Human lifespan presents a fascinating paradox. While average life expectancy has increased dramatically from around 47 years in 1900 to nearly 80 years today, the maximum human lifespan has remained remarkably stable at around 100 to 120 years. This suggests there is a biological ceiling that we cannot easily surpass.
The fundamental cause of aging is the accumulation of cellular damage over time. Our cells constantly face damage from metabolic processes that produce harmful free radicals, environmental toxins, radiation, and errors during DNA replication. While our bodies have sophisticated repair mechanisms, these systems become less efficient as we age, leading to progressive deterioration.
Several specific biological mechanisms contribute to the aging process and our lifespan limit. Telomeres, the protective caps on our chromosomes, shorten with each cell division until cells can no longer divide. DNA damage accumulates over time, cellular senescence increases as cells stop functioning properly, proteins misfold and lose their function, and mitochondria become less efficient at producing energy. These interconnected processes create a cascade of declining function.
From an evolutionary perspective, our lifespan limit makes sense. Natural selection primarily favors traits that enhance survival and reproduction during the fertile years, roughly from adolescence to middle age. There has been much less evolutionary pressure to maintain perfect bodily function beyond the typical reproductive and child-rearing period. Evolution prioritizes resources for reproduction and early survival rather than indefinite maintenance, which explains why aging accelerates after reproductive years.
To summarize, human lifespan is stuck around 100 years due to fundamental biological constraints. Cellular damage accumulates faster than our repair systems can handle, multiple aging mechanisms work together to limit function, and evolution has not prioritized indefinite lifespan maintenance beyond reproductive years. While we can extend average lifespan through medicine and lifestyle, breaking through the maximum lifespan barrier requires addressing these deep biological processes.