A more developed version of this question.
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.
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.