How would these nocturnal humanoids evolve?

Question

I've designed a race of humanoids for a standalone worldbuilding project, but I'm beginning to wonder how they could have evolved. They are very similar to humans, but differ in the following key ways:

1. They are exclusively nocturnal, having the exact opposite circadian rythm to humans
2. Their skin and hair totally lack pigmentation (yes, they do burn very easily)
3. their eyes are totally black, allowing them to see in the dark very well, but in spite of this their colour perception is no worse than that of a human
4. Their basic dietary needs are the same as those of humans, but the vitamins and micronutrients they need are different. Namely, their bodies synthesise their own vitamin C, but they must source significant quantities of taurine from their diet
5. Their canine teeth are significantly larger than those of a human, but in all other respects their teeth are the same
6. The males are about the same size as human men, but the females are larger than human women, meaning they have less sex-related size difference
7. The males do not grow facial hair
8. The mammary glands of the females only swell into breasts when they are pregnant or breastfeeding, with the females being effectively flat-chested the rest of the time

So, how could this species evolve? There are no humans in their world, so if they evolved from humans there must be a reason that humans went extinct but they survived.

Answer 1

It won't be albino.. for lack of camouflage !

Otherwise I don't see an issue for humans to develop large, "nocturnal" eyes..

One half is Loris, https://en.wikipedia.org/wiki/Loris

Answer 2

Giant lemur!

https://alpenglowstudios.wordpress.com/2015/10/31/archaeoindris-the-giant-subfossil-lemur/

https://en.wikipedia.org/wiki/Archaeoindris

Archaeoindris fontoynontii is an extinct giant lemur and the largest primate known to have evolved on Madagascar, comparable in size to a male gorilla. It belonged to a family of extinct lemurs known as "sloth lemurs" (Palaeopropithecidae) and, because of its extremely large size, it has been compared to the ground sloths that once roamed North and South America.

Lemurs generally do not have sexual dimorphism and so that accomplishes one of your variables. They have the canines you request. They have a flat muzzle so might look more human in that respect. They are thought to have had a gorilla-like diet of leaves plus whatever else they run across.

The giant lemur was probably diurnal. There are lemurs which are nocturnal but I suspect they are to avoid predation by hawks. A primate this big would probably be preyed upon by cats in our world but you could have big diurnal aerial predators. Some humans (street people) are nocturnal because there are predators that come out at night (other people) and they do not want to be caught sleeping. Another reason to be nocturnal is if it is very hot during the day.

Answer 3

Consider their environment and why some of these adaptations occurred.

So far it sounds like these beings have not seen the surface world for quite some time, or that the sun never shines on the part of the planet they reside around.

1. This tells me that at some point, something that occurred during the day was learned to be avoided at ALL costs to the point their biological rhythm followed suit; a creature that hunted them, a cancer or effect from the sun. Or something about night time is required for them to survive that was important enough to focus on night exclusively, a prey that they hunt only comes at night, it only gets cool enough for them to function without over heating at night.

2. Does that mean they are albino and white? We see this in nature in places completely devoid of light, where visible spectrum camouflage would be useless. Without contacting any sunlight, the need for melanin in their skin would be removed and over time the resources required to produce the melanin could go toward some other body function. Diseases and other conditions can also result in their pigmentation vanishing, potentially a condition shared by all members at this point.

3. Without light, you can't see. In areas of very little light, larger lenses are required to gather that light into the eyes. A very reasonable adaptation to the dark, but it would become a weakness in the presence of bright light, which further convinces me these creatures don't even have the option to see sunlight if they wanted to for this trait to evolve.

4. They would need to survive on what ever was available, and without sunlight, most plants are off the menu. Omnivores that humans are, we can eat some fungi, most animal prey, we can supplement minerals and deficiencies from non-living sources like salt and iron which would be reachable within darkness. The methods of digestion would likely be different, so feeding a human diet to these beings could be detrimental to their health. Producing their own Vitamin C, they would want to avoid the sun, else have too much!

5. With the amount of hunting required to live underground without fungal farming (which may be a solution as some leftcutter ants are found to farm fungus) and possible infighting that requires self defense, any offensive adaptations would be preferred. Having sharper teeth (in lieu of claws) makes you sharper and more deadly to enemies. Teeth length may become a part of mating rituals too, longer teeth giving more chances to mate producing ever longer teeth.

6. Physical weakness in such a harsh, lightless world, would likely lessen your chances of survival. In the animal kingdom we see all sorts of size differences for different reasons. It may also be advantageous to be gender neutral from afar if there still exists differences in strength, females would benefit from an attacker confusing it for a male if males had some sort of advantage.

7. You can grab hair in a fight, making baldness an advantage. Their diet may matter, leaving certain fungus to expire or rot can get you sick, so those spores in your mustache after eating a bunch of mushrooms may get you sick if you fall asleep without cleaning it away. Over time, evolution granted the non-facial hair members survival advantages.

8. See number 6, advantages of being confused for a male.

The how of them evolving, depends on other factors of the environment, or if they started off as humans or not. Do they have cities? Are they wild animals? Uncertain. There certainly isn't a lot of sunlight in their daily lives though.

Perhaps there are underground jungles that are pitch black or only lit by bio-luminescence, with ecosystems not too different than that of the rainforests, though fungus would replace plants in most all cases.

If they used to be humans, something drove them underground, they ended up getting stuck underground or unable to return to the surface for a long time (nuclear winter) and over generations or intentional bio-engineering changed into the form we see now.

Answer 4

"if they evolved from humans there must be a reason that humans went extinct but they survived"

If your species is derived from humans the obvious way to explain your nocturnal evolutionary adaptions would seem to be an environment that strongly selects for nocturnal activity.

A ubiquitous diurnal apex predator might do the trick for most other species, but it's a bit hard to rationalise that for humans given all the large predators we've dealt with & wiped out in the past with our big brains and social cooperation.

So I'd probably go straight to the source, sunlight.

If your world lacks a strong enough magnetosphere then radiation will get anyone who spends substantial amounts of time in sunlight with cancers before they have much time to reproduce.

But you will probably need to ballance it carefully, if the radiation is too weak they'll be more likely develop dark pigmentation as a defence instead and if the radiation is too strong you can't have an otherwise relatively normal world so you need a sweet spot where dark skin colour isn't going to cut it and the radiation isn't strong enough to kill shorter lived (than us) wildlife before they reproduce.

Chernobyl may be a good indicator for the sort of daytime radiation levels you're looking for.

If you're OK with setting your world somewhere other than Earth (perhaps they're inhabiting a planet colonised many thousands of years ago) then this might work for your story.

The minor non nocturnal bits can easily be hand-waved with genetic drift and sexual selection in a population isolated from the rest of humanity so doesn't really need any further explanation.