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How different would a human-like creature be in order to need only half the calorie consumption that we need?

Constraints:

  • The species must be capable of surviving in an Earth-like planet.
  • The differences between this species and a human should be a possible result of evolution, rather than handwaving.
  • The actual environment they'd live in on the planet is unspecified, but they should be able to travel (even if it's costly or unpleasant).
  • Extended periods of sleep are off the table. Members of this species should be able to stay awake as long as a human can, although the actual sleep habits can differ.

The following similarities with an average human must be maintained:

  • General structure (similar enough to be mistaken for a human when fully clothed)
  • Physical strength
  • Intellect

Making those creatures lighter and/or shorter is also on the table as long as it doesn't get in the way of any of the constraints. Any other possible change is on the table.

Edit: Despite the difference in calorie intake, they should be as healthy and as strong as a human being who eats normally.

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    $\begingroup$ I feel like a "normal" 1m80 man can easily eat double of what a "normal" 1m60 woman eats. Could you give a more precise example of what your "standard human" should be able to do and look like ? $\endgroup$ – Echox Sep 19 at 15:40
  • $\begingroup$ @Echox that's tricky. All the essentials I could think of I already put in the question. If it's not there, I guess they don't need it or it doesn't matter. $\endgroup$ – Nightingale Sep 19 at 15:47
  • $\begingroup$ @Echox as for your example, I guess the same differences should apply. A tall male would need more food than a short female, and the difference between those and humans should remain consistent $\endgroup$ – Nightingale Sep 19 at 15:56
  • $\begingroup$ Would you be fine with a realistic offshot of homo genus, derived from homo floresiensis (almost called homo hobbitus) but who evolved a bigger brain? Evolution forces organism to be roughly energy efficient, so there is no realistic way of having all advantages of our specie but for half of the (energy) price. en.wikipedia.org/wiki/Homo_floresiensis $\endgroup$ – Shadow1024 Sep 19 at 18:52
  • $\begingroup$ Well, technically speaking you may be offered Pygmies - they would more or less need half whites calories intake. Complaining that they are not human-like enough by your standards, presumably may be considered as racism. :D $\endgroup$ – Shadow1024 Sep 19 at 19:16
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Very Different

This is a bit of a frame challenge, working on humans alone won't help. If you draw a box around a human and say 'I want the same output with half the input', that is impossible. It will not happen because you can't do that. But if you play with the externals, we can reduce the number of calories necessary. So let's start going through things humans do, and figure out what you need to do to make it lower.

Eliminate Digestion - This can take anywhere from 10 to 20% of your daily calorie intake. Your body needs to be able to break down the complex food you eat. So we just removed it. On this world, plants have evolved to create mana-fruits that contain everything a human needs. All your humans need to do is eat the mana-fruit, and that's that. So we get to removed their digestion system.

Weaker gravity - A lot of calories go to your muscle mass. Your heart needs to be able to take blood from all parts of your body and pump it to all parts. It's easier if it doesn't have to fight gravity so much. Same with muscles - it's easy to have equivalent mass strength to a human if the mass doesn't weigh as much. Less muscle mass, less calories.

Make 'em Mesotherms - A kind of cross between warm and cold blooded. These humans have somehow reverse back to being mostly cold-blooded with the ability to generate a regulate some heat. That way they can draw the heat from the planet's stable 95 degree climate (by the way, the planet has no axis and humans live near the equator for this to work), and don't need to burn all those calories for body heat.

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  • $\begingroup$ Making them mesotherms seems like a great idea $\endgroup$ – Nightingale Sep 19 at 21:46
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    $\begingroup$ Yes. Until the temperature drops in which case they'd probably need full body protection in as little as 50F, and kind of cease functioning once it got below zero. This also sounds weird, but they would have to be careful around consuming cold or hot foods, like ice cream or hot chocolate. $\endgroup$ – Halfthawed Sep 19 at 23:28
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Technically speaking, what you're after evolved as other offshoot of hominids:

homo floresienisis head

In real life we had Homo floresiensis, which as adults were barely taller than 1 m.

Smaller body == lower calorie intake: You need your speculative species to be at least as small as Pygmies (~1 m 45 cm for an adult male). That's the only realistic way for a human like-species to cut its calorie intake.

Reasonably big brain - we're using roughly 20% of calories in a resting state just for our brain and moreover our brain is picky and accepts glucose as its energy source, thus consuming fully half of our sugars.

When you try to keep this species smart you'd face two issues:

  • a smaller body requires a slightly smaller brain, but your brain to body size ratio would go up
  • small pelvis, big brain - giving birth would be even more problematic than for our species

Passing as homo sapiens - Neotenous facial features and less developed secondary sexual characteristics may help your species to pass as homo sapiens kids. Such changes tend to co-evolve together with increasing intelligence, so they would be absolutely fine to justify.

Justifying their evolution

  • Our species evolved on the savanna and by standards of the animal kingdom, we're master long distance runners. This species should have most of its evolutionary history in the jungle or other thick forest.
  • A group of homo sapiens with even spear throwers is not only the apex predator, but a walking extinction event for most megafauna. Here you have two realistic options:
    • either they evolved mostly as gatherers not hunters (more sneaky and careful than us, preferring plant food)
    • they evolved a more sophisticated social structure much earlier than we did, to be able to take down large prey in a group of a hundred hunters (more team players, later better at coordinating any social structure)
  • their mass / surface body ratio would give them edge in a very hot climate, and would be serious problem in a cold one.
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  • $\begingroup$ If only one or a few needed to pass, they might not even need to disguise as children... we do have adults who are quite short, even if ones as short as children are not as common $\endgroup$ – Megha Sep 26 at 7:08
  • $\begingroup$ @Megha Out specie had some recent evolutionary pressure towards neotenous facial features (especially East Asians), thus it's not big stretch to imagine the some parallel. In contrast to keeping the same for half calories, this part is easy. $\endgroup$ – Shadow1024 Sep 26 at 7:38
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This already exists.

People can survive on scraps. People can survive on less than 1000 calories a day. So it would be easy to imagine a human-like species that can not only survive like this, but thrive.

No physical or biological differences would need to be present for your suggestion to happen.

If you want to make things interesting, however:
- Perhaps you can give your species little "humps" that contain fat, enabling them to go weeks without eating.
- Have the species eat a lot over a period of days, and then not need to eat for several weeks.
- Have the species' diet more versatile, able to use more of the food they consume. This is turn means they would gain more energy from the food, and less of it would be excreted as waste (poop)

There are many different ways to explain what you're describing, most of which already exist in some animals around today.

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    $\begingroup$ They can survive, but that doesn't exactly mean they're as healthy or as strong as a human who eats normally. I guess I should add that to the question. $\endgroup$ – Nightingale Sep 19 at 15:57
  • $\begingroup$ @ayrton nope. In fact, many people who eat less than the average first world citizen are stronger than the average first world citizen. Take sherpas for example. I'd bet my ass that most users of this site can't take a backpack up base camp in Everest. Sherpas can, and the whole time they will be eating less than you. $\endgroup$ – Renan Sep 23 at 12:21
  • $\begingroup$ @Renan And they suffer like 10x the mortality rate. Survive, yes, but not thrive. That's why these places suck so bad. Basic needs aren't met. $\endgroup$ – 458 Sep 23 at 15:24
  • $\begingroup$ @fredsbend well, they exist now and have existed for millenia, no? I think they fit into an example of how we don't need another species to answer the question. $\endgroup$ – Renan Sep 24 at 12:46
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Provided that the environment in which these Low-Energy Human Analogs (LEHAs for short) live is suitable, the LEHAs could easily have a metabolic rate a mere 5 to 10% that of a human's... by the simple expedient of poikilothermy.

However, this falls far short of the maximum permitted energy throughput of 50% that of a human, so by increasing the metabolic rate to the required level, the problems associated with poikilothermy can be avoided.

So... what do we get...? Marsupials have a significantly lower metabolic rate per unit mass than placental mammals, on the order of half that of an equivalent sized placental mammal. Marsupials are still warm-blooded, they just require far less food than placentals, though they are perhaps not quite as capable of extended periods of high exertion. Kangaroos appear on first glance to be an exception, until you realise that their hopping gait is very efficient, given the springiness of the tendons in their legs: much of the energy expended in each leap is recovered upon landing.

So... these LEHAs would be sort of like lazier, more food-efficient humans.

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To preserve the functions of a human requires ton of energy. Without absorbing qi, mana and chakra, the few scientific ways i can think of are:

1) A plant-human symbiosis: The plant would take roots within the human or grow on the skin. It could be a tree like a human venusaur or just plain green moss or fern covering the skin. The plants would carry photosynthesis and absorb water, CO2 and nutrients it needs from host while photo synthesizing and providing nutrition the host needs in exchange. Now, symbiosis requires both sides to obtain something from the other that they cannot on their own. The Human gains inbuilt food supply, the plants gain mobility. You can probably think of some other advantages too.

2) Skin cell mutation that allows photosynthesis: Basically Namekians from DBZ. This would be a human that doesn't actually need to breath out much (during daytime at least) but will require quite a bit of water intake. It could even be a humanoid that grows scale like leaves instead of hair. These body leaves can essentially carry out photosynthesis as well as the cooling function of sweating, so no need to sweat. Instead the sweat channels will evolve into water and nutrition channels.

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What you are asking for is a biological entity that would have the all the same critical physical characteristics of a human being but require less energy to achieve those characteristics. The organ in humans that soaks up the most energy (by far) is the brain. In order for evolution to favor smaller brains in a species that required the same intellect, I would suspect a technological compensating factor would have to be in play. Brain functions that were offloaded to energy efficient devices as part of a species culture may allow for genes that code for development of those parts of the brain to switch off as part of evolutionary adaptation. That's the only way I can see to cheat biology.

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Short answer, but what do humans have the capacity for that we don't actually use? Running. We just don't do it like we are built to. So if you're having this Alien fit in to a modern society, just have them be even bigger sitters/sedentary than humans are. You could also have them move slightly slower when they don't need to be fast, with the ability to speed up as needed for short bursts.

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