Without oxygen- about four to six minutes, without water- approximately three days, without a space suit in space- it would only take fifteen seconds for humans to freeze to death. These answers showcase the abstract nature of the question posed and give us countless ways to interpret it.
 
The human body is a complex machinery made of fundamental building blocks called cells. These cells are the basis of our existence. The structures they possess determine our day-to-day functionality. It would not be surprising to conclude that the death of the human form is somewhat connected to the death of these cells. Leonard Hayflick, a former Professor of Anatomy at the UCSF School of Medicine, researched an interesting topic in 1961.[1] By studying both normal human cells and human cancer cells, he established that while cancer cells have a characteristic to divide in an uncontrollable fashion, the cells of a healthy body can only divide up to sixty times. With each division, the telomere, or the terminal end of the chromosome containing a portion of the DNA, damages.[2] This end of the chromosome prevents the activation of DNA damage response and prevents different chromosomes from sticking together.
 
Does this mean that cancer cells can make us live longer?
If this was really the case, then getting cancer would have been a celebratory moment for most. Let us imagine the Battle of Hogwarts in the famous fiction series, ‘Harry Potter.’ In the battle, a magical spell brought inanimate objects to life in the form of a soldier. These soldiers continued to increase in number and filled the grounds of the magical school. Comparably, cancer cells- although living- fill the human body in quite the same way and interfere with the normal functioning of our machinery.[3] This shows that both normal and cancerous cells put an upper limit on life.[4]
 
In the book, ‘How We Die?’ Nuland argues that the proof of being human lies in the inevitability of death and attributes death to killer diseases such as heart attack, cancer, and stroke. He outlines that each of them results in organ failure, dearth of oxygen and the eventual stoppage of the heart. The author also explores the change from the time of Hippocrates to now and argues that there has been almost no progress in the field of healthcare. Medical treatment might have increased life expectancy, but nothing is able to avoid death. He defines aging as a painful process damaging cells and describes it as synonymous with ensuing death. On the contrary, a recent statistical analysis conducted in 2018[5] depicted that the risk of death expectedly increases from the time we take our first breath and surprisingly flattens at a higher age. Siegfried Hekimi, a geneticist conjectures that body cells eventually reach a point where the repair mechanisms can counterbalance further degeneration to keep the mortality rate stable. 
                                                                                                                             
While we explored how long it takes for humans to die, we have not addressed how long human beings can live. One would say that the two are synonymous however, on evaluation we find the difference. We are aware that death is inevitable from the time of birth itself but what delays death till the last possible moment and how is it that people live long? If we compare our life span to that of the dogs, the mouse, or any other living species, we will realize that we have some sort of an evolutionary benefit that allows us to flourish for a longer period of time. According to the theory of natural selection, life span is the time between birth and the end of the fertility period. Taking this into account, a human female would be expected to die at the age of around forty-five which marks the menopause. However, that is not the case. Scientists propose three potential explanations for this. One argues that the genes that promote early fertility might also promote long life and since it directly affects reproductive success, natural selection would increase the presence of these genes with evolution. The second one compares the male reproductive period of roughly about seventy years of age and assumes that the genes for long life are similar in both male and female. The final alternative links post-menopausal survival to the improved quality of childhood for both the children and grandchildren of the women. If progenies benefit from the care that their mothers or grandmothers provide, then that would improve their survival rate and in turn their reproductive success. By theory, those who have higher reproductive success would produce more off springs and this method would naturally select the long survival of post-menopausal women as a healthy idea.[6]
 
Our assessment of the causes of death and the upper limit to life showed us that death can be delayed only till the time cells continue to function whereas, the evaluation of our lives allows us to understand that we can extend our life only till a point where it directly benefits our progeny and enhances the condition of our future generations. The combination of these two results helps answer the first question. The human life is analogous to a plant stem. The height of the plant is directly related to how well and for how long the roots function and is correlated with the amount of time it nurtures seeds and fruits for.   
 
Reference List

[1] “The Embryo Project Encyclopedia.” Leonard Hayflick (1928- ) | The Embryo Project Encyclopedia, embryo.asu.edu/pages/leonard-hayflick-1928.

[2] Yadav, Prem Singh, and Abubakar Muhammad Wakil. “Telomerase Structure and Function, Activity and Its Regulation with Emerging Methods of Measurement in Eukaryotes.” IntechOpen, IntechOpen, 24 Dec. 2019, www.intechopen.com/chapters/70617.

[3] “How Does Cancer Kill You?” Medical News Today, MediLexicon International, www.medicalnewstoday.com/articles/how-does-cancer-kill-you.

[4] Avi Roy PhD student. “Lust for Life: Breaking the 120-Year Barrier in Human Ageing.” The Conversation, 9 Aug. 2021, theconversation.com/lust-for-life-breaking-the-120-year-barrier-in-human-ageing-14911.

[5] Dolgin, Elie. “There's No Limit to Longevity, Says Study That Revives Human Lifespan Debate.” Nature News, Nature Publishing Group, 28 June 2018, www.nature.com/articles/d41586-018-05582-3.

[6] “Why Do Humans Live for so Long?” The University of Edinburgh, 10 Oct. 1970, www.ed.ac.uk/biology/news-events/news-2017/why-do-humans-live-for-so-long.

[7] Image Source: www.genengnews.com/topics/translational-medicine/16-biomarkers-may-predict-human-lifespan/