From Our DELHI Bureau
The transition to modernity – largely driven by the Industrial Revolution – provided us with easier access to food and clean water, with antibiotics, vaccines, and modern medicine. Yet modernity did not just bring fewer infectious diseases and longer life: it also created an environment radically different from the one we evolved in. Genes helpful in our evolutionary past may now predispose us to chronic disease – such as cardiovascular diseases and cancer – in old ages. In a paper published in the journal Nature Reviews Genetics an international team of five scientists collate the evidence for this mismatch between past evolutionary adaptation and our modern lives. They also ask whether natural selection linked to modernisation might reduce globally the burden of some chronic diseases.
Over the last four centuries human ecology, life styles and life histories have dramatically changed. The transition to modernity also altered the major causes of human death. Infectious diseases prevalent in childhood have given way to chronic diseases associated with ageing. Naturally – as all of us must die – if some causes of death decrease others must increase in proportion. However, the increasing differences between the circumstances our genes have adapted to and our new environment also plays an important role.
Ageing is, in part, caused by the combined effect of many genes that are beneficial when young, but have adverse effects at older ages. Genes can influence a variety of traits and can also express themselves differently as we age (pleiotropy). The term antagonistic pleiotropy describes genes that can carry both beneficial and detrimental effects. Somewhat counter-intuitively evolution by natural selection can lead to antagonistic pleiotropy spreading in populations: The benefits received when young can outweigh the evolutionary disadvantages in old age. However, women who carry one of such variants of BRCA1 will – more likely than not – develop breast cancer by the age of 90.
“Angelina Jolie’s decision to opt for a preventive double mastectomy instead of risking breast cancer was based on her carrying a high-risk BRCA1 variant,” Virpi Lummaa, professor at the University of Turku in Finland, explains. “This gene variant hasn’t been eliminated by natural selection in the past, precisely because it also has a great benefit for female fertility. Nowadays, the situation is much worse. Due to our much lower fertility levels and longer life spans the early benefits of such genes no longer play out.”
In contrast, the evolutionary impact of contemporary life on human health is difficult to establish: evolutionary change often requires many generations to leave an unambiguous trace in our genome. The review found “suggestive but not yet overwhelming” evidence that natural selection, the engine of evolution, is changing course in our modern times.
