What protein could humans lose to become more carnivorous?

Question

A more developed version of this question.

The Premise

As in the original question, 100k-200k years ago a population of homo sapiens ended up on an alternative version of Earth through a rift that closed afterwards, leaving the population stranded.

This alternate Earth has much the same gravity, solar system, geography and climate etc., with only few minor changes such as an extra volcano or two.

And so it stayed that way for the next 100k-200k odd years, two worlds once connected but now separate. That is until in the present day, the Rift opens again allowing our distant cousins to greet us face to face.

For most part they look like us, omnivorous, high forehead, distinct chin, same intelligence, a variety of ethnicities, the works. But the closer we looked, more differences were spotted.

While miniscule the eyes of these humans are slightly larger with an occipital bump to process the information. The incisors and piercing molars are larger and pointier, more reminiscent of fangs from vampire of our folklore though in comparison modest to some of the more outlandish depictions of vampiric dental design.

The most defining feature from us was the greater feats of physical ability compared to humans from out Earth, possessing greater strength and a denser skeleton, extra nerve endings on limbs, increased mitochondria density and metabolism to keep up with demand.

When asked why this was the case, we learned that the humans that managed to make it to the present day from this other Earth are result of two distinct populations that originated from the founder population 100k-200k.

These two populations were closely related to each other, though differed in one key aspect. Where the first population was for all intents and purposes to what we would call a homo sapiens 100k-200k years ago, the second population lost the ability too synthesis a protein and/or fatty acid that is vital to there development.

This protein and/or fatty acid has analogues in other animals that are close enough to be used by the body, resulting in the second population becoming more carnivorous. The issue is that while other animals had this protein and/or fatty acids are close enough to be used, it is nowhere close enough to what the first population can produce.

And so the second population became ghouls, vampires, succubae and incubi to the first population. Stronger and tougher than even there descendants, these humans ate animals when they there where no other humans to hunt and when available, stalk other populations before either over powering them, seducing them, or running them down to be eaten. And when times where lean, enter a state of torpor to lower there overworked metabolisms and wait for better conditions.

This was the state of affairs before a volcanic winter set in, killing many a forcing these tow populations to intermingle more than before. While predator and prey, were social enough to get along, so humans being humans (along with a few more 'adventurous' souls) had children with other members of the opposite populations. While comparatively weaker than there monstrous mothers or father, they did retain the ability to enter torpor and could overpower members of the first population easily.

What allowed them to succeed in this cold environment was that due to there comparatively lower strength had a lower metabolic rate, meaning they could consume less calories than there vampiric forbearers, making them less prone to starving as easily while regaining the ability to synthesis the lost protein and/or fatty acid form there more 'normal' parentage of the first population.

Eventually, this hybrid population outcompeted the each parent population and populated the alternative Earth that before crossing the Rift once more to meet us in the present day.

The Meat of the Question

My question is quit simply:

What is this protein and/or fatty acid that these hominids lost?

Things it requires:

• Have analogues in other animals, though is present/'higher quality' in other hominids.
• Be vital to some part of the development of these humans growing up.
• Can neurological or physiological.

Notes#

Inspired heavily form the conclusions drawn from the previous post, the vampires from Brightfield, Neanderthal evolution and every project/question that goes into depth of vampire physiology since I realized that the issue I originally had with the humans from the previous post are also confronted in vampire physiology.

i.e. standard human frame that somehow has increased physical prowess compared to us.

Answer 1

Taking a cue from Poul Anderson's The Sharing of Flesh: they don't lose the ability to produce anything basic, but instead lose the ability to produce human-specific hormones related to growth and maturation.

In Anderson's story, this is a problem for the entire population of a certain isolated space colony. "Fortunately", the missing hormones are only required to trigger male puberty, so they resolve it through a combination of slavery and intertribal warfare to acquire specific organs of adult males which can divided up to feed several boys per sacrificed man as a rite of passage into manhood.

In your case, the people with this deficiency aren't the only population, so they don't have to fight among themselves, and can specialize into a new species preying on the other humans. If you want them to be more regularly carnivorous, just decide that the particular hormones they are missing is something that's actually needed long term: e.g., something like Human Growth Hormone.

Alternatively, or additionally, you could take a cue from cats and make them incapable of synthesizing taurine, an amino acid which is not available in sufficient quantities from plant-based protein sources, and is what makes cats obligate carnivores. That would not require them to specifically hunt other humans, though.

Answer 2

Don't change Humans (right away), change the native organisms on the other world

As user104995 pointed out in his answer, humans can use pretty much any proteins to make any other proteins, as long as we can source all of the amino acids from something, we are fine. But every amino acid used in human biology is pretty common here on Earth; so, it's hard to run into a diet where we miss out on any of them in a significant way... but they don't have to be common on an alternate Earth.

Instead make it such that Earth 2 has life that diverged very early on from Earth 1 based life so that everything there produces and uses way less of one of the 9 amino acids that Earth based humans need, but can not produce ourselves (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, or valine).

When humans first crossed over, there was plenty of edible stuff on Earth 2; so, they ate through most of the rations they brought from Earth 1 expecting to be able to cultivate/hunt for what they need from Earth 2. The malnutrition issues were not apparent at first, but luckily one of the immigrants brought plenty one specific kind of edible seed (like peanuts) to snack on; so, the immigrants planted this specific crop to fulfill their dietary needs.

Unfortunately, this did not solve the starvation issue for ALL humans, because some humans were allergic or intolerant to this singular Earth-1 crop, and those humans were left to either starve, or cannibalize for their missing amino acids; so, it was not a mutation that happened on Earth 2 that was the problem, but rather an allergy to the one and only food source that we brough over with us that could stave off the malnutrition issues.

Answer 3

Humans require 8 essential amino acids, humans acquire amino acids by splitting protein molecules into smaller parts.

The amino acids we require to survive are so easily avaiable and common that most humans can survive for years or decades even on the poorest and most strict diets possible, people may even survive eating grass, literal green grass gathered from the floor, it may give you diarrhea, but it still contains enough protein to keep you alive.

Apparently this also applies to most herviborous mammals, that's why herbivourous animals can digest meat too, they just lack the claws and murderous instincts to kill prey.

also the protein requirement for humans goes from 0.65 grams of protein x 1 kilogram of bodyweight for the average athletic teen to around 2 grams x 1 kilogram of bodyweight for a very tall adult bodybuilder on steroids. So a 150kg (330lbs) world record giant bodybuilder with 10 times the normal blood testosterone still doesn't need more than 300 grams of protein per day and as of 2023, there isn't any proof that eating excess protein has any beneficial effect.

so the only way to make humans require more meat is to make us less effective at using proteins.

but you could also make eating meat more preferable by changing how the human gut functions.

as of now, after million years of evolution, primates tend to get butt-cancer from eating meat due to how our bacterial flora handles carnitine and carnosine when combined with choline.

carnitine, carnosine and choline alone can have good beneficial effects when introduced in the body alone, especially on brain development, focus and attention. (that's why they put them in almost every energy drink or stimulant)

but when mixed together, you get cancer, especially cancers related to digeston and metabolism of foods, and in nature carnosine, carnitine and choline almost never happen to come separated, they come already packaged together.

Edit

To give you some stats, even for the most carnivorous human societies Germanics and Anglosaxons so northern Europe and America, meat only makes a small part of their overall diet, yet buttcancer is very common, 1 out of 8 men to 1 out of 7 men in northern europe or america will get butt cancer at least once in their life. And these cultures are only PARTIALLY CARNIVOROUS.

Make your Fantasy humans fully carnivours and you'll get a species of super antentive, super smart, super focused and very tall humans that die of buttcancer at 24 years of age lol

**correction **

the most carnivorous human population are Mongolian nomads who's diet consists entirely of meat and dairy. plants make up such a small minority of their diet many goes years without eating any.

from google:

Mongolia has the highest rate of cancer deaths for men and women combined at 175.9 people per 100,000. The cumulative risk of incidence for all cancers is 27.7% and 20.8% in men and women respectively

Answer 4

So there's probably three steps to the process going from a purely biological and cultural stance.

Step 1: Population 2 loses/heavily reduces ability to synthesise the amino acid taurine, making them (almost) obligate carnivores - the same way cats are. The 100-200k years that follow would inevitably lead to natural selection of predatory adaptations resulting in a gut specialised in digesting meat, meat adapted teeth etc

Step 2: Population 1 heavily increases taurine synthesisation. Humans can already synthesise taurine, and to make these humans primary prey for a taurine-hungry community they'd need to be stuffed with it. A gene duplication event or something could easily cause an increase in taurine production. It might be worth putting in other 'prey' adaptations over time as the two populations would naturally adapt to each other, 200,000 years is a really long time (pop 1 is nocturnal while pop 2 is diurnal, pop 1 primarily lives in caves/burrows to hide, pop 1 gets slightly smaller to account for hiding behaviour etc, also pop 1 wouldn't need to consume meat nearly as much so could easily end up with a more herbivorous diet)

Step 3: Cultural changes - humans are intelligent, and while other meat may have sufficient amounts of taurine to supply a community, cannibalism is present in the real world for ritualistic or religious purposes. If this is something done repeatedly it's going to be a learned behaviour, so perhaps Population 2 will limit the amount of humans hunted due to the practice of hunting them being seen as important for some reason (eg rites of passage, festivals, other religious meanings). There's an issue with assuming that Population 2 would decimate a prey population and then hibernate until better conditions arose, as unless they were purely hunting rabbits or other fast-breeding species they'd die way before the prey population could bounce back. For me the more realistic option is that Population 2 stayed very small so there couldn't be excessive overhunting, and hunting other humans (which have a very slow reproductive time frame) was saved for 'special occassions'.

When the two populations eventually back-crossed to make hybrid offspring, those offspring would essentially have regular human levels of taurine synthesis (aka not obligate carnivores but not vegetarians either) or maybe sub-human levels to favour a more primarily meat-based diet, but maintain adaptations from whichever parent you want (it's fantasy, we can ignore the likelihood that they'd end up with sterile hybrids from such a long time undergoing speciation). Then the human hunting cultural practices could still be maintained as well, if Population 2 ends up being the more culturally dominant group in the arrangement. This doesn't really account for 'vampirism' or really specialist stuff, but with a bit of suspension of disbelief it would give a fairly solid reasoning for superhumans.