Why Does Biological Ageing Occur?

By Jasmine Gunton

Although morbid to consider, the fact of death is preordained and a constant across all forms of life on Earth. Much has been discussed about the subject, including what is likely to kill us, and what exists beyond death. However, biologists are still unsure about one of the most basic aspects of our life cycle: Why does biological ageing occur, and what is its purpose?

An image of cancerous human colorectal cells. Photograph by the National Cancer Institute on Unsplash.

Scientists already know why death is important in our current ecosystems. The death and following decomposition of organisms allow nutrients to be efficiently recycled within an ecosystem, increasing its overall net productivity [1]. Nevertheless, one must wonder why organisms would evolve to slowly decrease in fitness over their lifespan. Additionally, why do some organisms live for far longer than others?

As is common with many questions in science, several theories have been proposed to explain this process. What makes the study of ageing more complicated is that ageing rates vary across both species and related individuals. In addition, different parts of the body can age at different rates depending on several environmental and genetic factors. Much of the research associated with this subject has been focused on eukaryotes, of which humans are a part of.

Natural Immortality

In eukaryotes, there exist two broad theories as to how we age. These causes include the programming of ageing within our genome and the accumulation of damage to our cells [2]. Both of these theories can be grouped as sources of the phenomenon known as senescence, which refers to the inevitable decay of all eukaryotic organisms. However, a partial exception to this biological law exists in species with negligible senescence. Species under this category are seemingly able to avoid degeneration and are, therefore, potentially immortal.

An example of negligible senescence is displayed by Turritopsis dohrnii, also known as the immortal jellyfish. The life cycle of Turritopsis dohrnii includes four distinct stages, or morphoses, which the jellyfish can cycle through several times. In this way, Turritopsis dohrnii can be considered biologically immortal. However, this phenomenon has only been observed under laboratory conditions, as the process of morphosis occurs very quickly. Additionally, many medusae jellyfish in a natural environment will be killed by predators. Under laboratory conditions, it was found that only 20-40% of the mature medusae jellyfish transformed back into polyps [3]. It is not yet understood why only a small percentage of Turritopsis dohrnii display this phenomenon, and why similar organisms have not yet evolved this mechanism.

Telomeres

Turritopsis dohrnii is not the only organism thought to exhibit immortality. Previous research has suggested that lobsters may not weaken in strength or lose fertility with age like most other organisms. However, this does not mean that lobsters are truly biologically immortal, as it is known that lobsters are increasingly likely to die from shell moulting as they age. Although not immortal, lobsters do not senesce in a typical sense, most likely due to the presence of the enzyme telomerase. Telomerase is capable of repairing DNA sequences at the end of chromosomes — known as telomeres. Unlike other vertebrates, in lobsters, telomerase is expressed beyond the embryonic stage into the adult stage [4]. Therefore, lobsters are able to avoid most consequences of DNA damage and live to an estimated 45-50 years in the wild [5]. When considering lobsters’ longevity, one may wonder whether we can somehow adopt the lobster’s technique.

Biological Issues

To understand biological ageing, it is essential to link the discussion back to the species of which we have the most understanding: humans. Research has shown that as we age the body loses its ability to repair DNA damage. It is also known that telomeres shorten with age due to this damage, resulting in senescence and death [6]. What if we were able to develop a technique to prevent this process, lengthening our life spans indefinitely? Unfortunately, human immortality is almost impossible due to the nature of our cells and their biological processes. In cellular biology there are two main certainties: the function of cells shut down, and cells become more likely to turn cancerous with age. Attempting to alter either of these processes enhances the other, meaning that humans are certain to die of either organ failure or cancerous growths [7].

Philosophy of Death

We now know why and how we age, but this leaves an important question: why has natural selection not selected for immortality in all eukaryotes? The answer exists in how natural selection affects the macroevolution of a species. Natural selection not only selects for traits that increase the survival of the individual, but also the survival of the species that it belongs to. This means that for a species to survive, generations of individuals must constantly reproduce offspring, age, and then die so that the next generation can continue this cycle.

This explanation is relatively simple and also greatly unsatisfying. Although, from a philosophical point of view, we can view death as essential to our existence, as it allows us to appreciate life more, and for many more individuals to experience the joys that life can bring than would be possible with the existence of immortality. Nevertheless, with the development of new technology, we may be able to extend the period in which we experience life and the consciousness to value it.

Fibres and microtubules in human breast cancer cells. Photograph by the National Cancer Institute on Unsplash.

References

[1] E. Benbow, J. P. Receveur, and G. A. Lamberti, “Death and Decomposition in Aquatic Ecosystems,” Frontiers in Ecology and Evolution, vol. 8, pg. 17, Feb. 2020. [Online]. URL: https://www.frontiersin.org/articles/10.3389/ fevo.2020.00017/full

[2] J. Vina, C. Borras, and J. Miquel, “ Theories of Aging,” IUBMB Life, vol. 59, no. 4-5, pp. 249-254, Jan. 2008. [Online]. URL: https://iubmb.onlinelibrary.wiley.com/ doi/abs/10.1080/15216540601178067

[3] S. Piraino, F. Boero , B. Aeschbach, and V. Schmid, “Reversing the Life Cycle: Medusae Transforming into Polyps and Cell Transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa),” The University of Chicago Press, vol. 190, no. 3, pp. 302-312, June. 1996. [Online]. URL: https://www.journals.uchicago.edu/ doi/10.2307/1543022

[4] W. Klapper, K. Kuhne, K. K. Singh, K. Heirdon, R. Parwaresch, and G. Krupp, “Longevity of lobsters is linked to ubiquitous telomerase expression,” FEBS Press, vol. 439, no. 1-2, pp. 143-146, Dec. 1998. [Online]. URL: https://febs.onlinelibrary.wiley.com/doi/full/10.1016/ S0014-5793%2898%2901357-X#

[5] T. Wolff, “Maximum Size of Lobsters (Homarus)(Decapoda, Nephropidae),” Crustaceana, vol. 34, no. 1, pp. 1-14, Jan. 1978, doi: https://doi. org/10.1163/156854078X00510 [6] M. Blasco, “Telomere length, stem cells and aging,” Nature Chemical Biology, vol. 3, pp. 640-649, Sept. 2007. [Online]. URL: https://www.nature.com/articles/ nchembio.2007.38

[7] J. Mitteldorf, “Telomere biology: Cancer firewall or aging clock?,” Biochemistry (Moscow), vol. 78, no. 13, pp. 1054- 1060, Sept. 2013. [Online]. URL: https://link.springer. com/article/10.1134/S0006297913090125