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Over the course of Homo-sapiens, we have become larger, because of diet & selection. Even in the last couple centuries. Today, we vary across geography and cultures, too, of course.

Now it is thousands of years in the future (your call) and people live happily on Earth, but have evolved to be much smaller; still in proportion, just smaller even than our distant ancestors. We've grown in population, but have stabilized the Earth's issues and become highly sustainable: almost Utopia.

What contributing factors helped make us so much shorter over time - even those staying on an Earth?

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  • $\begingroup$ Maybe short became the new sexy? $\endgroup$ – Mikey Apr 19 '15 at 17:17
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    $\begingroup$ I think Kim Jong-Un could provide you with a good answer to this one. He has created a North Korean "utopia" populated almost entirely by short people. $\endgroup$ – Stephen Apr 20 '15 at 3:16
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    $\begingroup$ Can you define "much smaller?" 3ft tall? 6 inches tall? The latter would start running into processing limits due to smaller brains $\endgroup$ – Cort Ammon Apr 20 '15 at 3:30
  • $\begingroup$ @CortAmmon I think, yes, 1m or 1.5m++, but proportional and gradual, not due to some kind of 'happening'. $\endgroup$ – Mikey Apr 20 '15 at 6:46
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    $\begingroup$ frankly a thousand years is a drop in the bucket to evolution. You would need a major MAJOR effect, something capable of killing well over half of the population and focusing on only tall people, to have any effect in that time frame. Even then we wouldn't be much smaller then the shortest average person of today. It's just too short a timeframe for natural evolution to play much of a role without a huge killing-everyone disaster that probably doesn't fit your uropia. $\endgroup$ – dsollen Apr 20 '15 at 19:58
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Over that timescale and without a major crash in population (which could reasonably be recovered from later), the only real option is intelligent selection.

Important note: The change in average height over the last few centuries has nothing to do with evolution. Those genes for height already existed; the thing that changed was childhood nutrition. (Less important note: this gives an important hint about why height is usually considered attractive in humans, taller people in the ancestral environment had fitter parents, indicated by the fact that they were less malnourished than everyone else.)

Evolution is very slow; the number of generations required for a new gene to become fixed in a population is equal to $2 \frac{\ln(N)}{s},$ where $N$ is the size of the population and $s$ is the increase in fitness the mutation creates. For our population of 7,000,000,000, that means 45 generations even for a mutation that outright doubles the number of children you have. Clearly, small increases in fitness like "short people are sexier" (Maybe an increase in fitness of .01, most people can find mates even if they aren't that sexy. And it's probably much lower.) just aren't going to cut it even in several thousand years.

So how could it happen? Genetic screening! Or any other sort of technology that lets parents intelligently choose which of their genes to pass on to their offspring. If a whole bunch of people have access to the technology and are influenced by similar selection criteria they could easily massively increase the fitness of those genes which they want, making it very easy for those genes to become fixed in the population very quickly.

The main question you're left with is why they'd all have that same favor towards shortness. The first thing that comes to mind is some sort of tax break or subsidy put in place by governments for people who have shorter children. They have a smaller carbon footprint and you can fit more of them in your cities, seems pretty sensible to me.

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    $\begingroup$ On second thought, I should probably elaborate on the "major crash in population" thing. Essentially, if something happened which killed some large part of the population but was significantly less likely to kill someone with a certain gene or combination of genes, those genes become much, much more fit. And many genes have side effects, for example making people shorter. So maybe Earth is struck by a highly deadly disease that happens not to kill people with a gene that also causes dwarfism or something. $\endgroup$ – Saidoro Apr 19 '15 at 18:21
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    $\begingroup$ Alternately, the earth is attacked by magic, evil lawnmower blades which fly at exactly 5 feet 8 inches off the ground at all times. $\endgroup$ – Saidoro Apr 19 '15 at 18:22
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    $\begingroup$ The Dutch have gotten taller over the past 150 years . About a foot taller. I think that's pretty fast. Assuming the assumptions in the article are correct, the average human height can presumably fluctuate quite rapidly. $\endgroup$ – ton.yeung Apr 20 '15 at 2:02
  • $\begingroup$ Well, that's interesting. The article does overplay the role that natural selection played, if you look to the original publication it's based off of, you'll see that the effects are fairly small, most of the change comes from nutrition and medicine as it has in the rest of the world, but the increase in fitness is still larger than I predicted in my answer. I'll think about this some and then change my answer. $\endgroup$ – Saidoro Apr 20 '15 at 17:59
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In evolutionary biology, a dwarf subspecies can be created by isolating a population on an island (or creating an island effect). Since resources are limited by accident or design, the trend is for the population to gradually become smaller as smaller individuals have a greater chance of surviving and reproducing than large individuals in the limited resource setting.

Dwarf mammoths existed near the end of the last ice age in isolated islands in the arctic after their large brethren were hunted to extinction. Homo floresiensis is an example of a hominid species isolated on an island which gradually shrank until the average individuals were about 1 metre tall. Even today, deer in some parks and military training areas are much smaller than average since they live in restricted "islands" of resources (deer who wander off are subject to hunting or death by automobile), and should this continue for another century or two, a new race of deer will exist.

Humans might naturally shrink in the far future if they are living in asteroids or small colonies in unpromising environments in space. The need to conserve resources so you can survive and reproduce will have the same long term effect as being isolated on an island. (This ignores the effects of genetic engineering or the ability of future humans to harness advanced technology to increase their access to resources and energy even in these environments).

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    $\begingroup$ I love the oxymoron of "dwarf mammoth". $\endgroup$ – David Richerby Apr 19 '15 at 23:24
  • $\begingroup$ This is what I would have said, most dwarf or pygmy species evolve on islands. To achieve this effect on humanity you would presumably need to create a similar environment, most likely from some kind of global catastrophe. Extreme global warming might turn the world into actual islands, but I imagine even if there was enough water to do that the warming effect might have killed us off by then... $\endgroup$ – adaliabooks Apr 12 '17 at 11:36
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Many researchers have pointed out that our taller height is a product of over nutrition. Basically, we feed our children excessive protein and calories and these two elements promote growth in height, lean body weight and fat.

A report by The World Cancer Research Fund stated that we have grown taller, heavier and increased our chronic disease as a result of the industrial revolution and the associated Western diet.

Most children are overfed in terms of calories starting in infancy. A well-balanced diet with moderate protein and calories could reduce our size.

Birth weight is also a factor. Most studies have found that birth weight correlates with adult height, weight and body mass index. Birth weight is related to the mother's height, weight before pregnancy and weight gained during pregnancy. Thus, smaller mothers would produce smaller infants who wouldn't grow as large. Japanese women cut their calories when they get pregnant to avoid getting fat. As a result, their infants are lower in birth weight. This practice has been going on for many years and Japanese males have leveled off in height at about 5'7. This practice should be done under a doctor's supervision to assure adequate nourishment of the infant during pregnancy.

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Parents don't want their kids to die of cancer or diabetes. Nor do governments want to spend billions treating these conditions. Laron dwarfism is a genetic condition that causes the growth hormone (GH) receptor to work less efficiently. The resulting insensitivity to growth hormone, or "GH resistance", confers nigh immunity to cancer and type 2 diabetes and reduces the harmful effects of aging.

Based on other answers, I can think of a couple ways that the Laron allele might be selected for.

  • Genetic screening for the Laron allele could become popular. Parents could arrange marriages to Laron dwarfs, or governments could provide tax breaks or health insurance subsidies to families with at least one Laron parent or two Laron carriers. In the limit, Gattaca-style recombinant "designer babies" would have the GH receptor gene corrupted on purpose to the Laron genotype.
  • A more carcinogenic environment would cause a population crash, giving Laron dwarfs more chance to survive (make your time) and reproduce. So might the diabetes-prone high-calorie, high-carbohydrate Western diet.
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Howard Fast wrote a story "The Vision of Milty Boil" where a developer started making cheap housing with shorter and shorter ceilings. He also started mass adverts promoting shortness, made movies feature tall, hulking villians and short, attractive heroes. Eventually everyone was less than 3 feet tall.

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In 2312 by Kim Stanley Robinson, the engineered "littles" had small bodies and other significant features, all having to do with longevity. At the time of the story it was remarked that no "little" had ever died of natural causes.

These (or other options) were chosen by parents as developmental attributes for their children, so it did not require a genetic bottleneck or selective reproduction of parents with the desired traits.

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