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Premise

Inhabiting my earth-like world will be humanoid beings that have a skin tone that is as dark as Vantablack. Let's assume these human-like beings share the same ancestry as humans and we can safely assume cognition and physique to be identical to humans. The only difference being their skin is naturally as dark as Vantablack (well techinically, this may require some inner workings, like bio-mechanics, to be different as well). While Vantablack is not actually a color but rather a material, it's still the closest thing known to man to the effect I want to create in my world, and so as not to use the word inappropriately, I will just coin my own word for these fantasy humans: Vantaman. While the darkest known pigment of us humans has not been scientifically documented, I did want to provide a frame of reference. From a crude¹ series of image analyses I got the hex color: 1f1b1a for very dark-skinned homo sapiens. Hex colors, however, can't do Vantablack justice. Vantablack is often described as the closest thing to looking into a blackhole as we can get. It absorbs 99.97% of light. Here is a Vantablack material demonstration, both busts are identical in shape:

Actual human skin pigment evolutionary narrative

To help us tackle the task of how Vantaman came to be, it may be helpful to understand how actual humans took on the adaptation of dark skin in the first place. According to the understanding of human evolution at the time of this post, humans began to develop dark skin pigments through natural selection about 1 million years ago. The context for this adaptation was these hominids were beginning to move into environments that offered little to no protection from exposure to the sun -- the Savannah. It was at this point that the hominids began to have less and less body hair and began to develop perspiration as a means of cooling down. And it was at this time that humans began to take on the trait of having dark skin. The darker skin helped protect the bio-mechanics needed for healthy hominids. With only vestigial body hair, the hominids with lighter skin could not provide sufficient protection for embryogensis and so the melanin-dense darker skin was naturally selected to deal with harmful UV radiation. (it's thought that they would conceivably face DNA degradation as well). Of course as later descendants left this environment and ventured to, say, Europe, where humans embraced a more troglodytic lifestyle, then skin pigments became lighter to allow our livers to synthesize vitamin D more easily. And the rest is history.

Question

With all that in mind, it's all the more challenging to conceive of an evolutionary narrative to explain a vantablack skin pigment trait. Why would Vantaman need to absorb 99.97% of light?

My first inclination was to keep the narrative and up the ante; Vantablack pigment to cope with a sun that emits far more radiation than our sun. I lacked the knowledge of astrophysics to gauge whether such a world would still be earth-like my hunch was no. However maybe someone more knowledgeable can come up with some special case scenarios and revive this solution.

However the question is not only concerned with stellar emissivity -- anything rooted in science is fair game. To help maintain a reasonable scope, here is a quality metric:

Quality Metric: The closer to earth-like your evolutionary narrative for Vantaman is the better². If it turns out there is no earth-like solution in the truest sense of the word, then I can still accept an answer as long as it's scientifically plausible. Earth-like is ideal though; I don't want to go nuts with mother nature unless it's really, really necessary.

Further Clarifications:

• Even though we have not seen Vantablack in biology, we are assuming its possible; how the heck it works is out of scope. It could be nature's version of vertical carbon nanotubes or something else. Whatever it is, it is as dark as vantablack. Point being, it's not a xenobiology question.


• Assume the vantablack pigment trait requires negligible energy resources


• In this question, "earth-like" is not used in the most rigid sense. Flora, fauna and celestial bodies are earth-like in terms of initial conditions, but you may change them as per your answer's needs. Bear in mind the quality metric favors fewer or smaller changes.


• The orifices of Vantaman: mouth, eyes, ect, do not have to be vantablack.


• The evolutionary narrative is concerned only with prehistoric-times, we don't have to worry about high technology -- humans won't invent nano-tech for a million years


• Unless it's vital to your narrative, you don't have to provide an exhaustive nutrition plan for Vantaman, you can assume nature allows him to synthesize nutrients like vitamin D some other way. This assumption is optional, because it's related to the main problem, but I don't want to impose undue work for the answerer


• Ideally, we provide a narrative for Vantaman that revolves around blocking UV rays. However, if it this is a dead end, you may propose an alternate evolutionary need for vantablack skin.

_{1. crude because images not robust to different lighting or other external factors, but I did my best and averaged the hex value}

_{2. I think this is a reasonable quality metric; we can quickly tell what's plausible on Earth, but we are on the honor system to an extent. For example, who is to say what's more outlandish: UFO's beaming lasers at Vantaman to be dark or having billions of tiny blackholes continuously forming and decaying to create Vantaman's skin. If it comes down to this, we'll just have to use another metric: what is there documented evidence of?}

Simple:

Vantamen live on a vantaplain surrounded by vantagrass. Dark, nutritious volcanic soil sprouts grasses coated in carbon nanoforests evolved to suck up as much light as possible using [insert plausibly quantum term here]. The vanta colouration is simple defensive camouflage against predators that are exceptionally good at picking out non-vanta prey. They might even use the vantagrass somehow to gain or maintain their vanta colouration.

This entire area of the planet is incredibly hard to navigate as pretty much everything looks exactly the same shade of uniform black, hence why the vantamen exploit this particular niche.

Their art is incredibly dull.

Vantamen didn't evolve to block 99.97% of light, they evolved to absorb 99.97% of light.

Vantamen developed their dark skin tone in an arctic tundra, where you need to get the most out of your energy sources. As you said, the pigment to produce vantablack requires negligible resources, so the extra gain in solar radiation absorption makes up for the energy to produce the pigment.

So, much like the mighty polar bear, your Vantamen evolved the darkest skin they could to gather the most heat they can from the sun:

Why would Vantaman need to block 99.97% of light?

Vantamen evolved in a completely dark environment, much like the ocean floor. Predators hunt by using bioluminescence to illuminate and locate prey. By being completely dark, vantamen stay camouflaged against the dark background even if you shine a light on them.

Sure, they will cast a shadow, but they will still be harder to spot than a creature of any other color.

Being practically invisible to them is no problem because they find each other and communicate through non-visual means.

The only reason to have that kind of absorption is stealth.

From the perspective of absorbing something of value, the difference between 99% and 100% is rather negligible. If you are that strained for resources, you're probably already doomed.

From the perspective of not letting something in (such as UV rays), there's little advantage for pushing for such high absorption when reflection and scattering are more than efficient. It's much easier to layer up the biological equivalent of zinc paste, or even mud (as many animals do) than to try to come up with such a perfectly absorptive layer.

Vantablack doesn't emit anything, so the only thing that's left would be the cases where you specifically desire to not provide any emissions. It's a sort of "stealth mode" If you are capable of "looming," which is moving towards a prey without changing aspect angle, you appear to grow in size and it's hard to judge distance (dragonflies abuse this). THis would be even more effective if there was no useful information found in your image.

Of course, this would also generate a mighty sharp silhouette, so it would be bad during the day.

So I say you're looking for a predatory species that hunts at night. The dominant prey animal on the planet has developed an astonishing telescope-grade nightsight capability, which lets it see preadators coming a mile away as they graze. Vantablack may be just what the predator needs to be truly invisible when striking the prey.

You're probably looking for a really black owl.

Even though we have not seen Vantablack in biology, we are assuming
its possible

Actually, Vantablack is biologically possible, quite easily in fact: https://www.nature.com/articles/s41467-017-02088-w

Which I bring up because you could use all of the evolutionary mechanisms that have been proposed for hyper-black birds. Sexual selection being the lead choice.

1. Camouflage for night hunting, caves. Even more so with no moon or moon has different composition and size to give less illumination at night.



2. 
   

   Insane assassin/duelist What is even more scary about such being is you can not even tell it's shape, features or perceive it's attacks. They could use their remains to cover tools or paint them black.

   
   
   

   Just imagine you need to fight someone like that in a duel or war. You won't even see how he made a trust.

   
   


3. 
   

   Fear incarnate. It would scare shit out of other animals and species of humans. If you use tools and stand in group, no one will be able to tell your real size or count. You can pretend to be bigger or smaller, conceal real numbers.

   
   
   

   Other predators will consider you tot risky and uncertain.

   
   

For sure there is a problem to make your equipment close to your skin for daytime. Not a big problem in low-light situations.