No chimpanzees or gorillas are in training for a fall marathon — a reflection, perhaps, of the ways in which the hearts of apes and men look and function as they do, according to a major new study of the health and evolution of cardiac muscles.
The study, which involved scanning the hearts of untamed primates and a wide variety of men, indicates that hearts adapt in telling ways to the needs of their owners. The findings likewise suggest that not getting enough of the right kind of exercise could mean that our hearts start to look just a little bit less human, and could impact our long-term health.
In general, mammalian hearts are quite malleable. They change in response to the demands placed on them. Recently, I wrote about a study showing that the hearts of elite swimmers and runners differ from one another slightly in shape and function and differ substantially from the hearts of those who aren’t athletes.
But until now, researchers had not examined whether and how this changing of the heart, known as its plasticity, might have played out during our evolution as a species and what that process could mean for our heart health today.
So, for the new study, which was published in September in PNAS, scientists from Harvard University and other institutions decided to compare, for the first time, the looks and inner workings of human hearts and those of our closest primate cousins, chimpanzees and gorillas.
This comparison would be expected to reflect how our lives diverged over the millenniums, says Daniel Lieberman, an evolutionary biologist at Harvard and one of the study’s principal co-authors, together with the Harvard cardiologist Dr. Aaron Baggish and the exercise physiologist Robert Shave, from the University of British Columbia’s Okanagan Campus.
Chimps and gorillas, whether in the wild or zoos, rarely run or even walk much, Dr. Lieberman says, except for a mile or two in search of food. But they rapidly clamber up and down trees and grapple during fights. They are strength and power athletes.
We humans, on the other hand, tend to be built for endurance and frequent, moderate physical activity, he says, with a fossil record showing gradual changes in our skeletons likely caused by and promoting plenty of walking and occasional distance running as we hunted and gathered.
Hearts, however, “do not fossilize,” Dr. Lieberman says, so the best way to learn about their evolution would be not through excavations but via comparative appraisals of our organs with those of our erstwhile relatives.
To that end, Dr. Shave and his colleagues went to African nature preserves over the course of several years to scan dozens of chimpanzees’ hearts and check their blood pressures during the animals’ annual veterinary checkups. They also visited zoos to do heart scans on several gorillas (although they did not measure blood pressures).
In the meantime, Dr. Baggish and other researchers scanned the hearts and measured blood pressures of young, male distance runners and football players at Harvard, subsistence farmers in Mexico, and 40 sedentary but healthy young men in Boston. (Only males were included in this study, although the researchers would like to study women and female primates in the future.)
Then the scientists began making comparisons, which turned out to be stark. The hearts of the chimps and gorillas proved to be well adapted for short, sharp bursts of activity, with a rounded shape and thick walls inside their chambers that could withstand and respond to sudden, brief spikes in blood flow but resulted in relatively high baseline blood pressures in the chimps, compared to people (although primates, unlike us, do not seem to experience heart problems from such hypertension).
The human hearts, on the other hand, were more elongated and supple, with thinner chamber walls that could twist and pump greater volumes of blood at lower pressures than in the primate hearts, a necessity during sustained aerobic activities, like walking or jogging.
Perhaps most intriguing, though, were the differences within the various groups of people. The collegiate runners and subsistence farmers, whatever their age, harbored hearts that were endurance-ready, with the thinnest, springiest chamber walls and the lowest blood pressures among the human groups.
The hearts of the football players, meanwhile, whose regular exercise consisted mostly of weight training, and those of the sedentary young Bostonians, whose regular exercise consisted of not doing any, showed relatively thicker chamber walls and greater heart stiffness.
Their hearts had developed a subtly “chimpanzee-like phenotype,” in the words of the scientists.
That slight aping of the hearts of the football players and the sedentary young men was coupled with everyday blood pressures that, while still within the normal range, were higher than those of the runners and farmers.
What these findings suggest as a whole, Dr. Lieberman says, is that, when human lives diverged from those of other apes so long ago, so did our hearts, with ours evolving to allow us to be in steady, regular, aerobically based motion.
The same data also indicate, though, that when we do not get that kind of frequent cardiovascular exercise, our hearts begin rapidly to “remodel themselves,” Dr. Baggish says, in ways that could be related to later heart problems.
The sedentary Bostonians in this study, he points out, although still young, had hearts that already were hardening and blood pressures rising. “We are taught” that heart disease is related to aging, he says. “But maybe it starts here,” with not listening to our hearts and following in the footsteps of our forebears and heading out, whenever we can, for a walk.