Yes, there is a limit to the lifespan of a human. The reason is that after a human has procreated and raised children, and helped taken care of grandchildren a bit, the human is no longer affecting the outcome of its genes. The human is then no longer taking part of evolution, so there is no selection pressure to keep the human alive any longer. All cells and functions in the body slowly deteriorates and stop functioning until the system fails, and the human dies, unless it die from a disease.
There are many different mechanisms that fail. Cells stop dividing and become senescent zombie cells, DNA errors increase with time, mitochondria becomes less efficient, brain cells dies, hormone levels drop making muscles weak and bone brittle, teeth wear out. Basically every subsystem goes bad.
There might even be an evolutionary reason to not keep us alive too long. If we live too long in the wild, we would compete with our children and their children for resources, without contributing much. If that happened, there would be an evolutionary pressure to limit the lifespan.
The natural life span of a human is individual, but it’s estimated that 120 years is close to the limit.
Below is Jeanne Calment turning 120, but lived to 122 years and 164 days, the oldest recorded human in history.
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