing early hominins to expand into drier environments where heat stress is greater and finding food requires more work.
There is, however, another way our bodies might have changed to conserve water. Unlike apes, humans have external noses, which is thought to reduce water loss when we breathe.
These prominent snouts first appear in the fossil record about 1.6 million years ago, with the emergence of Homo erectus, and since then, such prominent noses have continued to diverge from the flatter snouts of apes.
More room within nasal passages gives water the opportunity to be cooled and condensed, allowing reabsorption of fluids instead of exhaling the liquid out into the air. In addition to our thirst response, these new noses might have been crucial in allowing humans to be more active in arid environments.
"There's still a mystery to solve, but clearly humans are saving water," Pontzer says.
"Figuring out exactly how we do that is where we go next, and that's going to be really fun."
The study was published in Current Biology.