Genes associated with coronary heart disease also play an important role in reproduction, influencing the number of children that people can have. This may explain why they have not disappeared with natural selection, despite the disease being the number one killer worldwide today.

Coronary artery disease is characterised by a build-up of fatty substances in the coronary arteries which blocks blood supply to the heart. According to the World Health Organisation's estimates, 17.7 million people died from cardiovascular diseases in 2015, including 8.9 million from coronary artery disease.

While addressing behavioural and lifestyle factors such as tobacco use, obesity and inactivity can greatly reduce the risk, genetic variations contribute to the development of the disease as well.

Past research has shown that coronary artery disease has been present in human populations for thousands of years - it can even be detected in ancient Egyptian mummies.

Why then, scientists wondered, have these genetic variations not disappeared with natural selection, considering the number of deaths they are associated with?

"According to the theory of natural selection, as proposed by Charles Darwin, genes for traits that improve individual survival or reproduction will increase or be maintained in populations, whereas those that reduce these will be selected against and gradually removed or reduced over time. So it is unclear why coronary artery disease is so common in modern humans and this is important to understand given the global health burden it represents", explains Dr Sean Byars, from the University of Melbourne.

With his colleagues, Byars attempts to solve this mystery, in a study now published in the journal PLOS Genetics.

Genes selected through evolution

The team analysed 56 loci (regions on the chromosomes) in twelve populations from around the world, identifying 76 candidate genes linked to coronary heart disease. The scientists then used a statistical method to see if there had been recent selective changes to the DNA associated with coronary artery disease.

The results suggest that many of the genetic variations associated with the disease have positively been selected through evolution, as they also influence male and female reproductive function, ensuring reproductive success. People with these particular genes are more likely to have many children.

heart attack
Coronary heart disease can result in people suffering heart attacks. SEBASTIAN KAULITZKI/Getty Images

"After further research, we found coronary artery disease genes are also important for reproduction and that these genes are involved in important functions in male and female fertility being expressed in the testes, ovaries and endometrium, for example," says Pr Michael Inouye, based at the Baker Heart and Diabetes Institute, who was also involved with the study.

The findings do not imply that people with many children are more likely to develop the condition, but it suggests that the disease is a by-product of being able to reproduce well.

Dr Byars explains: "Evolution it seems is involved in a trade-off where CAD only begins to appear at around 40-50 years of age when the potential beneficial effects of these genes on reproduction will have already occurred. That will tend to compensate for any negative effects these genes also have on CAD later in life".

The study highlights how complex the effects of genes can be, and more research will be needed to understand exactly how these genes associated both with heart disease and reproduction function.