Mammals — from the enormous blue whale to the tiny shrew — live in nearly every corner of the planet. Their ability to adapt to wildly different environments has long fascinated scientists, and each species has evolved its own survival tricks. For years researchers assumed the heart looked and worked the same across mammals. A team at Swansea University has now shown that humans are the exception: our hearts differ markedly from those of our closest relatives — the great apes (chimpanzees, bonobos, orangutans, and gorillas).
So what makes the human heart unique? Humans split from chimpanzees (Pan troglodytes), our last common ancestor, about five to six million years ago. The human line evolved to walk upright and to perform a wider range of activities while our brains grew much larger. Those shifts raised our metabolic needs and demanded more blood flow to both muscles and brain, said lead author Amy Drain, a senior lecturer at the Faculty of Medicine, Health, and Life Sciences. The Swansea team found that the human heart adapted in specific ways to meet those demands.
For a decade the researchers studied great-ape cardiovascular systems around the world using ultrasound. That let them measure heart structure and function — size, and how the heart muscle contracts, twists, and rotates. Earlier they suspected the human heart might differ from chimpanzees’. Ultrasound showed that chimpanzees’ left ventricle — the heart’s main pumping chamber — contains bundles of muscle arranged into a mesh, supported by septa and plates called trabeculae. The team checked other great apes and found the same trabecular pattern. Humans, by contrast, have a smooth left-ventricle wall. The smoothness is most pronounced in the lower part of the ventricle: the human wall is nearly four times smoother than that of great apes.
There was more. The team also found a clear functional difference. Using speckle tracking echocardiography — a technique that follows heart-muscle movement during contraction and relaxation — they measured how the muscle thickens, twists, rotates, and lengthens. Humans, who show the weakest trabeculation, had much more twisting and rotation at the tip of the heart during contraction. Non-human great apes with trabeculated hearts showed far less motion. The researchers concluded the human heart evolved away from the trabecular pattern seen in other great apes to improve twisting and contractile efficiency. That smoother wall and stronger twist likely let the human heart pump more blood per beat, matching the higher demands of upright movement and a larger brain, according to Science Alert.
The study upends the idea that mammal hearts are all the same. Subtle anatomical and functional shifts in the human heart appear to be adaptations to our unique ecological and physical challenges.
