Why is the liver so weirdly regenerative?
4 hours ago
- Many human body features seem poorly designed: fragile kidneys, shrinking telomeres, inability to regrow limbs, brain neuron loss, immune rejection of transplants, and buildup of cellular junk.
- The main unifying theory is that these apparent defects are evolutionary tradeoffs to reduce cancer risk: limiting cell division and regeneration makes cancer less likely.
- Telomere shortening is a deliberate cancer defense, not a mistake; it prevents mutated cells from reproducing indefinitely, though some cancers reactivate telomerase.
- The liver is unusually regenerative because it constantly encounters toxins and needs to repair itself, accepting a higher risk of liver cancer.
- The small intestine achieves both high regeneration and low cancer by using protected stem cells and a conveyor-belt system of surface cells that are quickly shed—at a high energy cost.
- Regeneration, cancer risk, and energy usage form a three-way tradeoff; organs have different balances depending on their structure and exposure to the environment.
- The pancreas is a notable exception that doesn't clearly fit the proposed evolutionary tradeoff.
- In the modern world, some tradeoffs may be misaligned: injury recovery may be over-calibrated, while telomeres and brain fragility may still be roughly balanced.
- Even if cancer never develops, its evolutionary pressure still harms us by forcing the body to maintain costly cancer-suppressing mechanisms.