Homo sapiens: the water-saving ape

It’s an age-old question in evolution: how did humans become the dominant primate, able to venture out from tropical rainforests to the savannah and then populate the entire world? In other words, what makes us unique?

Science offers many explanations, including our large brains, bipedal gait and ability to craft complex tools – but a new study suggests that we’re also more efficient at conserving water.

For humans, water is life: our bodies are 60% water and we need to drink 2–3 litres per day to replenish what we lose through sweat, urination and even breathing. And yet, according to an international team of scientists, humans actually use about 30–50% less water per day than our closest primate cousins.

The study, published in the journal Current Biology, may have interesting consequences for our understanding of human evolution. An ability to conserve water could have allowed our hunter-gatherer ancestors to travel further from water sources in order to find food.

“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 Herman Pontzer, lead author and evolutionary anthropologist at Duke University, US.

Pontzer and team studied the water intake (from food and drink) and output (via sweat, urine and the GI tract) of 309 people with lifestyles ranging from office workers to farmers to hunter-gatherers. They compared this to 72 apes living in zoos and sanctuaries, to find that while the average human processes three litres (12 cups) of water per day, chimpanzees and gorillas process twice as much.

This is surprising for two reasons.

Firstly, humans are much sweatier than our primate cousins, with 10 times more sweat glands than chimpanzees per square inch of skin.

Secondly, we tend to spend more hours active than the great apes.

“Most apes spend 10 to 12 hours a day resting or feeding, and then they sleep for 10 hours,” Pontzer notes. “They really only move a couple hours a day.”

The study, however, controlled for factors such as differences in climate, body size, activity level and calories burned per day. The results suggest that humans have gradually evolved to use less water to remain healthy.

The researchers put forward a couple of hypotheses for how this change evolved. Perhaps our body’s thirst response adapted to crave less water per calorie, or perhaps our prominent noses – developed around 1.6 million years ago – may help us retain more moisture from our breaths than the flatter noses of other primates.

“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.”