William ParkFeatures correspondent•@williamhpark
Fernando Trabanco Fotografía/Getty Images
Most creatures begin aging and eventually die after reaching sexual maturity – but what are the fundamental reasons behind this universal biological phenomenon? While certain jellyfish and their relatives offer intriguing clues about the possibility of biological immortality, the question remains: why is death an inevitable part of life for the vast majority of species, including humans?
Among the diverse array of aquatic life forms inhabiting our oceans and freshwaters, the hydra, a freshwater relative of jellyfish, anemones, and corals, stands out for its remarkable regenerative capabilities. Named after the mythical Greek serpent that could regrow its heads, the hydra, resembling a dandelion seed with a tubular body and a crown of tentacles, possesses an extraordinary ability: it can regenerate completely from fragments. Cutting a hydra into multiple pieces results in each fragment developing into a fully formed, new individual.
This exceptional regenerative capacity has long fascinated biologists investigating the potential for immortality in nature. Why do certain species, like the hydra, seem to escape death from natural causes? And more broadly, is death truly an unavoidable aspect of life?
The Evolutionary Trade-Off: Reproduction vs. Longevity
The concept of aging was first articulated in the mid-20th century as an evolutionary trade-off between reproduction and cellular maintenance. Initially, an organism’s body prioritizes growth and health, allocating resources to cell maintenance and development. Throughout childhood and adolescence, the biological imperative is survival and achieving peak physical condition. However, upon reaching sexual maturity, the organism’s priorities shift towards reproduction. For most species, resources are finite, and this prioritization of offspring production can come at the cost of somatic maintenance and longevity.
Consider the example of salmon that undertake a strenuous upstream migration to spawn and subsequently perish. These fish expend all their energy reserves to reach their spawning grounds and maximize their reproductive success once there. The probability of a salmon successfully returning downstream, surviving another year at sea, repeating the arduous upstream journey, and spawning again is statistically so low that natural selection would not favor individuals with such traits. Furthermore, having already successfully passed on their genes once, the evolutionary pressure for continued survival diminishes.
However, the contemporary understanding of mortality delves deeper into the nuances of natural selection. The prevailing theory posits that as organisms reach reproductive maturity, the force of natural selection weakens, initiating the aging process and ultimately leading to death. This is not necessarily a mechanism to “make way” for subsequent generations, as appealing as that altruistic notion might seem, according to Alexei Maklakov, a professor specializing in evolutionary biology and biogerontology at the University of East Anglia.
Natural Visions/Alamy
Named after the Ancient Greek mythological serpent, the freshwater hydra has a remarkable ability to regenerate (Credit: Natural Visions/Alamy)
The Accumulation of Mutations and Senescent Cells
Throughout an organism’s lifespan, its genes accumulate mutations. These genetic alterations can arise randomly, or be induced by environmental factors like diet or UV radiation. The majority of these mutations are either neutral or detrimental, with beneficial mutations being rare. Prior to sexual maturity, any gene mutation that diminishes an organism’s reproductive prospects or causes premature death before reproduction is strongly selected against, explains Gabriella Kountourides, an evolutionary biologist at the University of Oxford. However, once an organism successfully reproduces and passes its genes to the next generation, the intensity of natural selection decreases.
Returning to our salmon example, having reached adulthood and reproduced, it has already achieved significant evolutionary success. Its offspring are now equipped with a reasonable chance of reaching reproductive maturity themselves. If a gene mutation were to occur in this post-spawning salmon, randomly extending its lifespan by an additional year (an improbable scenario), its offspring would not gain a substantial evolutionary advantage over their siblings lacking this mutation. The salmon has already ensured the continuation of its genetic lineage with the first generation.
From a natural selection perspective, there is minimal benefit in sustaining the considerable effort required for continued health and survival after reproduction. Consequently, genes that promote longevity beyond the reproductive phase are not subjected to the selective pressures necessary for them to become more prevalent in the population. As Kountourides notes, “An individual would ‘like’ to stay alive. But at that point, natural selection doesn’t work so hard on it, because there’s nothing more to keep giving to the next generation.”
Aging: A Two-Fold Process
While the salmon example represents an extreme case of post-reproductive death, many organisms, including humans, survive for a considerable period after reproduction, potentially having further offspring. Most DNA mutations have neutral or negative consequences. Our bodies possess mechanisms to repair some of this DNA damage, but the efficiency of these repair processes declines with age due to the weakened forces of natural selection.
Aging and death, therefore, manifest through a dual mechanism: the accumulation of detrimental mutations due to diminished natural selection and the emergence of mutations that may have been advantageous during reproductive years but become detrimental to long-term survival.
BRCA gene mutations serve as an illustrative example of this latter phenomenon. These mutations are well-established risk factors for breast and ovarian cancers but have also been linked to increased fertility in women carrying them. This suggests a potential evolutionary trade-off where BRCA mutations confer a reproductive advantage early in life, followed by heightened health risks later in life. Because natural selection is less potent after sexual maturity, the early reproductive benefit may outweigh the later-life disadvantage from an evolutionary standpoint.
Kaitlin McHugh, a biologist at Oregon State University, emphasizes this point: “Whatever happens earlier in life is going to outweigh whatever happens after the age of reproduction, because reproductive potential is really what matters.”
Cellular senescence, a process where cells cease dividing, provides another example of a potentially advantageous early-life mechanism with adverse late-life consequences. Senescence acts as a protective mechanism against cancer by preventing cells with damaged DNA from proliferating uncontrollably. However, in later life, senescent cells can accumulate in tissues, contributing to inflammation, tissue damage, and age-related diseases.
Exceptions to the Rule: Negligible Senescence and Biological Immortality
While aging is a near-universal phenomenon across species, exceptions exist. Many plant species exhibit “negligible senescence,” meaning they show no discernible decline in function with age, and some can live for thousands of years. The Pando tree, a vast colony of genetically identical quaking aspen trees in Utah’s Fishlake National Forest, exemplifies this longevity. This interconnected colony, spanning over 100 acres and weighing thousands of tons, is estimated to be over 10,000 years old.
The “immortal jellyfish” (Turritopsis dohrnii), a relative of hydra, presents another remarkable strategy for longevity. When faced with physical damage, starvation, or stress, this jellyfish can revert back to its polyp stage, essentially transforming back into its earliest life cycle stage. This process of cellular transdifferentiation raises profound questions about the very definition of individual identity and biological immortality, as McHugh points out: “Though at a certain point, you have to ask yourself is it the same individual or something different?”
Arch White/Alamy
The arduous journey up river from the ocean takes such a toll on the salmon that make it that they die shortly after spawning (Credit: Arch White/Alamy)
The Complex Role of Sex and Reproduction in Aging
The relationship between sex and aging is complex and multifaceted. Some research suggests “negative senescence,” where reproductive success increases with age in certain species, but Maklakov considers the evidence for this phenomenon to be weak and potentially flawed.
In species where reproduction is inherently limited or delayed until later in life, the dynamics of natural selection can shift. Animals like walruses or deer, which form harems, might illustrate this. A dominant male’s control over a large group of females, and consequently his reproductive output, may increase with age and size. However, even in these cases, true negative senescence is unlikely, as a walrus’s dominance over a harem is not indefinite.
Intriguingly, sex itself may play a role in human aging. Studies suggest that women who engage in regular sexual activity experience menopause later in life. This could represent another evolutionary trade-off, where energy allocated to ovulation can be redirected to somatic maintenance when pregnancy is less likely.
Conversely, in much of the animal kingdom, higher fertility appears to accelerate aging. Bats with larger numbers of offspring tend to have shorter lifespans compared to those with fewer offspring. This suggests that when given the opportunity to reproduce, organisms may invest all available resources into reproduction, at the expense of longevity. However, once again, hydra appear to defy this rule, as their fertility rates do not seem to decline throughout their lifespan.
Social insect colonies, like ants, bees, and termites, exhibit striking differences in lifespan between reproductive and non-reproductive castes. Queens and kings, responsible for reproduction, often live significantly longer than sterile worker castes. In these cases, the evolutionary trade-off between reproduction and lifespan may not apply equally to all castes, potentially due to the queen’s protected environment and different lifestyle compared to workers.
Brooke Fasani/Getty Images
Having grandmothers around can bring many advantages to a family (Credit: Brooke Fasani/Getty Images)
The Grandmother Hypothesis and Human Longevity
Given the profound influence of reproduction on lifespan, the extended post-reproductive lifespan in humans, particularly in women, presents an evolutionary puzzle. The grandmother hypothesis offers a compelling explanation. It proposes that the continued survival of older female relatives after menopause is evolutionarily advantageous because grandmothers can contribute to the survival and reproductive success of their grandchildren. By investing in grandchildren, grandmothers indirectly promote the propagation of their own genes. Studies have shown that families with grandmothers present exhibit higher reproductive fitness, potentially because grandmothers assist in child-rearing, allowing mothers to focus on having more children.
However, as grandchildren share only 25% of their genes with their grandmothers, the evolutionary benefit might extend beyond direct kinship, potentially encompassing nieces and nephews as well.
Ultimately, as Maklakov concludes, “It could also simply be that in the past not enough women survived to be able to reproduce at the age of 50. And so the selection on what happens to female reproduction at the age of 50 was very, very low,” reiterating the central tenet of aging: after reproduction, the strength of natural selection diminishes. While the consequences of aging in later life may be undesirable, there is limited evolutionary pressure to protect us from them. Death, therefore, is not necessarily a programmed event but rather an evolutionary consequence of the declining force of natural selection after reproductive age and the accumulation of cellular damage over time.
* William Park is a senior journalist for BBC Future and tweets at @williamhpark
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