Why do we age? Can we stop it? Could we someday live forever?
Why Do We Age in the First Place?
Ageing is a deleterious trait and an evolutionary paradox. It leads to physiological deterioration and increased likelihood of death and/or decline in fecundity. This means that an ageing organism’s ability to make a genetic contribution to the next generation decreases with time. So why is ageing not under the pressure of negative selection? The research field of gerontology – the study of ageing – might be able to help.
It is thought that ageing has evolved as a side effect of natural selection – the pleiotropy/trade-off theory of the evolution of ageing. Mutations which are beneficial during youth can be harmful later on. And since the force of natural selection grows weaker with age, these eventually harmful mutations remain in the genome.
For example, we have some genes that limit the proliferation of cells (tumour suppressors) and some that promote it (proto-oncogenes). There is a fine balance between the two – too much cell growth can cause cancer; too little would cause tissues not to function properly and die. This is important to our survival because it balances the need for regenerative capacity while guarding against an increased risk of cancer. It has been suggested that as we age, this balance skews causing a decline in stem cell function, meaning we lose the ability to regenerate tissue.
Immortality Already Exists!
Believe it or not, ageing is not inevitable. You have probably observed a great biodiversity in lifespan in such that humans can live past 100 years old, while mice generally only make it to 3 years. But what might not be so apparent is the extremity of this diversity. For example, the creatures pictured below and even plant life, like the Bristlecone Pine which can survive for around 5,000 years. Comparing the biology of ageing between different species allows us to ask whether particular factors influence the rate of ageing.
Take the famous 5 mm “immortal jellyfish” pictured at the top. Its cells can undergo a process called transdifferentiation, allowing it to revert to its juvenile state once it becomes sexually mature and mates which is creating quite a stir among marine biologists fearing a “worldwide silent invasion” as a consequence.
When Hercules tore through one of the Lernaean Hydra’s nine heads, two grew back in its place. Its real life counterparts belong to a genus of cylindrical freshwater polyps called a Hydra cell (pictured below). These creatures seem to show no signs of ageing.
The Dahlia Anemone (also pictured below) appears also to be non-ageing and is more closely related to humans than the Hydra. They seemingly live forever and just keep getting bigger. Astonishingly, if you cut them in half, you will get two fully functioning anemones. Even more interesting is that their cells don’t mutate how you would expect; they seem to be constantly replenished. Despite being radically different from humans they are still multicellular and have specialised organs like us.
Some fish, such as the Atlantic Cod, even undergo negative ageing and become less likely to die as they grow older. But whether that is a biological phenomenon or a statistical artefact, I’m unsure.
The only problem is that these differences in lifespan are confounded by the many other differences that occur between species. Nevertheless, it demonstrates that immortality can indeed exist.
Creating the Elixir of Life
Immortality is a stable rate of mortality as a function of chronological age. In other words, no increased risk of dying as you get older. In the world of biology, it is genuinely believed that the first organisms didn’t age.
It’s thought that the Hydra’s immortality is achieved through a pool of continuously proliferating stem cells. In fact, all cells in the Hydra are continually dividing, replenishing any defective cells. This is in contrast to humans and other animals where stem cells eventually lose the ability to proliferate and form new cells, meaning ageing tissue can no longer regenerate. However, keep in mind that Hydra have only been proven to live past 4 years. It is only because it outlives its predicted lifespan by such an incredible magnitude and shows no apparent signs of ageing that we believe it to be immortal. More time will be needed to confirm whether these Hydra age and eventually die. In addition, experiments are yet to measure the sexual reproductive output which could show age if it slows down with time.
If you look at pretty much any cell in the Hydra or the Dahlia Anemone, they are germ line cells which have unlimited lifespans. Germ lines are immortal and are therefore subjected to a constant force of natural selection. This means damaged cells are selectively eliminated, allowing an undamaged and rejuvenated lineage to live on forever. Interestingly, the germ line is segregated from the rest of the mortal body at different stages in the development in different organisms and this may be the key to immortality. Although it would come at a price. Of course, the future could also behold technological immortality in the form of transhumanism (but I’ll cover than in a later post…).