Why is the liver so weirdly regenerative?
TL;DR
Most of the body heals poorly because regeneration means cell division, and cell division feeds cancer. Organs that face the environment, like the liver, skin and colon, pay a cancer tax to stay repairable. The small intestine renews fast with little cancer. It pays in energy instead, which makes the trade-off three-sided.
The article
- Shrinking telomeres are a deliberate anti-cancer brake. Cells carry the gene for telomerase but switch it off after development. A mutated lineage then runs out of divisions before it can grow large. The price is one of the clocks of aging.
- The same logic explains missing limb regrowth and fragile kidneys. A body that can restart rapid growth keeps every cell closer to cancer. The liver is the odd one out because it sits downstream of the gut and gets damaged as part of its job.
- Skin and colon also turn over constantly. Their cancers are common. Neurons and heart muscle never divide and almost never become cancer. Structured organs like the heart and brain may not have a regeneration option at all.
- The lungs combine low regeneration with high cancer. Their fractal surface, about 100 square meters in 5 liters, is hard to rebuild and breathes in toxins. The author admits the pancreas does not fit the theory.
- The small intestine breaks the rule. Protected stem cells feed a conveyor belt of cells that die at the villus tip within days, so mutations get flushed out. The price is energy: it sheds about 40 billion cells a day and uses roughly 20% of the body's energy.
- For the modern world, the author guesses the body tries too hard to recover from injuries. He calls telomeres and brains about right. He has no answer for the liver or menopause.
Why it matters
Read the original for the organ-by-organ table of regeneration against cancer risk. A footnote answers biologists who object that 90% of cancers turn telomerase back on. Several commenters also praised the author's jokey tone. Treat it as a speculative essay; commenters flagged several outdated claims.
The discussion
A busy thread that liked the writing more than the biology. Doctors, patients and amateur biologists corrected claims, argued over whether regeneration really raises cancer risk, and attacked the essay's talk of evolution 'deciding' things. Long side threads covered vitamin A toxicity, liver myths in Greek and Persian culture, and why human bodies reject transplants.
Does regeneration really cost cancer?
Some commenters took the essay's core trade-off as obvious, since cancer is just growth that has lost its controls. Others said limb regrowth never paid off for humans: losing a limb used to mean death, and regrowing one takes years. A separate camp said the hard part is complexity, not cancer, because rebuilding bones, nerves and tendons in place needs machinery mammals never evolved. One noted that the simplest animal branches can all regrow whole bodies, so bilaterians probably lost the trait.
Because in a nutshell cancer is uncontrolled proliferation while regeneration is controlled proliferation.
Humans can repair most of simple tissue-level damage well enough. The complexity equivalent of placozoan regeneration is in place. Rebuilding complex anatomy is what's often unimplemented.
Claims the essay got wrong
Readers fixed specific facts: adult neurogenesis exists, per a 2026 review, and heart muscle turns over about 1% a year in young adults. A Science blog post gives a more involved cause for the lack of heart cancer than the essay's. Type 1 diabetes mostly strikes the young; age-related diabetes is Type 2 and not autoimmune. A wound-care commenter disputed that skin over-invests in healing, since surgery itself depends on it.
This is a very outdated view. Neuroscience has shown that neurogenesis continues throughout the entire life span of humans, albeit at a much slower rate.
It’s a good trade-off: nearly everyone will eventually get cut open by accident or on purpose.
Evolution does not decide anything
The essay's framing of 'design decisions' irritated many readers. Some objected only to the wording, which they said teaches people that evolution follows a plan. A longer critique said reasoning from organ function misses cell-level constraints set two billion years ago, such as why DNA became linear. The harshest replies called the essay unscientific garbage, and a moderator answered that speculation belongs on the site even when wrong.
Evolution is a blind watchmaker. Ignore that and you’ll end up with nice sounding ideas that may not quite fit what we see.
There's nothing wrong with speculation on HN. Speculation is intimate with curiosity, and curiosity, not correctness, is the guiding value of this site.
Why the liver, and what it's like to need one
One explanation was structural: the liver works like a battery pack of interchangeable cells, so a new cell fits almost anywhere, unlike in a kidney. Others pointed to ancestors who ate spoiled food and toxic plants because food was scarce. A transplant recipient described a donated half-liver that regrew within months, but down his side instead of across. Repeat living donation does not work, he added, because the regrown liver lacks the vessel and bile duct layout surgeons need.
The liver's great ability to regenerate largely boils down to how homogenous and simple the liver actually is.
Within a few months of transplant, the new liver had regrown, but (probably because of the transplant orientation) rather than growing across to the left, it grew down the side of my body.
Menopause and transplant rejection
Commenters offered the grandmother hypothesis for menopause, and others pointed out that the essay already shows most social mammals don't have it. Alternatives included the danger to human infants, and humans raising several children at once, so a late pregnancy takes from the ones already born. On transplant rejection, a kidney transplant patient and others said the immune system can't judge cells by what they do. So it attacks anything not present before.
If your prediction is, social animal => menopause, it's wrong 99% of the time.
The only useful signal is "This wasn't here yesterday".
Consensus
The essay is fun and well written, and its regeneration-versus-cancer idea explains some things. But biology is messier than any single trade-off, and several of its facts are out of date.
Dissent
A vocal minority called the essay unscientific. They said its 'design' language and engineering lens misrepresent how evolution works, and that it shouldn't be treated as education.