Why the Human Heart Differs From Great Ape Hearts

Mammals, ranging from the enormous blue whale to the diminutive shrew, live in almost every part of the world. Their striking capacity to adapt to varied habitats has fascinated scientists for decades, as every species has evolved particular characteristics that enable it to survive and flourish.

Yet, despite this immense biological variation, scientists had until recently assumed that mammalian hearts shared the same structure and function. Research by my colleagues and me shows that the human heart is an exception: it differs markedly from the hearts of our nearest relatives, the great apes - chimpanzees, bonobos, orangutans and gorillas.

Why, then, are humans so unusual?

How the human heart adapted

Humans separated from chimpanzees (Pan troglodytes), our most recent common ancestor, around five to six million years ago. Humans subsequently evolved to walk upright and undertake more sustained activity, including persistence hunting. We also developed substantially larger brains.

These physical changes brought far greater metabolic requirements, meaning more blood needed to reach both the muscles and the brain. Our findings indicate that the human heart adapted to accommodate upright posture, movement and an enlarged brain.

For the past decade, we have assessed the cardiovascular systems of great apes around the world. We have been privileged to work alongside committed veterinary teams and animal-care staff in the UK, Europe, Africa and Asia.

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

Left ventricle differences in great apes

Earlier research indicated that the human heart may have a different structure from that of chimpanzees. Cardiac ultrasound revealed that chimpanzees have muscle bundles in the left ventricle - the heart's principal pumping chamber - arranged in a mesh-like pattern called "trabeculations".

In our latest study, we examined whether trabeculations were also present in other great apes, and found that they were. Humans, by comparison, have a smooth left-ventricular wall. The contrast is particularly clear at the base of the left ventricle, where the human heart is almost four times smoother than those of our great ape relatives.

Our study identified more than structural distinctions between human and great ape left ventricles; it also found an important functional difference. We used a specialised method, "speckle-tracking echocardiography", which follows heart-muscle movement as the heart contracts and relaxes, to assess how the muscle thickens, twists, rotates and lengthens.

The findings were notable. Humans, whose hearts show the least trabeculation, displayed considerably greater twisting and rotation at the apex, or tip of the heart, during contraction. Non-human great apes, whose hearts are heavily trabeculated, exhibited far less movement.

We think that the human heart evolved away from the trabeculated form found in other great apes in order to improve the efficiency of its twisting and contraction. This greater twisting action, combined with smooth ventricular walls, is likely to enable the human heart to pump a larger amount of blood with each beat.

This would satisfy the increased demands created by our physical activity and bigger brains.

Our findings question the idea that heart structure is consistent throughout mammals. Instead, small but significant variations in cardiac anatomy and performance appear to have developed in response to particular environmental pressures.

Cardiac disease in great apes

Although this research has provided insight into the evolution of the human heart, our work investigating the hearts of endangered great apes is ongoing. Sadly, cardiac disease is the main cause of death among captive great apes.

Unlike people, great apes do not seem to develop coronary artery disease. Instead, their heart muscle experiences fibrosis, or thickening, which leads to poor contraction and makes arrhythmia - an irregular heart rhythm - more likely. The cause of this condition remains unknown. At the International Primate Heart Project, we are therefore assessing great ape cardiovascular physiology worldwide to improve understanding of the disease.

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