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In a universe I am creating, there are many living species of humans (mammals from the Homo genus) (for example, there are ogres, giants, merfolk, halflings, dwarfs, and goblins). One of them is called gnomes (their scientific name is Homo minimus) (that means "tiny human"). Their basic characteristics include:

  1. Adults are as small as a domestic cat;
  2. They are as social as meerkats;
  3. They have an improved sense of smell, and an improved sense of eyesight, but a worse sense of hearing;
  4. They are omnivores with herbivorous tendencies like gorillas;
  5. They have a higher risk of osteoporosis, and senile dementia, and a lower risk of type 2 diabetes, noninfectious cancer, and noninfectious cardiovascular disease;
  6. Males are larger than females (mandrill-like sexual dimorphism);
  7. They are MUCH more likely to be rhesus negative than anatomically modern humans (62 % of gnomes are rhesus negative) (I do not mean rhesus null, I mean at least the simple absence of D antigen), and they are also more likely than anatomically modern humans to have blood type O (70 % of gnomes are blood type O, 14 % are type A, 14 % are type B, and 2 % are type AB) (to be exact, 46.2 % of gnomes are O rhesus negative, 7.3 % are A rhesus negative, 7.3 % are B rhesus negative, 1.8 % are AB rhesus negative, 24.4 % are O rhesus positive, 6.7 % are A rhesus positive, 6.7 % are B rhesus positive and 0.2 % are AB rhesus positive);
  8. They have an average lifespan of 9.5 decades (95 years old), the world record is 14 decades (140 years old);
  9. They are more fertile than anatomically modern humans (the twin birth rate for gnomes is 13 to 22 twin sets per 1,000 births) (the twin birth rate for anatomically modern humans is 9 to 16 twin sets per 1,000 births);
  10. They can interbreed with anatomically modern humans, but it rarely happens, because if a female gnome reproduces with a male anatomically modern human, giving birth can be a nightmare EVEN by human standards (this is comparable to a puppy that has a Scottish terrier as a mother, and an Irish wolfhound as a father) (nevertheless, the resulting offspring between a gnome and an anatomically modern human is often fertile, except that female hybrids are more likely to get PCOS, and male hybrids are more likely to get oligospermia).
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    $\begingroup$ A critter the size of a domestic cat cannot have a brain as big as humans. And so, without some major adjustments to brain arrangement, cannot be as smart as humans. A dismal thought. Cat-sized brain means cat-sized intelect. Also, cat-sized mammals tend to have cat-sized life spans. Again, it requires major changes. $\endgroup$
    – Dan
    Feb 22 at 20:46
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    $\begingroup$ @Dan cats are dumb. Size isn't everything. measure like encephalization quotient try to quantify this, but they don't do an excellent job. Naked mole rats are smaller than cats and outlive them. I suspect the OP doesn't want to hear "they should be more like naked mole rats" though, somehow, despite the fact that NMRs are objectively awesome. $\endgroup$ Feb 22 at 21:17
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    $\begingroup$ @Dan - women have brains somewhat smaller than those of men, but they are not any less intelligent. H. floresiensis had brains a third the size of modern humans, but they were still smart enough to hunt, cook, and eat giant prehistoric komodo dragons that would send most modern humans fleeing in terror. $\endgroup$ Feb 22 at 22:24
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    $\begingroup$ @Dan there is a known case of a woman who died of unrelated causes. At post-mortem it was found that 90% of her skull was filled with a congenital benign tumour. She had lived a completely normal life with just 10% of the neurons of a "normal" human being. (Normal, including intelligence not noticeably outside average). Brains include a lot of redundancy. Now, imagine evolution selecting the best physically small brains. $\endgroup$
    – nigel222
    Feb 23 at 12:07
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    $\begingroup$ @nigel222 A similar story is of a man whose cranium was mostly filed with fluid for most of his life, with the brain taking up less than half the usual space. $\endgroup$ Feb 23 at 19:15

6 Answers 6

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Insular dwarfism

See wikipedia

TL;DR:

Insular dwarfism, a form of phyletic dwarfism,[1] is the process and condition of large animals evolving or having a reduced body size[a] when their population's range is limited to a small environment, primarily islands. This natural process is distinct from the intentional creation of dwarf breeds, called dwarfing. This process has occurred many times throughout evolutionary history, with examples including dinosaurs, like Europasaurus and Magyarosaurus dacus, and modern animals such as elephants and their relatives. This process, and other "island genetics" artifacts, can occur not only on islands, but also in other situations where an ecosystem is isolated from external resources and breeding. This can include caves, desert oases, isolated valleys and isolated mountains ("sky islands").[citation needed] Insular dwarfism is one aspect of the more general "island effect" or "Foster's rule", which posits that when mainland animals colonize islands, small species tend to evolve larger bodies (island gigantism), and large species tend to evolve smaller bodies. This is itself one aspect of island syndrome, which describes the differences in morphology, ecology, physiology and behaviour of insular species compared to their continental counterparts.

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Underground nutrients

Your future gnomes dispersed onto a small continent with a highly porous karst topography, full of caverns and hidden water sources. The surface is poor in rivers, high in elevation. The trees that grow there would be small and stunted, save for one remarkable species whose taproots can navigate small deep cavernous passages and pierce dozens of meters of solid limestone. These trees have full access to deep reserves of fresh water, and the nutrients that can be found there. Growing to immense size, spreading by stolons across the landscape, they diversified into a highly successful genus.

In this process, the trees compete with one another, but not so much that they prevent one another from seeing to one of their basic ecological necessities: they need fixed nitrogen. Nearly all the trees pump excess glucose and fructose down their taproots into the caverns, where they nurture symbiotic bacteria acting in a loose association comparable to the root nodule of a legume. However, these bacteria have not evolved into a tight physical association with the trees as of yet, and the underground ecosystem is far more open-ended. There are many puddles of sugary water laden with bacteria and yeasts, fed by the tree roots so long as they contain sufficient fixed nitrate. The dead taproots decay into hollow tubes that ensure a vigorous air circulation that brings in fresh nitrogen to fix, and also permits the habitation of some organisms. The gnomes are small due to the need to traverse small passageways as well as "insular" dwarfism within individual cave systems.

Just so, we see the colonization of the underground landscape not just with bats and the rich fertilizer they produce, but also with humans that evolve to become small and capable of seeing by what to us is the dim light of autofluorescent bacteria. They feast occasionally on bats, but more often consume bits of the sugar and alcohol-rich nectar the trees leave behind, being careful not to destroy the resources that sustain them. They are unfortunately prone to osteoporosis because although they receive an endless supply of calcium from the limestone, they have very low intake of phosphate. In the end their feces and their corpses return their nutrients to the ever-circling roots around them. But perhaps their greatest contribution is in their mining - scraping new passages into the stone that improve the carrying capacity of their environment for untold years to come.

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    $\begingroup$ These humans would have to be very small. A gnome- (cat-)size cave or gateway would only be accessible by small children, at first. Then, longer stay would kill them, because children would grow. You've created a paradise with plenty of food, and same time a chicken-and-egg problem for humans. To get into these passageways, you'd need to be small. How small. I don't think evolution works in a preparatory way.. it's an adjustment to change in the environment, not a targeted change beforehand.. I won't vote it down, but I wonder what the story is, which would result in population underground. $\endgroup$
    – Goodies
    Feb 22 at 23:27
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    $\begingroup$ A time scale wasn't specified. We know from Homo floresiensis that humans can become very small. Alas, they went extinct and we'll never know cat-sized is possible, but then again ... we may never know it isn't. The passageways are presumably all sizes, so selection can proceed quite gradually, one "bottleneck" at a time, with many interesting founder effects along the way. $\endgroup$ Feb 23 at 1:05
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Evolutionary Pressures are not science-based

(One thing you ask for evolution to cause is better eyesight. That was answered here. Evolution works this way for everything else you ask.)

The tags “Evolution,” “Biology,” and “Science-based” can’t be applied to this problem because evolution does not lead to anything; it has no goal and no “survival instinct.” In other words, DNA doesn’t have a brain. What this question asks for is Lamarckian inheritance. That can’t have the science-based tag, an evolution tag, or a biology tag.

None is the science-based answer.

But to help your story-based problem of having naturally occurring dwarves as a species (which of the 23 science-based definitions of that you are using here is not stated), the problem seems to have already been partially solved in the real world by deleterious and disadvantageous genetic mutations.

Chandra Bahadur Dangi

was born in Nepal, grew to a height of 54.6 cm in height (the average cat is 46 cm—a comparison asked for in this problem) and lived to a ripe old age of 75 years.

The medical condition causing Chandra's size has never been determined, but obviously some condition does exists which can do it.

Speciation of Chandra's dwarfism

It is impossible to say if Chandra's condition could at some point become a new species without diagnosing the condition. His woman counterpart however, Jyoti Kisange Amge, was diagnosed with one of the 5 known types of primordial (genetic) dwarfism, which would be the only ways a new genetic species could arise; which are:

  • Achondroplasia (Frequency = 1/40,000; hereditary 20%)
  • Taybi-Linder syndrome (
  • Majewski osteodysplastic primordial dwarfism type II (Frequency = rare; heritibility: autosomal recessive pattern requires both parents to carry the gene - a normal human donor would dominate the condition)
  • Seckel syndrome (Frequency = 1/10,000; hereditability: autosomal recessive pattern requires both parents to carry the gene - a normal human donor would likely dominate the condition)
  • Russell-Silver syndrome (Frequency = estimates of 1/30,000~1/100,000; heritability: the problem arises from improper methylation. It has been identified as an autosomal recessive pattern however, rarely, Russell-Silver syndrome can run in families. In some affected families, the condition appears to have an autosomal dominant pattern of inheritance (but not confirmed). This would mean that one single donor parent may cause the condition in the children.

No evolutionary pressure can cause any condition to happen, however, as mutations are a stochastic process, the probability of a dominant form of dwarfism surviving can be affected through the evolutionary pressure of a reduced gene pool. Diversity will likely prevent primordial genetic conditions from gaining a foothold in a population because normal, healthy humans have rigorous DNA repair mechanisms which can correct these mutations in later generations, and wipe out the mutations which created your "species."

Thus the process of creating a dwarf species is four-fold:

  1. Reduce the gene pool to a single clade or family to promote genetic diseases.
  2. Wait several generations for deleterious recessive mutations to manifest, and by chance, several with your desired characteristics may or may not occur
  3. Provide advanced medical and security support to protect the disadvantaged population from predation, tribal war, disease, and any other natural inhibition to procreation
  4. Remove people without the desired traits from the gene pool, as their children can correct the recessive deficits. (You don't have to kill them, just exile them as outcasts and prevent cross-breeding)
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    $\begingroup$ This isn't really an answer $\endgroup$
    – Atog
    Feb 23 at 0:03
  • $\begingroup$ That's a comment, not an answer. I think 'evolutionary pressure' is ordinarily comprehensible - you might say selective pressure to be more formal. I should also add that "Lamarckian" inheritance has been on something of a roll recently - on top of a now-robust field of epigenetics, pol theta is a reverse polymerase, and then there are aberrations like this. $\endgroup$ Feb 23 at 1:12
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    $\begingroup$ Go to PubMed and look up "selective pressure". Lots of articles like this: ncbi.nlm.nih.gov/pmc/articles/PMC3778006 Even evolutionary pressure pulls up lots of articles describing the things it causes to happen. $\endgroup$ Feb 23 at 1:19
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    $\begingroup$ "lead" or have led? Lead, I've no idea. Have led, would be from a lack of pressure. Why are there different size felines? They're gluttons living in a land of plenty; make your early mammals have an even harder time than ours did, leaving no possibility that eventually being six feet tall is a good idea. Everything else would also have to never scale up. How to have gnome sized humanoids? Start 60M years ago and make some really big changes. $\endgroup$
    – Mazura
    Feb 23 at 11:31
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    $\begingroup$ @VogonPoet I don't see where they said 'cause mutations' though. They want to know what pressures could lead to a pre-determined set of traits. Simplifying the scenario, assume a primate species with diverse alleles that influence neck length. Now, part of species is forced by predators to hide in caves with low, spiky ceilings :) Within a few decades, long-neck alleles are absent from local breeding population. Further, some homozygous (frmr)recessives and/or previously unsuitable short-neck mates thrive in the new environment, leading to a specialized sub-species of our original primate. $\endgroup$
    – Rab
    Feb 23 at 23:30
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You can merge two of the answers to get a somewhat good answer...

Lets say their ancestors were normal people from a society that lived in the surface. They used to outcast or abandon their deffective offspring (I think spartans used to do so, throwing them from a cliff). So, babies with dwarfism would be left out and the few that survived ended up forming a little society in the caves where big people could not follow and kill them.

Their normal children would be left in the surface, die suffocated in the tunnels as they grow, or be sacrificed, as you like. The point is that the "big people" genes would be slowly removed from the gene pool until only the ones with dwarfism were left. Then something happend to the big people and left the small people, lets say they were decimated by orcs or something.

The species could have then evolved privileging smaller individuals that could navigate further in the tunnels, slowly becoming your current gnomes. I can think a lot of wrong thing with this... For example, a stable society living in a fixed place would most likely be farmers, so not so far in time to let a new species evolve from them.

Another option, you can say you have a peacefull society with a tendency to dwarfism, so its normal. Then a new species appears and hunts your humans (lets say orcs again) and the only ones having a chance of survival are the ones that can fit into the tunnels and hide/live there. As before, normal humans would not survive outside because of orcs nor inside because of their size, effectively letting live only the smaller individuals. The most difficult part sould be having a big enough dwarf population in the tunnels to start with...

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The discovery of the aplinium tree forever changed human society. The aplinium fruit was a potent aphrodisiac that guaranteed the greatest pleasure and fertility. Wild trees that grew in harsh forests resisted every form of domestication; the fruit of such trees blossomed once per season and only for a few hours, as the fruits were also favored by the local wildlife. Only the tallest humans could harvest them, and harvesting meant proliferation.

The shortest humans were quickly rejected from society as being unfavorable mates. Often expelled from local communities, these humans formed their own colonies in the dense forests, determined to develop their own means for harvesting the coveted fruit. Without access to forges and tools, these humans evolved to use the bounty of the forest for their survival. Over generations they become shorter, as the taller of them were scorned for being like the others - the foresakers of the kin.

The shorter humans soon discovered that the roots of the aplinium trees were rich in the same active chemicals of the fruits, and developed various means for extracting these chemicals without harming the trees. Shallow underground caverns wound gently around the roots of the trees, tapping the sap of the tree at various locations under the earth. Shallow tunnels led to deep and vast complexes, providing shelter and communal space for the society hidden away from the predators above.

With the the aid of the aplinium sap, gnomekind was borne, with little regard for those who banished them.

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You don't want evolutionary pressures that lead to gnomes

For some reason, I think a lot of people think evolution works like this:

  1. The environment changes
  2. Organisms adapt
  3. Now the organisms are fit to the environment

But that's very wrong. As in this answer, the idea of evolution having a goal is wrong. Evolution is not about survival of the fittest or creating better organisms. Instead, organisms that are fit enough survive and those that are not fit enough do not. How evolution (in general works) is more like this:

  1. Organisms become more genetically diverse through random changes
  2. The environment changes
  3. Organisms that are not fit enough die off or don't reproduce

#3 is a problem if you still have humans in your world; if humans die off and gnomes survive THEN that's evolution, but if humans and gnomes co-exist that's not evolutionary pressure, that's just random variation.

Having a group of more gnome-like people separate from humans at some point in the past should be enough to make them genetically distinct. The rest of your requirements (blood type, etc.) could easily just happen by random chance.

So to have gnomes, the only real requirements are:

  • random genetic variation
  • gnomes can survive and reproduce
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  • $\begingroup$ neither is actually true, individuals and genes that are not fit enough die off. And evolution does not require other species to go extinct at all. the existence of humans has no impact on where gnome populations continue to evolve. $\endgroup$
    – John
    Mar 9 at 22:00
  • $\begingroup$ @John I edited my answer to make what I'm trying to communicate more clear (hopefully). $\endgroup$ Mar 10 at 0:09
  • $\begingroup$ "What evolutionary pressure would lead to X" is still aa completely legitimate question and does not imply evolution is goal oriented just that it is to some extent predictable, which is true. humans and gnomes at the same time does not mean no evolution is happening. If anything the exact opposite, It can't even be random, that drastic of a difference requires very different evolutionary pressures. $\endgroup$
    – John
    Mar 10 at 0:38

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