The Hayflick Limit: A Biological Theory Explaining Human Lifespan states why the human body cannot live beyond 125 years. While ancient texts from various cultures have long taught us that humans are not immortal, our ancestors reportedly lived for hundreds of years.
We are human, and we will age; however, our cells do not. Humanity has turned to religions, cultures, and various historical sources to understand why we age. At that time, science did not truly grasp the concept, thus failing to provide reasons behind the natural aging process. This changed in 1961 when a biologist – Leonard Hayflick – made a discovery that forever altered the field of medicine.
Leonard Hayflick working in the laboratory. (Source: Irish Time)
Who is Leonard Hayflick?
Leonard Hayflick was born in Philadelphia, Pennsylvania, on May 20, 1928. His parents, Edna Hayflick and Nathan Hayflick, both worked in the healthcare field, a factor that fueled Hayflick’s passion for science and biomedicine. What truly inspired him to become a scientist was a chemistry set gifted to him by his uncle on his ninth birthday.
As a teenager, Hayflick’s parents built him a small biology and chemistry laboratory in their basement. When he started attending John Bartram High School in Philadelphia, his chemistry knowledge was so profound that he could correct his chemistry teacher’s mistakes.
Hayflick is believed to have begun his studies at the University of Pennsylvania in 1946, but he postponed his education to serve in the military. Upon returning in 1948, he resumed his studies. After graduating in 1951, he was hired as a research assistant in bacteriology. Despite a good job, he loved the environment at the University of Pennsylvania, which is why he returned to obtain his master’s degree. Right after graduation, he earned a PhD scholarship in the medical microbiology and chemistry program, receiving his doctorate in 1956.
Leonard Hayflick in the 1960s. (Source: University of Pennsylvania Archives)
An Accidental Discovery!
At the Wistar Institute in 1958, Hayflick began researching whether viruses could cause cancer in humans. This led him to extract suspected cancer-causing viruses and introduce them into healthy human cells in search of confirmation. To avoid skewing the results, he had to use multiple samples, which meant developing more cells. Working through the process of cell culture, Hayflick observed something unusual: a group of older cells stopped dividing, and he could not understand why.
The cells did not die as they continued to metabolize, but they would not divide anymore. Upon examining other cultured cells, he found that most would cease dividing after about 50 population doublings.
Illustration of cell division with shortening telomeres. (Image: CleanPNG)
According to previous understanding, all of our cells continuously divide, a process that cannot be halted. However, through this experiment, Hayflick discovered that after each division, the telomeres found at the ends of each chromosome would gradually shorten, and when they reached a limit, cells would cease dividing.
Until that point, scientists believed that the natural aging process was linked to the origin of life, a concept we still struggle to understand or comprehend today. Upon discovering this about the cells, Hayflick stopped researching cancer cells and focused on what is now known as gerontology (the study of aging).
Over two years of research, he found that cellular aging correlates with the age of the human body, which is why we can only live to around 125 years. His paper published in 1961 was titled “Continuous Cultivation of Human Diploid Cell Strains.” In another study, he examined cells collected from different parts of the body and compared cells gathered from adults and fetuses.
Results showed that cells would divide around 40 to a maximum of 60 times before stopping. Once they stopped, they would degenerate and die. The same applies to humans as they age, and this is the reason for natural death. The body degenerates, and thus over time, we will die. This theory is described in detail in his paper, where he mentions that the length of the telomeres presented in different cells may take varying amounts of time to shorten to the point of cell division cessation.
Scientific Implications of the Discovery
Some cells only divide 40 times before they stop due to telomere length, which also demonstrates that each DNA will possess unique characteristics. This means that the reason some individuals age faster than others is all due to genetics. When correlating with a person’s age, when cells divide for the 60th time, it signifies that the individual is about 125 years old, and thus, if their genes contain longer telomeres, they might theoretically have a higher lifespan.
A cell can complete mitosis, or cell doubling and division, only from forty to sixty times before undergoing apoptosis and subsequently dying. Since our bodies are made up of these cells, this explains why death due to old age is a given. Moreover, the paper indicates that with each doubling and cell division, the cells themselves become more fragile, weaker, and less efficient during mitosis.
Above, we can see the research conducted by Hayflick in 1961, where he attempted to determine how many times a cell could double and divide during cell culture. After completing the 50th mitosis, the cells would begin the apoptosis process and gradually die.
This is a perfect representation of human aging. Over time, as we grow older, our bodies weaken; all senses, such as sight and hearing, deteriorate, and most importantly, the healing process slows down because cells take longer to regenerate. Over time, everything becomes slower and more challenging.
