An international research team from UBC Okanagan and Swansea University has uncovered a new insight into human evolution by comparing humans' hearts with those of other great apes.
While humans evolved to travel long distances, likely to hunt, other great apes developed to meet the demands of their immediate forest environment and moved to seek shade if overheated, explains Bryony Curry, a doctoral student in UBCO's School of Health and Exercise Sciences.
The human species' greater bipedal locomotion and larger brains are two pieces in the puzzle that explain humankind's evolutionary divergence from its common ancestor.
Now, through a comparative study that examines the differences between the human heart and those of several primates, researchers think they may have found another piece of the puzzle regarding how the hearts of primates and humans adapted differently over time.
The international study, published this week in Nature's Communications Biology, compared the human heart to those of chimpanzees, orangutans, gorillas and bonobos. These non-human great apes share more than 98 per cent of their DNA with humans making them the closest evolutionary relatives of humankind.
The non-human great apes were being cared for at wildlife sanctuaries in Africa and European zoos. During routine veterinary procedures, the team used echocardiography—a cardiac ultrasound—to produce images of the great apes' left ventricle, the lower left chamber of the heart.
Within the left ventricle are bundles of muscle, called trabeculations, that extend into the chamber.
"The left ventricle of a healthy human is relatively smooth, with compact muscle as compared to the more trabeculated, mesh-like network in the non-human great apes," explains Curry. "The difference is most pronounced at the apex, or bottom of the heart, where we found approximately four times the trabeculation in non-human great apes compared to humans."
The team also examined the twisting and rotation of the heart. When a heart contracts, it deforms and then springs back to its original shape. The researchers measured this deformation and the velocity of this motion using an imaging technique called speckle-tracking echocardiography, which traces the speckled pattern of the cardiac tissue as it contracts and relaxes.
"We found that the degree of trabeculation in the heart is related to the amount of deformation, rotation and twist," says Curry. "In humans, with the least trabeculation, we observed their hearts have the greatest function. This finding supports our hypothesis that the human heart may have evolved away from the structure of other non-human great apes to meet the higher demands of our unique ecological niche."
Curry explains that humans have larger brains and are also more physically active compared to other great apes and this equates to a higher metabolic demand. This requires a heart that can pump a relatively higher volume of blood to the body. Higher blood flow also contributes to humans' ability to cool down as blood vessels close to the skin dilate —observed as flushing of the skin—and lose heat to the air.
"In evolutionary terms, our findings suggest selective pressure was placed on the human heart to adapt so it can meet the demands of walking upright and managing thermal stress," adds Dr. Robert Shave, Associate Dean of Research with UBCO's Faculty of Health and Social Development. "What remains unclear is how the more trabeculated hearts of non-human great apes may be adaptive to their own ecological niches. Perhaps it's a vestigial structure of the ancestral heart, though in nature form most often serves function."
The research team is grateful to the staff and volunteers who care for the animals in the study, including the teams at Tchimpounga Wildlife Sanctuary in Congo, Zambia's Chimfunshi Wildlife Sanctuary, Tacugama Chimpanzee Sanctuary in Sierra Leone, Borneo's Nyaru Menteng Orangutan Rescue and Rehabilitation Centre, the Zoological Society of London of the United Kingdom, Paignton Zoo in Devon, England, the Bristol Zoo Gardens, England, Burgers' Zoo in the Netherlands and Germany's Wilhelma Zoo.
