Venki Ramakrishnan asking and explaining the question—Can humans live forever?
The diversity of lifespans in nature suggests that aging isn’t fixed. Despite being constructed from the same chemical building blocks, species can have dramatically different lengths of life. Mayflies may live only days, mice about two years, and bowhead whales and Greenland sharks for centuries. Galapagos tortoises live so long that one of them hobbling around today could have encountered Charles Darwin in 1835.
These differences are not random. Lifespan is closely related to metabolism. Smaller animals generally have higher metabolic rates and shorter lives; larger animals tend to metabolize energy more slowly and live longer.
Natural selection doesn’t care how long an organism lives. It favors traits that increase reproductive success. Every species makes trade-offs in how it allocates resources among growth, reproduction and maintenance. A mouse doesn’t improve its ability to produce more offspring by investing in longevity if starvation or predators are likely to kill it first. For a mouse, investing in rapid growth and reproduction is more advantageous. But larger animals, such as whales, mature slowly and reproduce later. Their bigger size also reduces the risk of predation, making long-term maintenance more worthwhile.
The same logic explains some notable exceptions. Birds and bats often live much longer than similarly sized terrestrial mammals because flight reduces the risk of predation and also allows them to forage over a wide area, cutting the chances of starvation. When surviving longer increases reproductive success, evolution favors slower aging.
Evolution can also reward genes that are beneficial early in life even when they become harmful later. For example, the same genes that promote rapid growth or prevent cancer during youth can contribute to aging later on. This evolutionary theory of aging is known as antagonistic pleiotropy. “Pleiotropy” simply means a gene can have more than one effect, and “antagonistic” means some of those effects can be harmful. The theory suggests that aging is not an adaptation in itself but an unintended consequence of evolutionary compromises. The upshot is that each species has a maximum natural lifespan.
This makes sense. If natural selection favored longevity instead of reproductive success then evolution will stop.




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