Many animals are known for their longevity. Sea turtles can live up to 50 years or more, while the Greenland shark can live for more than 400 years. Some animals have the privilege of living long, others are not so lucky.
Some species have a significantly shorter life expectancy. like the pygmy goby, an Australian fish, which lives only 8 weeks. Adult mayflies, on the other hand, live on average only 24 hours.
Overall, it can therefore be very difficult to calculate the life expectancy of different species. And that of many is still unknown, because scientists rely on a sample of wild animals in captivity, therefore in conditions very different from those of their natural habitat.
Now, scientists are looking for new methods to estimate life expectancy: reading DNA is the most explored approach in recent years.
Understanding life expectancy from DNA
In a 2019 paper published in Scientific Reports researchers have predicted the life expectancy of vertebrates, including reptiles and mammals, by looking at specific parts of their DNA. They collected genetic information from 252 organisms and focused on DNA sequences that can explain the large differences in lifespan observed across the animal kingdom. With this data, they built an algorithm to predict the maximum lifespan of living and even extinct species. They called it the “lifespan clock” and used it to refine the average lifespan of some long-lived species.
With real-world applications and the potential to reveal new details about the past, reading DNA is a promising technique for geneticists who want to understand how animals (including humans) age and how to help them live longer. Beyond that, draw on the methylation it could help indicate when an animal experiences environmental stress and risks having a shorter life expectancy, dying earlier.
The ticking clock in your DNA
DNA is the biological model that makes every living creature unique. Everything from a person's height to the iconic orange scales of a clownfish can be traced back to DNA instructions. When it comes to determining lifespan, DNA may also be relevant. More specifically, methylation, a biological process within cells, may hold clues to lifespan and the aging process in humans and animals.
A few words about methylation
During our life cycle, genes turn on and off. This is critical for healthy growth and development, and methylation is an important process that governs gene expression. During methylation, enzymes add a methyl group to a gene, which prevents it from being transcribed. It's a bit like putting temporary handcuffs on a gene: the gene is still there, but the cell's machinery can't read it or turn it into a protein. So methylation turns off genes, while demethylation (the removal of the same methyl group) turns them on, and it all affects life expectancy.
Although methylation patterns and how they are regulated remain poorly understood, studies have shown that methylation declines with age. In centenarians it is reduced to a minimum. Does that mean having more methylation is a good thing? Well it depends. A number of normal cellular processes rely on methylation. But it could happen that deactivating certain genes prevents their expression, reducing the risk of developing certain diseases.
DNA methylation can also be used as a marker to determine age in animals. Studying wildlife that can significantly outlive humans can be a challenge. Using their methylation technique, the researchers who wrote the Scientific Reports study learned that le bow whales have a maximum natural duration of 268 years. This is new information: previous measurements had put their lifespan at 211 years. The researchers also applied the same technique to predict the life expectancy of some now extinct species. For example, revealing that the woolly mammoth could live up to 60 years. A life expectancy similar to that of African elephants, which still populate the African savannahs today.
Humans: Wired to Live 38 Years?
The same study also looked at the genomes of our ancient hominid cousins: Neanderthals and Denisovans. And he found that both ancient hominid species had a life expectancy of 37,8 years. Interestingly, the lifespan of the first Homo sapiens, our species, was likewise 38 years. (Maybe we're not that different from our early ancestors, after all.)
It may seem strange that humans have such a short life expectancy written in our DNA. Does this mean we should die at 38? Not exactly. As the lead author of this study writes, Benjamin Mayne, “Humans may be considered an exception to this study because advances in medicine and lifestyle have extended life expectancy.”
After all, genes are not destiny
You may still be wondering why there are such stark differences between species in terms of life expectancy and methylation. Could controlling methylation improve life span among individuals of the same species?
Different animals have different mechanisms within their cells that regulate methylation rates. These differences also occur between animals within the same species, because methylation depends on numerous factors, including different environments and underlying diseases. Someone with cancer will have different DNA methylation patterns than a healthy person, simply because the disease is associated with genetic alterations.
A healthy and active lifestyle is also likely to go a long way. Studies have reported that people who exercise and eat more fruits and vegetables often have higher levels of methylation, which is in contrast to the age-related declines normally seen.
