Are human bodies programmed to die?

From a certain point of view you could argue that our bodies have an inherently limited lifespan;

• Telomeres are extensions to the end of chromosomes that prevent damage or loss of genetic information during cell division. Telomeres are not replaced (in normal cells), which gives rise to a replicative lifespan; the number of times a cell can divide before permenantly leaving the cell cycle (senescence).
  • This is generally viewed as an anti-cancer mechanism to protect against errors creeping in to the genome through many cell divisions. In order to become cancerous, a cell must first overcome its replicative lifespan [ref.]. This is achieved by activating the (normally inactive) telomerase enzyme that extends the telomeres - embryonic stem cells are one of the few cell types that normally express this enzyme.

There are other ways in which you could argue that our lifespans are fundamentally limited, but it is important to note that the objective is not 'to die', but to increase fitness (in a Darwinian sense) earlier in life. This is known as antagonistic pleiotropy; when an advantageous trait early in life is disadvantageous later in life.

Telomere shortening is just one example of antagonistic pleiotropy (protects against cancer when younger, but limits the number of times your cells can divide).

Other traits that inherently limit lifespan include;

• Neurons (as a rule) do not replicate, and last for your whole life time. This certainly excludes them from replicate senescence, however it means they are highly susceptible to 'wear and tear'; oxidative stress is a natural by-product of respiration, and the vast majority of damage done by these species (e.g. reactive oxygen species) is repaired by the cell, however some will always remain unchecked, and eventually this leads to neurological dysfunction and cognitive decline. Without intervention this is inevitable in each individual (the rate of aging differs between people, but aging and age-dependent diseases are disorders are a natural part of living).
• The same is true for cardiac and smooth muscle - whilst much repair can be carried out, it is inevitable that damage will creep in over a life time, and thus the vast majority of human age-related deaths are due to cardiac problems in one way or another.

So there is no 'programmed' limit to life span, in that we have not evolved to die, but our bodies are inherently limited by the systems that have evolved. Life expectancy a few thousand years ago was ~20 years (if you lived beyond infancy!), whereas now in the developed world this is ~80 years, so our bodies can already survive way beyond our 'natural' life span, and thus we now succumb to age-related disease. Evolution has spent millions of years giving us every possible advantage that leads to reproductive success. Natural selection of traits beyond reproduction are secondary to those beforehand, and thus we have fundamentally limited life spans.

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There is an argument for an evolutionary advantage of limited lifespan. This seems at first counter-intuitive, until you consider that natural selection does not act on individuals, but genes. It is proposed to be advantageous (in some circumstances) for an organism to have a shorter life span, as this increases the turn-over of individuals in that population. This in turn increases their evolvability - clearly advantageous for the gene(s) influencing this trait if it increases the likelihood of successful reproduction and thus passing on that gene/allele/trait...

I like this hypothesis, and can see that natural selection may favour it. I think mice are great examples here; they have much shorter lifespans than us, yet they 'age' (biologically) the same (cardiac problems, diabetes, cancer) but at a faster rate to give a higher population turnover). In a high mortality environment, the most adaptable animals will be more successful.

However, I think this is likely to be secondary to the pressures on other survival traits that more directly increase the chances of successful reproduction.

In Richard Dawkin's book "The Selfish Gene" it seems that there is a programmed purposeful death of the body. The genes are in control. They seek out a mate to produce offspring that will be even better suited to pass it's genes on again. The meaning of life as we have seen and observed it is really for the genes to perpetuate their existance into the future. Once passed on the body that did the passing is of little use. The more complex the organism like a human mates may be selected by life span of parents and relatives. There doesn't seen to be any Newtonian law that states a cell cannot just keep replicating and essentially avoid aging. Our bodies are doing that for a purpose. One we just don't understand.

I'm not very knowledgeable about evolutionary biology, so I'm going to answer this from the perspective of molecular/cell biology.

The short answer is: No, there isn't any evidence that I know of for a built-in "death".

The long answer:

1. Telomeres are shortened over repeated copying of chromosomes (so as cells divide repeatedly, the telomeres shorten). Shorter telomeres make chromosomes more unstable, and can lead to some of the symptoms of aging (like limiting organ regeneration). But this is more a side effect of humans not having the most efficient telomerase ever.

In the end, while shortened telomeres contribute to limiting the number of times a cell can replicate (Hayflick limit), we also have stem cells that can then replace those senescent cells. (This is also true in the brain and heart: stem cells are present in the heart and even in highly restricted areas of the brain, and produce neurons throughout life: https://pubmed.ncbi.nlm.nih.gov/10834848/.) So the telomere problem alone doesn't predetermine death.

2. Other factors that contribute to aging can be divided into two categories, genetic factors and accumulation of damage.

But in both of those groups, the cause isn't that genetically we're programmed to die at a certain time. It is always a case of the systems used to repair or regenerate failing after a time: DNA mutation repair becoming incapable of keeping up with the rate of mutations; failure of clonal deletion leading to autoimmunity; inhibition of autophagy by mTOR leading to accumlation of old and damaged cell parts; failure of senescent cells to undergo apoptosis.

There are many other hallmarks of aging, but the common thread we see is not that there is a set program leading to an organism's death, but rather a failure (through accumulation of damage) of the systems that keep us alive.