The average life expectancy had doubled since the prehistoric period to the 19th century and doubled again over last hundred years. But the maximum duration of human life hasn’t changed so much. In fact, it’s known that many centenarian lived in the ancient world.
All mammals, including humans, age in a very similar manner and unfortunately die of old age. Why does this happen, and what are the fundamental biological principles responsible for this?
View from Evolutionary Perspective
It may sound rather sad, but we are evolutionarily preprogrammed to die of old age. Sad to say, dying of old age is required to make possible alternation of generations. If humans or any other organisms didn’t perish, their populations would soon grow to its maximum, and there would be no place for new organisms with new traits.
Efficient adaptation of a biological species to changing environments requires emerging new organisms with new traits. Probably, if organisms were able to adapt to new environments much better during their lives, there would be no need for the appearance of new generations.
The species reproducing predominantly by asexual reproduction don’t actually die of aging. For instance, bacteria split but don’t die of aging. In the case of asexual reproduction, the adaptation to environmental change mainly occurs due to a very large number of organisms, for example, like microorganisms. Because as a result of asexual reproduction, offspring don’t differ too much from parents, asexual reproduction might be more valuable in stable environments.
The emergence of sexual reproduction caused interchange of generations much more valuable trait, in evolutionary terms. Bigger organisms like humans tend to reproduce sexually and therefore change generations. You can read more about evolutionary aging theories here.
Mechanisms of aging
There are many different mechanisms of aging, and much still remains unclear in this process. Aging is partly caused by the accumulation of errors at the genetic and cellular level. But there are other mechanisms.
There is a notion of the existence of the so-called genetic and epigenetic aging clock in multicellular organisms like humans. As we get older, our genes tend to express in a different manner – some genes turn off and some turn on (more genes turn on as we age). If we fully understand these processes, we can in principle slow down and even reverse the aging clock. You can read more about this here.
Some single genes play a bigger role in aging. These genes are conditionally divided into lifespan regulators, lifespan mediators, and lifespan effectors. For example, some lifespan regulators are responsible for rapid growth when we are young, but these genes can cause cancer in later stages of life. Sometimes mutations in a single gene may lead to very adverse effects.
It’s known that aging is associated with telomere (structures at the end of chromosomes) shortening. Presumably, if we kept our telomeres stable, we wouldn’t age.
External, environmental factors can influence internal mechanisms taken place in the body and alter the rate of aging in the organism. For instance, caloric restriction is a well-known factor that slows down aging, presumably, by retarding metabolism. Also, some chemicals can alter the rate of aging. In some organisms, the alteration of aging has been shown by such chemicals as resveratrol, STACs (Sirtuin Activating Compounds), sirtuins, and some other compounds. Notably, that using any drugs has never beat a simple calorie restriction in the effect on prolongation of a lifespan. (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4004650/).
Damage to large macromolecules such as proteins, lipids, and nucleic acids by different external and internal factors such as radiation or free radicals can result in aging and death. There are different mechanisms in the body for repairing such molecules, but these mechanisms function worse as the organism gets older.
As we get older, the different functional deteriorations of our tissues increase due to different reasons. In most cases, these deteriorations occur as a result of impairment of cell reproduction in an older body, which in turn are caused by different factors mentioned above.
As we see, there are many different factors of aging. All of them matter and interact with each other, determining the average biological age.
However, we can trick our biology, at least to some extent. In fact, we do it already. For example, we get vaccinated for many various severe diseases, from which millions of people would die in the recent past. Many leading experts in the field of life expectancy prolongation think of aging as of a curable decease.
Major potential strategies to significantly prolong the maximum duration of life or even achieve immortality of the human body
For example, artificial nanorobotic white blood cells were proposed. These nanodevices could travel throughout the blood system, detect, and destroy harmless bacteria or cancer cells. Other similar devices could remove atherosclerosis plaques.
But many nanotechnologists aren’t very optimistic about medical nanodevices. There are many constraints to design and manufacture such machines, even in the foreseeable future.
We can cure many devastating diseases by means of gene therapy, but these diseases are mostly monogenic (caused by one gene). Effective treatment of polygenic disorders by gene editing is still quite challenging.
Transplantation of biological or non-biological organs and entire body parts
Stem cells are the cells that can transform into any type of cells in the body. Scientets can grow various organs of the human body from them. Artificial organs have been already obtained by using stem cells. These artificial organs are not still flawless, but this is a very promising area.
At the same time, there is another option – creation of artificial organs from non-biological materials. An artificial heart, for example, has been already in use for a long time. Also, various prosthetic devices for different body parts like limbs or even teeth are very common. Such devices are getting better and becoming far more sophisticated. For instance, because of neural interfaces, people can manipulate such devices like an artificial hand in an almost natural way.
There is even an opinion that in the future people’s entire bodies will be almost totally artificial. Some believe that, in principle, we can even replace natural neurons in the brain to artificial ones.
But today we still have only one reliable method for significant lifespan prolongation and this is simply a caloric restriction. The restriction of an intake of calories can extend the duration of a lifespan to 40%, but all amount of necessary biologically active substances like vitamins and minerals should be provided.