Of all the fascinating creatures inhabiting our planet, the hydra stands out as a biological marvel. Resembling a tiny dandelion seed with tentacles, this freshwater relative of jellyfish possesses an extraordinary ability: regeneration. Cut a hydra into pieces, and each fragment will develop into a new, complete organism. This remarkable trait has captivated biologists, sparking questions about immortality in nature. Why do some species appear to escape natural death, and is death truly an inevitable part of life for most?
The mystery of aging and death has been a subject of scientific inquiry for decades. A significant theory emerged in the mid-20th century, proposing that aging is essentially a biological trade-off. Organisms initially invest resources in growth and cellular maintenance to ensure survival and development. Throughout youth and adolescence, the body prioritizes staying healthy and strong. However, upon reaching sexual maturity, the biological imperative shifts towards reproduction. For most living things, resources are finite, and dedicating energy to producing offspring can come at the cost of maintaining bodily health.
Consider the dramatic example of salmon. These fish undertake arduous journeys upstream to spawn and then often die shortly afterward. Their bodies expend every ounce of energy to reach the spawning grounds and maximize reproductive success. The odds of a salmon surviving the return journey, enduring another year at sea, and repeating the spawning migration are so slim that natural selection would not favor such individuals. Furthermore, they have already successfully passed on their genes.
Current scientific understanding offers a more nuanced perspective on the reasons behind death. Once organisms reach reproductive age, the power of natural selection diminishes, and the aging process commences, ultimately leading to mortality. This isn’t necessarily to make room for the next generation, as appealing as that altruistic idea might sound, explains Alexei Maklakov, an evolutionary biology and biogerontology professor at the University of East Anglia.
Throughout our lives, our genes accumulate mutations. Some mutations occur randomly, while others result from environmental factors like diet or UV radiation. The majority of these mutations are either neutral or harmful, and only a tiny fraction might be beneficial. Before sexual maturity, any genetic mutation that reduces an organism’s reproductive potential or causes death before reproduction would be strongly eliminated by natural selection, notes Gabriella Kountourides, an evolutionary biologist at the University of Oxford. However, once an organism has reproduced and passed its genes to the next generation, the selective pressure weakens.
Imagine our spawning salmon again. It has successfully reached adulthood and reproduced. Its offspring are likely to have a reasonable chance of doing the same. If a gene mutation were to arise in this salmon after spawning, increasing its lifespan by another year (though statistically improbable), its offspring would not gain a significant advantage over their siblings. The salmon has already ensured the continuation of its genetic line in the next generation without this hypothetical mutation.
From a natural selection standpoint, there is limited evolutionary benefit in continuing to expend energy on maintaining health after reproduction. Consequently, genes that promote longevity after reproduction are not subjected to the same selection pressures that would make them more prevalent. “An individual, of course, would prefer to live longer,” Kountourides states. “But at that stage, natural selection isn’t working as diligently because there’s nothing more to contribute to the next generation.”
While salmon represent an extreme example of post-reproductive mortality, many organisms do survive longer and reproduce multiple times. Most DNA mutations are either inconsequential or detrimental. Our bodies possess mechanisms to repair some DNA damage, but this repair capacity declines with age due to the reduced force of natural selection.
Later in life, senescent cells can accumulate in tissues, contributing to damage and inflammation, and are precursors to age-related diseases.
Aging and death, therefore, arise from a combination of factors: the accumulation of harmful mutations due to weakened natural selection and the presence of genes that might have been beneficial for reproduction but become detrimental to long-term survival.
BRCA gene mutations exemplify this concept. While significantly increasing the risk of breast and ovarian cancers, these mutations have also been linked to higher fertility in women carrying them. It’s plausible that BRCA gene mutations provide a reproductive advantage early in life, followed by increased health risks later. Because natural selection weakens after sexual maturity, the early reproductive advantage outweighs the later disadvantage from an evolutionary perspective.
“Events occurring earlier in life will always take precedence over events later in life because reproductive potential is paramount,” explains Kaitlin McHugh, a biologist at Oregon State University.
Cellular senescence, where cells cease dividing, could be another example of a trait with early-life benefits and late-life drawbacks. Senescence acts as a safeguard against cancer by preventing cells with DNA damage from multiplying. However, in later life, the accumulation of senescent cells in tissues can lead to damage, inflammation, and age-related illnesses.
Despite the prevalence of aging in most species, exceptions exist. Many plants exhibit “negligible senescence,” and some species can live for thousands of years. The Pando tree in Utah’s Fishlake National Forest is a remarkable example. This “tree” is actually a vast colony of genetically identical male quaking aspen trees connected by a single root system. Spanning over 100 acres (400,000 sq m) and weighing an estimated 6,613 tons (6,000 tonnes), some estimates suggest it could be over 10,000 years old.
The immortal jellyfish, a relative of the hydra, has evolved an even more astonishing strategy for longevity. It can revert from its adult medusa stage back to its juvenile polyp stage if injured, diseased, or stressed. “At some point, though,” McHugh questions, “you have to wonder if it’s still the same individual or something entirely different?”
There is also the notion of “negative senescence,” where certain species become more reproductively successful with age, but the evidence for this is limited, according to Maklakov.
“If a species’ ecology is such that reproduction is inherently low or impossible early in life, the dynamics of natural selection shift,” Maklakov explains. Animals like walruses or deer, which form harems, might exemplify this. A dominant male might control a large group of females, and his harem size, and thus his offspring count, could increase with age and size. His reproductive output, therefore, continues to rise.
However, Maklakov argues that while some species may maintain reproductive capacity with age, they are not true examples of negative senescence, and studies suggesting otherwise are likely flawed. Eventually, even the most dominant walrus will lose control of his harem.
Sex itself might play an unexpected role in aging. A study by Megan Arnot and Ruth Mace at University College London suggests that women who engage in regular sexual activity tend to experience menopause later in life. They propose this as another example of a trade-off: energy spent on ovulation might be more efficiently utilized by the body if pregnancy is unlikely.
In contrast, within the broader animal kingdom, higher fertility often seems to accelerate aging. For instance, bats that produce more offspring tend to have shorter lifespans than those with fewer offspring. Perhaps, given the opportunity to reproduce, they invest all their resources into it. “There’s this trade-off in timing,” McHugh observes, “where organisms that reproduce exceptionally well early in life don’t fare as well later in life.” (Hydra, again, are an exception, as their fertility rates do not appear to decline over their lifespans.)
Lifespans can also vary dramatically between sexes within a species. Typically, in ant, bee, and termite colonies, the queen or king, who is often highly fertile and long-lived, contrasts sharply with the sterile, short-lived workers. In their case, the reproductive cost doesn’t seem to shorten the lifespan of the queen or king. This could be because the queen or king is shielded from many of the dangers faced by workers, leading to such divergent lifestyles that traditional aging theories may not apply equally to both castes.
Given the powerful influence of reproduction on lifespan, why do humans live so long beyond our reproductive years?
The grandmother hypothesis proposes that the extended post-reproductive lifespan in humans is evolutionarily advantageous. Reproduction is a resource-intensive and risky endeavor. Grandmothers can contribute to the survival of their genes by investing in their grandchildren. A longer lifespan, therefore, becomes beneficial from a natural selection perspective. “Families with grandmothers present often exhibit significantly higher reproductive fitness,” Kountourides notes, “possibly because mothers can focus on having more children while grandmothers assist in raising existing offspring.”
However, grandchildren only share 25% of their genes with a grandmother, the same degree of relatedness as nieces and nephews. This raises questions about the specificity of grandmotherly investment versus broader kin selection benefits.
“It could also simply be that in the past, not enough women survived to reproduce at age 50,” Maklakov suggests, returning to the fundamental principle of aging – the weakening of natural selection after reproduction. “Therefore, selection pressures on female reproduction beyond age 50 were very, very weak.” Much of what we experience in later life may be unpleasant, but evolutionary forces haven’t strongly acted to protect us from it. Death, in this light, is not a biologically programmed necessity but rather a consequence of life’s evolutionary trade-offs and the diminishing power of natural selection after we have fulfilled our primary biological imperative: to reproduce.
