Human hearts evolved for endurance

They measured and compared heart function in apes and four groups of humans (ranging from sedentary through to elite runners, and including indigenous subsistence farmers). Their research is presented in the Sept. 2019 issue of Proceedings of the National Academy of Sciences of the United States of America (PNAS). Baggish's collaborators on this paper included Robert E. Shave, PhD, School of Health and Exercise Sciences, University of British Columbia; and Daniel E. Lieberman, PhD, Department of Human Evolutionary Biology, Harvard University.

Chimpanzees are humans' closest known relatives, based on genetics and evolutionary studies. There are, of course, some stark differences between these species. For example, in terms of exertion, chimpanzees mainly engage in short bursts of activity, such as climbing and fighting, which put intense pressure on the heart but only for a limited time. In contrast, it's believed that, up until the industrial revolution, humans were active for longer bouts of time in order to hunt and farm. Survival of pre-industrial humans, it is thought, depended on moderate-intensity, endurance-like activity (e.g. hunting, gathering, and then farming).

It is also well established that some physical features of the heart change in response to certain physical challenges. Walking and running, for example, require more blood to be pumped to deliver fuel to active muscles. In contrast, brief but intense exertion from activities such as climbing or fighting, create pressure in heart, which over time can makes the heart chambers develop stiffer and thicker walls.

"The heart remodels in response to two main forces: pressure and volume," says Baggish. As a result, "Humans have longer, thinner- and more flexible-walled hearts, while chimps have smaller hearts with thicker walls." What Baggish and his collaborators wanted to know was: Could those differences have evolved in response to humans' new activity levels? And if so, what implications does that have on human health today?

Using a group of more than 160 study participants, the researchers carried out detailed heart function studies, including measuring blood pressure and using ultrasound to examine the heart's structure and function during many different activities. Their subjects were fairly evenly divided into elite runners, American football players, indigenous Mexican subsistence farmers and people who engage in little physical activity. They made similar measurements in about 40 semi-wild chimpanzees and five gorillas.

"The goal was to compare heart structure and function in each "type" -- whether the subject was very active, to barely active," Baggish says. In addition, the investigators sought to determine if adaptation to either pressure or volume comes at the expense of the ability to handle the alternative form of stress. This was done by giving pressure adapted (football linemen) and volume adapted (long distance runners) both a "volume challenge," by giving them a large intravenous saline infusion and a "pressure challenge," by sustained, forceful handgrip, and simultaneously measuring heart function. The goal was to see if there is a tradeoff between having a heart that is adapted for endurance versus having one that performs better for short bursts of intense activity. Or could the heart adapt for both?

Baggish and his collaborators found that indeed, human hearts appear to have evolved to be better at handling endurance type activity, as opposed to short intense bouts. The researchers also confirmed that people who train specifically for endurance sports have hearts with longer, larger, and more elastic left ventricles, which is the part of the heart that pumps the blood out to the body. Those features, and others, make the heart better able to cope with pumping higher volumes of blood over a sustained time. In contrast, sedentary people, even at a relatively young age, have hearts that appear more "ape-like" that are better suited to cope with short bursts of high activity.

These findings help answer that question about the heart's evolution. "The human heart has evolved over hundreds of thousands of years as our activity levels gradually became more sustained," Baggish says. "We now understand that the human heart, coupled with changes in the musculoskeletal and thermoregulatory system, evolved to facilitate extended endurance activity rather than spurts of intense exertion." This study has important implications for understanding heart health today. For example, people who live a sedentary lifestyle appear to develop more ape-like hearts and are more prone to hypertension. That disease process then causes further changes and a negative feedback loop that raises risk of disease.

This study was unique for several reasons, Baggish says. "Not only were we able to study heart function in three types of primates, but we also had the opportunity to work with people who are among the last groups of truly subsistence-based farmers, the Tarahumara in Mexico's Copper Canyons." The research team also included a cardiologist (Baggish), an expert in exercise physiology (Shave) and an evolutionary biologist (Lieberman).