From an evolutionary perspective, it isn’t immediately obvious why animals should experience degeneration and illness as they age. Shouldn’t natural selection favor those who are healthier and live longer? Shouldn’t we eventually evolve to live forever since immortal individuals are clearly more “fit” than those who age?
But evolution only cares about reproduction and genes, not individuals and their health. The more offspring an individual is able to have, and the more these offspring reproduce themselves, the more successful that individual’s genes become. That’s all that matters from the perspective of natural selection.
For example, if a dog has genes that make it very fertile, and that dog bears or fathers dozens or even hundreds of offspring that survive to adulthood, those genes are going to be carried by those offspring. They too will be very fertile, and over time those “fertility genes” will be the most common in the population. If another dog has genes that make it less fertile, and it only has a few offspring, then those genes will gradually become less common or disappear.
But what if the fertile dog is so good at reproducing because its body doesn’t bother putting much energy into repairing or maintaining itself? What if nearly all its resources go to reproduction, and that dog gets ill and dies very young? The less fertile dog, in contrast, has a body that is very good at taking care of itself, and it stays healthy and lives twice as long. Shouldn’t evolution favor the healthier dog?
Unfortunately, evolution favors the dog that reproduces more even if it has a shorter, less healthy life. If the genes that make a dog fertile also makes a dog age fast and die young, these genes will still become more common over time. The dog population would eventually consist mostly of individuals that are shorter-lived but more fertile rather than those with greater lifespan and healthspan who have fewer offspring.
Of course, humans control much of dog breeding today and most dogs do not evolve with pure natural selection but the legacy of evolutionary history on dog aging is still very much present in modern dogs. This is an oversimplification of a complex and still-evolving idea but it works pretty well as a general explanation of why resilience against age-related deterioration is not favored by evolution.
Some species do live much longer than others, so there is variation in how evolution shapes different species. Humans, for example, live much longer than many other mammals. Our brains give us our competitive edge, but also require more time to develop and train, giving us less time for reproduction. This tradeoff worked for us but the best strategy for a given species depends on the details of the context in which it evolved.
Victory, with some exceptions, in the evolutionary race has gone to the swift and fertile, not to the slow and enduring.