Back in the day, humans had to travel on foot, and lift heavy loads.

Now we kick back in planes and bullet trains, and sink into cushy chairs.

Making more time to recline, has come with consequences (like heart disease).

A new hypothesis suggests that relaxing isn't actually so bad, though. In fact, we evolved to chill. Nowadays we just relax the wrong way.

Estimates suggest that Americans spend about 6.4 hours each day sitting (teens spend about 8.2 hours seated per day). If that number seems like a consequence of sedentary society, consider that other humans living in hunter-gatherer societies spend comparable amounts of time being sedentary .

A study published Monday in the Proceedings of the National Academy of Sciences examined the sedentary habits of the Hadza of Tanzania, a present-day hunter-gather society. The team found that the Hadza spend about 9.9 hours each day inactive, on average.

However, the Hadza don't have the same high risk of heart disease we see in industrialized countries, says David Raichlen, the study's lead author. That could be, in part, because when they're chilling, they're not actually sitting — hey're squatting, kneeling or resting on the ground.

Those are "active rest" positions that keep muscles more active, and help them avoid the consequences of sitting, Raichlen tells Inverse.

"We think that this consistent level of muscle activity they experience both in rest and when they’re active may explain why long periods of inactivity aren’t associated with cardiovascular disease risk in this population," he says.

Taken together, this research suggests it's not the time we spend doing nothing that leads to risks. It's the way we do it.

Hadza participants in resting postures. Image courtesy of David A. Raichlen.

The downside of cozy furniture? Bad health

As Raichlen explains, researchers once thought that inactivity wasn't a huge part of human evolutionary history.

That has led to an assumption: The reason sitting seems to be linked to conditions like death and heart disease today, is because we spend too much time inactive. Our current habits don't fit what we evolved to do, the theory goes.

Hunter-gatherers, in turn, are assumed to be more active than we are, he notes. And it pays off: Some societies, like the Tsimané, a hunter-gatherer society in Bolivia, have extremely healthy hearts. According to a 2019 study published in The Lancet, an 80-year-old Tsimané heart looks more like a 50-year-old American heart.

"That seems to be the mismatch with our evolutionary past."

Raichlen's work, though refutes the idea that hunter-gatherers don't have sedentary time. His work showed that the Hadza also have periods of long inactivity. In fact, their inactive time is "comparable" to what's seen in the US, he notes.

Notably, this inactivity doesn't seem to affect them . "They don't have the kind of biomarker risks we see," says Raichlen. "Like cholesterol or triglycerides."

This paper suggests that the reason inactivity is harmful to us isn't because humans didn't evolve to become sedentary creatures. It's because we were never supposed to get this cozy.

"What we suggest is that resting occurred in our evolutionary past in probably these kind of postures that elicit more muscle activity than chair sitting," says Raichlen. "Somewhere in our past, we invented these really comfortable pieces of furniture that let us be inactive with basically zero levels of muscle activity.

"That seems to be the mismatch with our evolutionary past."

The Hadza don't sit in comfortable chairs, instead opting for the ground or squatting positions. Image courtesy of David A. Raichlen.

Is sitting the whole problem?

Sitting for 10 hours per day has been linked to as much as a 48 percent increase in risk of death in industrialized societies. But when it comes to death risk in the industrialized world, it's hard to separate how much is due to sitting and how much is due to our unhealthy diets.

For instance: According to a 2019 study published in Circulation, if Americans kept their added sugar consumption below 200 calories per day, an estimated 354,400 cases of cardiovascular disease and 599,300 cases of diabetes would be prevented over two decades.

"Clearly diet is going to play a role in minimizing cardiovascular disease biomarkers."

Sitting and eating badly are intertwined, particularly when screens are involved. A 2019 study published in the International Journal of Behavioral Nutrition and Physical Activity examined 33,000 Brazilian teenagers, their phone use, and their health. Scientists linked spending more than six hours in front of screens with developing a metabolic syndrome (think: pre-diabetes). That link only remained significant for teens who also ate snacks while sitting.

The Hadza differ from us in many ways other than their ability to squat, not sit. They're simply not eating the same way we are.

Raichlen says that they didn't look at diet in this study, because they didn't have detailed data outlining what the Hadza eat. That said, he says that from previous work, they know that the Hadza probably eat foods like lean meats, plants, tubers, honey, and no processed foods at all.

"Clearly diet is going to play a role in minimizing cardiovascular disease biomarkers," he says. "But I think if you look at the entire lifestyle of a hunter-gatherer today, it fits well within the heart-healthy lifestyles we recommend to people in industrialized societies."

In other words, they're active, and they tend to eat well. The fact that they don't spend hours on end reclined on the couch is simply one additional way that they're not punishing their hearts.

Still, the results are enough to make you at least consider a standing desk. Or if you want to stay true to the results, the admittedly niche habit of squatting at your desk.

Abstract: Recent work suggests human physiology is not well adapted to prolonged periods of inactivity, with time spent sitting increasing cardiovascular disease and mortality risk. Health risks from sitting are generally linked with reduced levels of muscle contractions in chair-sitting postures and associated reductions in muscle metabolism. These inactivity-associated health risks are somewhat paradoxical, since evolutionary pressures tend to favor energy-minimizing strategies, including rest. Here, we examined inactivity in a hunter- gatherer population (the Hadza of Tanzania) to understand how sedentary behaviors occur in a nonindustrial economic context more typical of humans’ evolutionary history. We tested the hypothesis that nonambulatory rest in hunter-gatherers involves increased muscle activity that is different from chair-sitting sedentary postures used in industrialized populations. Using a combination of objec- tively measured inactivity from thigh-worn accelerometers, observational data, and electromygraphic data, we show that hunter- gatherers have high levels of total nonambulatory time (mean ± SD = 9.90 ± 2.36 h/d), similar to those found in industrialized populations. However, nonambulatory time in Hadza adults often occurs in postures like squatting, and we show that these “active rest” postures require higher levels of lower limb muscle activity than chair sitting. Based on our results, we introduce the Inactivity Mismatch Hypothesis and propose that human physiology is likely adapted to more consistently active muscles derived from both physical activity and from nonambulatory postures with higher levels of muscle con- traction. Interventions built on this model may help reduce the neg- ative health impacts of inactivity in industrialized populations.