(CNN) One-third of global deaths are due to cardiovascular disease caused by atherosclerosis, when arteries are clogged with fat deposits. Although it's common for humans to experience heart attacks, the same event is rare in mammals. Now, researchers believe that they understand why.

Risk factors for cardiovascular disease in humans can include age, hypertension, obesity, smoking, blood cholesterol and inactivity. However, in 15% of cardiovascular events caused by atherosclerosis, none of these risk factors is at hand.

Closely related mammals like chimpanzees living in captivity share some of the same risk factors for cardiovascular disease: hypertension, inactivity and high blood lipids.

But these chimps rarely experience heart attacks. If they do, it's not due to atherosclerosis; the cause is scarring of the heart muscle, which has yet to be explained.

Researchers think that there was the loss of a single gene, CMAH, in human ancestors between 2 million and 3 million years ago, leading to a higher risk of cardiovascular disease. This gene included a sialic acid sugar molecule called Neu5Gc. Apes and chimps have maintained this gene and molecule over time.