Why the Human Heart Differs from Great Apes

Mammals occupy almost every part of Earth, ranging from the enormous blue whale to the diminutive shrew. Scientists have long been intrigued by their extraordinary capacity to adapt to contrasting habitats, as every species has evolved particular characteristics that enable it to survive and flourish.

Until recently, however, the mammalian heart was thought to share the same structure and function throughout this huge biological range. Research by my colleagues and me shows otherwise: the human heart is an exception, differing noticeably from the hearts of our nearest relatives among the great apes - chimpanzees, bonobos, orangutans and gorillas.

So what makes humans so unusual?

Human evolution and the heart

Humans separated from chimpanzees (Pan troglodytes), our last common ancestor, around five to six million years ago. People subsequently evolved an upright posture, allowing greater levels of activity including persistence hunting, and developed substantially larger brains.

These bodily changes brought far higher metabolic requirements, meaning more blood had to be delivered to both the muscles and brain. Our findings indicate that the human heart adapted to accommodate bipedal posture, movement and an enlarged brain.

For the past decade, we have assessed the cardiovascular systems of great apes worldwide. This work has been possible through collaborations with committed veterinarians and care teams in the UK, Europe, Africa and Asia.

A key part of these investigations involves cardiac ultrasound. This allows us to examine the heart's size, structure and performance, including the way its muscle contracts, twists and rotates.

Human heart structure compared with great apes

Earlier work suggested that the human heart's structure may not match that of chimpanzees. Cardiac ultrasound revealed that the chimpanzee left ventricle - the heart's principal pumping chamber - contains muscular bundles organised into a mesh called "trabeculations".

In the current study, we investigated whether trabeculations are also present in other great apes, and found that they are. Humans, by comparison, have a smooth left-ventricular wall. The distinction is particularly marked at the base of the left ventricle: the human heart is almost four times smoother there than the hearts of our great ape relatives.

Our study identified more than structural variation between human and great ape left ventricles; it also found an important functional difference. Using the specialist method "speckle-tracking echocardiography", which follows cardiac muscle motion as the heart contracts and relaxes, we assessed how the muscle thickens, twists, rotates and lengthens.

The findings were clear. During contraction, humans - whose hearts have the fewest trabeculations - displayed considerably greater twisting and rotation at the apex, or tip, of the heart. Non-human great apes, which have heavily trabeculated hearts, showed far less motion.

We think that the human heart evolved away from the trabeculated form retained by other great apes in order to improve how efficiently it twists and contracts. This greater twisting action, combined with smooth ventricular walls, is likely to enable the human heart to pump a larger quantity of blood with every beat.

That capacity meets the increased demands created by human activity and larger brains.

Our work disputes the idea that heart structure is consistent across mammals. Instead, small yet important changes in cardiac anatomy and function appear to have evolved in response to distinct environmental pressures.

Cardiac disease

Although this research has provided insight into the evolution of the human heart, we are continuing to analyse the hearts of endangered great apes. Tragically, cardiac disease is the main cause of death among captive great apes.

Great apes, unlike humans, do not seem to develop coronary artery disease. Instead, their heart muscle undergoes fibrosis, a thickening process that impairs contraction and increases vulnerability to arrhythmia - an abnormality in the rhythm of the heartbeat. The cause of this condition remains unknown. At the International Primate Heart Project, we therefore carry out assessments of great ape cardiovascular physiology around the world to improve understanding of the disease.

Before we became involved, very little was known about normal great ape cardiovascular physiology. Working alongside veterinary practitioners, our research has produced essential data that has greatly advanced understanding of human heart evolution and improved the understanding, diagnosis and management of heart disease in great apes.

Aimee Drane, Senior Lecturer and Clinical Academic in Faculty of Medicine, Health & Life Sciences, Swansea University

This article is republished from The Conversation under a Creative Commons licence. Read the original article.