Ela

I would push back gently on the evolutionary framing though, because it proves less than it appears to.

Humans have functioning night vision and spent plenty of time in low light. The visual system handles both, and the specific advantage above comes from pupil optics in the moment, not from an ancestral commitment to daylight. "We evolved under the sun" is doing rhetorical work rather than explanatory work — the pupil argument stands on its own and does not need it.

Also worth noting that reading itself is evolutionarily brand new. Nothing about our visual system was shaped by text of any polarity, so appeals to what our eyes were built for run out very quickly once the question is about interfaces.

The error is in the second option and it is the most common misconception about evolution: it does not optimise for health or longevity. It optimises for reproduction.

The damage you are describing accumulates over decades and its most serious consequences appear predominantly later in life — largely after the years in which our ancestors were having and raising children. A trait that harms you at sixty exerts very weak selective pressure, because your genes have already been passed on. Selection is close to blind to anything happening after reproduction is finished.

That single point resolves most of the puzzle. It is the same reason many age-related conditions exist at all: not because they are harmless, but because they are invisible to the process that shapes us.

The second half is that we do have protection, and it is quite good — you are simply not counting it.

Melanin is a highly effective adaptation. Populations with long ancestral histories in high-sun regions have deeply pigmented skin that substantially reduces damage. That is exactly the defence you were expecting to find.

What has changed is not the biology but the situation:

• People moved. Skin pigmentation is adapted to a specific latitude and light environment, and modern populations frequently live nowhere near where their ancestors did.
• Behaviour changed. Long indoor periods followed by intense intermittent exposure — a week of holiday sun — is a pattern the system never encountered. Gradual continuous exposure builds tolerance; intermittent burning does not.
• Lifespans doubled. The damage now has decades to express itself in people who are still alive to experience it.

So the honest summary is a well-adapted system in an environment it was not adapted to.

A few more, spanning very different mechanisms:

Cnidarians — jellyfish, anemones, corals — fire stinging capsules. Each is a coiled, barbed thread under enormous internal pressure that everts explosively when triggered. Among the fastest accelerations measured in biology, and there are thousands of them per square millimetre of tentacle. This is the most widespread projectile system on Earth by a very large margin.

Some sea cucumbers eject sticky tubules that entangle a predator. Structural material, launched, single use.

Certain fungi launch spores ballistically with strikingly precise aim, using surface tension released suddenly. Not animals, but the same category of solution.

The pattern across all of these is that biology has no good rotary or spring mechanism at small scale, so it reaches for stored pressure released suddenly almost every time.

The second factor is severe survivorship bias in which cases you hear about.

Introductions of small numbers of animals happen constantly — deliberate releases, escapes, stowaways — and the overwhelming majority fail. They fail quietly, nobody writes about them, and there is no population left to study.

The cases that become famous are precisely the ones that succeeded, and they succeeded for a combination of reasons that include luck. Reasoning from them to "founder effects are not a problem" is like reasoning about business risk from the companies that are still trading.

The conditions that let one succeed are usually: abundant food, no predators, no competitors, a climate that suits them, and enough founders to avoid the very worst of the bottleneck. That is a demanding list and most introductions do not get it.

The answer to the number question is much less about optics than you would expect: it is inherited body plan, not optimisation.

Most animals you are thinking of are bilaterally symmetrical — a left side and a right side, an ancestral arrangement shared by an enormous group of animals. Paired structures are the default in that body plan: two eyes, two nostrils, two ears, limbs in pairs. Two eyes is not a solution somebody converged on; it is what you get when the ancestor had a pair and every descendant inherited the arrangement.

The evidence for this is that animals outside that body plan do it differently. Spiders have eight, many insects have compound eyes plus simple ones, scallops have dozens along the mantle edge, box jellyfish have clusters of them. Where the constraint is absent, the number varies enormously.

So the question "why not four" has the slightly deflating answer: because our ancestors had two, and there has been no pressure strong enough to justify rebuilding the head.

The underlying reason is worth understanding because it is counter-intuitive: you do not inherit DNA from most of your ancestors.

Go back ten generations and you have over a thousand ancestral slots. Your genome is broken into a limited number of inherited segments, and with each generation those segments are fewer and larger. Beyond roughly eight or nine generations, the number of distinct segments you carry is smaller than the number of ancestors you have.

The conclusion is unavoidable: many of your documented ancestors from that era contributed nothing to your genome. They are genuinely your ancestors, genealogically, and genetically they are absent.

That surprises people badly the first time they meet it. Genealogical descent and genetic descent are different relationships, and they diverge quickly.

One piece of evidence worth knowing, because it is the strongest thing we have: the shared arbitrary choices.

All known life uses the same handedness of amino acids and the same handedness of sugars, from two chemically equivalent options. All known life uses substantially the same code mapping genetic triplets to amino acids, out of an astronomical number of possible mappings.

Those are arbitrary. There is no strong chemical reason to prefer one handedness, and the code's specific assignments are not forced. Universal sharing of arbitrary choices is exactly the signature of common descent rather than of convergence — the same argument that tells you two documents were copied from one source rather than written independently.

So whatever happened at the origin, everything alive now is one family. That does not rule out other origins having occurred and lost.

It is also worth saying that your model is not wrong — it is describing what happens when the containment fails, and those cases exist.

Some infections do produce visible destruction: blistering lesions where cells are killed in a patch, and severe infections where large areas of tissue are damaged. Severe respiratory illness involves exactly the process you describe, at scale, in tissue that cannot be replaced fast enough.

And there is an important twist: a great deal of the damage in severe infections is caused by the immune response rather than the virus. Inflammation is destructive by design, and when it runs hard in delicate tissue, the collateral damage can exceed what the pathogen would have done alone.

So the reason ordinary infections do not leave holes is that containment usually works, and the visible cases are the ones where it did not.