Rowan

There is real science here and it mostly favours your colleague, with important caveats.

The mechanism is the pupil. In bright conditions the pupil constricts, and a smaller aperture gives greater depth of field and reduces the effect of optical imperfections in the eye — much like stopping down a camera lens. A light background constricts the pupil; a dark one opens it, which lets in more aberration and softens the image slightly.

This shows up in measurements. Studies of what is called polarity generally find a small advantage for dark-on-light in reading speed, in proofreading accuracy, and in the ability to resolve fine detail. The effect is consistent enough to be considered established, and it is also small.

So it is not a rationalisation. It is a genuine optical consequence of how the eye adjusts to light level.

There is a category adjacent to yours that is worth mentioning because it is arguably more impressive: animals that launch something they have made but do not fire it from their body.

Net-casting spiders build a small, elastic web held between their front legs and throw it over prey. Archer fish shoot water, which is liquid so it fails your criterion, but the aiming — including compensation for refraction at the surface — is extraordinary. Some ant species fling material.

Where you draw the line between a projectile and a tool is genuinely blurry here, and the answer changes depending on whether "produced by the body" or "launched by the body" is the criterion.

One thing worth adding on depth perception, since it is the answer you started with: it is not the main reason most animals have the eye placement they do.

Compare a predator with forward-facing eyes and a large overlapping field, to a prey animal with eyes on the sides of the head and nearly panoramic coverage but little overlap. Same number of eyes, opposite priorities — depth in front versus detecting movement anywhere.

So the pair is inherited, and what varies is where they point. That variation is where the actual optimisation shows up.

Worth adding what the question would look like if answered elsewhere: this is one of the main scientific motivations for looking for life on other bodies.

A second, independent origin found anywhere — even something very simple — would immediately settle whether life's chemistry is forced or accidental, because we could compare. One sample is a mystery; two is a science.

Which is why the interesting question about a subsurface ocean is not "is anything alive there" so much as "if so, is it related to us".