In prosperous and relatively secure nations, the duration of human life now likely hinges as much on the genetic predispositions inherited from parents as on personal environmental influences and daily habits. This perspective emerges from a recent study that re-examined existing twin research data from Denmark and Sweden.
For individuals residing in these countries, it might not be entirely unexpected that their life expectancy is roughly divided between genetic contributions and external factors. However, earlier analyses of the same twin datasets, conducted decades ago, had suggested that genes accounted for only about a quarter of the variability observed in human lifespans.
“This finding shifts the balance slightly,” commented Joris Deelen, a researcher at Leiden University Medical Center in the Netherlands and a member of the study team. “It suggests a more significant role for genetics, while diminishing the environmental contribution somewhat. Nevertheless, at least 50 percent is still attributable to environmental factors, underscoring its continued major importance.”
Heritability serves as a metric to quantify the extent to which variations in a specific trait are driven by genetic factors, as opposed to environmental influences. As the research team points out, the heritability of any given trait is not a universally fixed value. Instead, it is specific to a particular population operating within a defined environment.
The growth of wheat provides a classic illustration of this concept. If seeds are sown in a uniform, level field, nearly all differences in plant height will be attributable to genetic makeup. However, if the same seeds are planted across a varied landscape, almost all variations in height will be a result of differing conditions related to soil, sunlight, and water. Consequently, the heritability of height would differ dramatically in these two scenarios.
To estimate heritability in human traits, geneticists frequently employ comparisons between twins raised together in the same household and those raised separately. For this particular study, Deelen and his colleagues primarily utilized data from twin studies involving individuals born in Sweden or Denmark between 1870 and 1935.
When the researchers excluded deaths resulting from accidents or infectious diseases, focusing instead on age-related conditions such as heart attacks, the heritability of lifespan increased to approximately 50 percent.
This revised figure aligns more closely with established knowledge regarding aging in animal species, according to Deelen. “I believe a figure closer to 50 percent is more realistic than 25 percent,” he stated.
Peter Ellis from the University of Kent in the UK noted that the study specifically examines “the heritability of maximum lifespan under ideal conditions, assuming only age-related processes contribute.” He elaborated that this represents a “much narrower question than overall lifespan” and consequently, it is not surprising that the heritability is higher for this more focused inquiry.
João Pedro de Magalhães at the University of Birmingham, UK, concurred with this assessment, remarking that “The results are not entirely surprising.”
Findings of this nature indicate the existence of numerous genetic variants influencing variations in human lifespan. Identifying these variants could potentially pave the way for the development of life-extending pharmaceuticals. However, to date, relatively few such genes have been discovered.
“It remains a significant enigma why so few human genes associated with longevity have been identified,” de Magalhães commented.
One contributing factor is that a majority of participants in large-scale studies, such as those conducted by the UK Biobank, are still alive. This lack of mortality data limits the necessary statistical power for definitive conclusions. Deelen also suggests that the complexity of the genetic interactions plays a part.
Ellis further pointed out the possibility of trade-offs at the genetic level. For example, variants that modulate the immune system might lower the risk of autoimmune disorders but simultaneously reduce an individual’s defense against infections. This complicates the study’s assumption that deaths from infection are entirely unrelated to lifespan.
De Magalhães also highlighted a crucial distinction: the role of genetics appears markedly different when comparing various species rather than individuals within the same species. “If you possess the genome of a mouse, you cannot anticipate living more than three or four years,” he explained. “Conversely, if you have the genome of a bowhead whale, you might live for over two centuries.”
Journal Reference: Science DOI: 10.1126/science.aee3844