Zeynep

The caveats are where this gets practical, because the small average advantage does not settle what anyone should use.

Ambient light matters more than polarity. A bright screen in a dark room is uncomfortable regardless of scheme, and matching the display to the surroundings does more for comfort than the choice between them.

Some people genuinely do better with light-on-dark. People with certain visual conditions, and many with light sensitivity, report substantial benefit. That is not preference, it is a real difference in requirement.

Halation. On a dark background, light text can appear to bleed at the edges, particularly for people with astigmatism. That effect is individual and it can outweigh the general finding entirely.

Which is why the sensible design position is to support both properly rather than to pick a winner from the average result.

Several do, and the best example is a snail, which is not where most people expect to find one.

Cone snails hunt with a modified tooth — a hollow, barbed, harpoon-like structure held in the proboscis. It is loaded, fired at close range into the prey, and delivers venom through the hollow shaft. It is a genuine single-use projectile that the animal manufactures internally, and a fresh one is loaded for the next attempt. Some species take fish this way, which is remarkable for a snail.

The firing mechanism is hydraulic pressure built up behind the harpoon and released suddenly, which is the general solution in animals that lack anything resembling a spring-loaded launcher.

Worth adding the standard caution that several cone snail species are genuinely dangerous and should not be handled — this is one of the few "interesting animal" answers that comes with a real safety note.

The other half of the answer is that eyes are expensive, which is why nobody adds spares.

Measurements in fish suggest that vision can consume a substantial fraction of resting metabolism — the figures are striking, and the cost is highest in small animals. Neural tissue to process the image is costly, the eye itself is costly to build and maintain, and it is a vulnerable opening in the body.

The evidence that this cost is real and continuously felt: cave animals lose their eyes, repeatedly and independently, in lineage after lineage. If eyes were cheap, they would simply be retained and unused. They are not, so they are dismantled within surprisingly few generations.

Given that, adding a third eye would need to pay for itself against an already expensive system. Two is what the body plan supplied and evolution has generally spent its budget improving those rather than adding more.

Worth adding the one line of inheritance that does not behave this way, since it can be useful.

Mitochondrial DNA passes down the direct maternal line essentially unchanged, and the Y chromosome down the direct paternal line. Those do not get shuffled, so they persist over far more generations than autosomal segments.

They only test one thread each through your tree — your mother's mother's mother, or your father's father's father — but for those specific lines they are far more informative at distance than the general test you have taken.

There is an active line of research that follows directly from your question: looking for a shadow biosphere — organisms with different biochemistry, surviving in places our incumbent life does not reach or in forms our detection methods would miss.

The methodological problem is severe and worth appreciating. Almost every technique for finding life looks for the biochemistry we know: our genetic material, our proteins, our metabolic signatures. Something built differently could be sitting in a sample and be invisible, because we are running the wrong test.

Nothing has been found. Given the detection problem, absence of evidence here is weaker evidence of absence than usual, which is why the idea remains taken seriously rather than dismissed.