Mortality, Evolution, and the Temporary Individual

Key idea:
From an evolutionary perspective, the continuation of a species does not depend on the indefinite survival of individual organisms, but on reproduction, genetic variation, and selection across successive generations.

After asking what a human being actually is, we are inevitably confronted with a characteristic that affects the entire human condition: mortality.

Why are human beings mortal? And why does nature appear to be organized in such a way that living organisms eventually age and die?

An important part of this process takes place at the cellular level. Throughout life, cells are continuously exposed to damage from internal and external influences. The body attempts to repair this damage by replacing old cells with new ones.

During each cell division, genetic material has to be copied, and other cellular processes also have to be maintained with great precision. These processes are highly reliable, but they are not perfect. In addition to DNA damage, other mechanisms also play a role in aging, including changes in gene regulation, telomere shortening, loss of proteostasis, mitochondrial changes, cellular senescence, and declining stem-cell function. Aging is therefore not simply the consequence of repeatedly copying DNA, but the result of a complex interaction of different processes.

More information about the aging process is provided in Appendix 4.2 (The Aging Process in Humans) .

Yet this raises an intriguing question.

When a woman gives birth to a child, a new organism emerges that, despite the age of the parents, can potentially live a complete life of its own. This shows that reproduction involves mechanisms through which genetic information is reorganized and passed on to a new generation.

Why, then, does the body not simply use these mechanisms to continuously renew itself completely?

Perhaps part of the answer lies in the trade-offs that have emerged during evolution between growth, reproduction, repair, and maintenance. The body possesses powerful mechanisms for repair and maintenance, but these are not unlimited. From an evolutionary perspective, natural selection also does not necessarily have to produce an organism that is maintained optimally for an unlimited period of time.

Human beings exist today because the species has continuously adapted to changing circumstances. This adaptation usually does not occur because a single individual fundamentally changes itself, but because new variations arise within the DNA of populations over many generations.

When people live for long periods in regions with limited sunlight, lighter skin can be evolutionarily advantageous because it can contribute to more efficient vitamin D production. In regions with very intense sunlight, darker skin can provide greater protection against harmful ultraviolet radiation. However, the evolution of human skin pigmentation is complex and is likely influenced by multiple factors, including UV radiation, vitamin D, folate, genetic variation, and cultural circumstances.

This variation arises through a combination of processes, including mutations and new combinations of genetic material during reproduction. Some of these combinations may prove better adapted to prevailing conditions than others. Individuals with relatively advantageous characteristics have, on average, a greater chance of reproducing, causing those characteristics to be passed on more frequently to subsequent generations.

From this perspective, evolution can be understood as a process in which genetic characteristics and populations continue through ongoing variation, selection, and reproduction across generations.

The individual human life is therefore temporary in an evolutionary sense. The species continues because successive generations follow one another, with new individuals introducing genetic variation into the population. Evolution therefore does not operate primarily through the permanent adaptation of a single individual, but through continual replacement and selection within successive generations.

Perhaps a limited lifespan is partly an evolutionary consequence of the fact that natural selection acts less strongly on characteristics that have harmful effects only later in life. Characteristics that provide an advantage early in life, for example, may be retained even when they have detrimental effects at older ages. There may also be a trade-off between the energy an organism invests in reproduction and the energy available for long-term maintenance of the body. These are important ideas within evolutionary theories of aging, but they do not provide a definitive answer to the question of why humans age.

Therefore, we cannot simply say that older organisms become less flexible and that new generations are evolutionarily advantageous solely for that reason. Reality is more complex. Aging arises from a combination of biological processes and evolutionary trade-offs, whose precise contribution may differ from one species to another.

From this perspective, the human being can be seen as a temporary biological organism in which genetic information is maintained and can be passed on to subsequent generations.

In nature, there are also organisms that appear to follow a different evolutionary strategy. Some species of flatworms, for example, see Appendix 4.3 (The Aging Process in Flatworms) , possess strong regenerative abilities and can therefore exhibit an exceptional capacity for tissue renewal. Some planarians are consequently described in the literature as organisms showing very little or difficult-to-detect aging under certain conditions. This does not mean, however, that they are literally biologically immortal.

Human beings do not possess such abilities on a comparable scale.

This need not be regarded as a deficiency. It may instead be related to the evolutionary trade-offs between repair, maintenance, growth, and reproduction.

The question therefore may no longer simply be why human beings die, but why natural selection has not produced an unlimited individual lifespan.

Perhaps mortality is not a “flaw” in the system, but rather a consequence of the way evolutionary selection operates, involving continual trade-offs between maintenance, reproduction, and adaptation to changing circumstances.

And with that, we once again arrive at a fundamental tension that affects not only biological processes, but ultimately human societies and moral systems as well: the tension between the individual and the whole, between stability and change, and between preservation and renewal.