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Imagine that the average number of children born per birth without any artificial intervention was three (i.e. triplets were the norm) and that 98% of pregnancies gave rise to 2 to 4 children, with a single child happening only 1% of the time and more than 4 children happening 1% of the time.

This has been the case for as long as recorded history and oral traditions extent.

Otherwise, people are as they are today with today's level of technology and conditions.

How would human anatomy, fetal development and infant development differ?

If there are multiple plausible possibilities for one or more aspects of these questions, focus on the most likely ones.

Consider cultural consequences only as necessary to make plausible assumptions about the physical ones.

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    $\begingroup$ Very important detail... Would the siblings born this way be identical or not? $\endgroup$ – Hyfnae Jun 29 '17 at 19:29
  • $\begingroup$ @Hyfnae Identical siblings would be only as common as they are today as a proportion of all live births and even if some siblings in a birthing were identical, others could be fraternal. $\endgroup$ – ohwilleke Jun 29 '17 at 19:31
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    $\begingroup$ A very big question: are "humans" still optimized for maximum brain power, to the point where a fetus' head barely fits into the birth canal (and even then, only if it rotates when it's half way through), and then still needs an extra trimester outside of the womb before its brain matures enough to be "functional?" (women often talk of humans having a 4 trimester gestation, of which 3 trimesters are in the womb, and 1 outside). The requirements for maintaining that head size will be substantial. $\endgroup$ – Cort Ammon Jun 29 '17 at 21:53
  • $\begingroup$ @CortAmmon Good question. Not sure what the most sensible resolution of it would be. I would think that the selective pressure for maximum brain power would still be similar. $\endgroup$ – ohwilleke Jun 29 '17 at 21:56
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    $\begingroup$ It may be relevant to note that the Yoruba ( of which I am a member ) Have the highest rate of twinning in the world to the extent that twins are a major part of Yoruba culture , and yet do not have any meaningful dissimilarities between them and other human groups $\endgroup$ – user15036 Jun 30 '17 at 8:23
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Anatomy.

Anatomically nothing much has to change in a human to accommodate triplets as the norm. However, food and energy intake for the duration of the pregnancy is likely to be higher than it is with humans in the current state of affairs as well as shorter pregnancies on average. twin pregnancies we can assume follow the same concepts as triplet pregnancies though in triplet pregnancies the things that are more pronounced in twin pregnancies, might be even more pronounced.

if people evolved in the way to have triplets as the norm these are the most pronounced anatomical and physiological features that can be associated with it:

  • Narrower birth canal as the average size of each individual baby is smaller. This might lead to the necessity of a C-section for women who bear a single child or death as the only other alternative. (Or a very, VERY strenuous birth which might be touch and go for both baby and mom.)
  • Larger uterus in proportion. There has to be room for three instead of the current standard of one.
  • More breasts. I'm not kidding, Look at animals who have larger litters, they all have enough teats to facilitate their entire litter drinking, unless they have an atypically large one. This little bit gives a little explanation on that matter.

Culturally.

This mostly boils down to economics. Baby food and diapers would have to be significantly cheaper than they are now else such births would financially ruin plenty of people if the situation were as it is now. Also, I think there would be a lot more attention for anti-conception. Lastly, if we take into account current birthrates. (1 to 2 in the West versus up to 10 in developing countries) that would translate to 3-6 in the West versus 30 in developing countries. The ratio would remain the same but the population explosion and the sheer biomass needed to facilitate so many human would pose even greater trouble than the world has to deal with nowadays.

Speculation.

Let's enter the realm of the hypothetical and say humans have the same life expectancy in your setting as they have in the current world. Projected is that the average amount of children per woman declines over the next century.

As mentioned above birth control will become incredibly important to manage population growth and biomass to facilitate the amount of children born per woman. It might be entirely feasible that if humans reach a certain amount, countries are going to set limits to how many births someone may have, or in extreme cases screen which families are permitted to have children.

This of course is entirely up to you as writer of your story, I just wanted to have my own go at the hypothesis and imagine a possible reaction of society on the matter.

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    $\begingroup$ You're assuming women go through the same number of pregnancies. In reality, they'd probably just have 1 pregnancy in developed nations, and might even limit things more in the developing world. Therefore, birth rates likely won't triple. $\endgroup$ – Bert Haddad Jun 29 '17 at 21:08
  • $\begingroup$ Your thoughtful answer suggests that breast feeding related birth control effects might be much stronger. $\endgroup$ – ohwilleke Jun 29 '17 at 21:22
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    $\begingroup$ @BertHaddad user121330 thinks we get lots of war/homicide to control population explosions, but as you note the number of pregnancies per woman might be lower (perhaps just one in a lifetime in most cases) and one could also have a scenario in which a significant minority of women (perhaps 30% at replacement level) never have children which is larger than the status quo as of the twenty-teens which is that about 11% of U.S. women don't have children and a larger percentage in many other countries in the developed world. The numbers might fit well to a 1 child per litter doesn't reproduce norm. $\endgroup$ – ohwilleke Jun 30 '17 at 18:58
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    $\begingroup$ @BertHaddad Birth control is a blip on the evolutionary radar. I get that it's our blip, but no animals that have litters reproduce just once. $\endgroup$ – user121330 Jun 30 '17 at 20:15
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More mammaries

Probably obvious, but you will need more than two breasts to feed more than two children.

More fat

Women tend to add fat during pregnancy and keep it while breast feeding. This ensures that they can continue to feed their children even if their food intake is interrupted while the child is still breastfeeding (for a certain amount of time). To provide this cushion for multiple children will take more fat, and probably result in more significant weight gain for women during and after pregnancy.

I would suggest that this implies a more sedentary lifestyle. Women that have to march across the savanna after giving birth aren't really in a great position to be adding 20 or 30 more lbs of fat. On the other hand, a forest-dwelling human species would be more able to stay in one place and could support heavier pregnant women better. This suggests human evolution in the forest rather than on the savanna.

Slower development

To feed 4 times as many children, the women would need a big jump in caloric intake. This can be challenging for a variety of reasons. I would argue that a better strategy would be to slow down the development of the children so that their demands for milk are lower and there isn't too much extra stress put on the body.

This slower development would only apply to the first two years or so. A 4-year-old multi-birth human might be only as developed as a two-year-old normal human, but then could catch up once the child was eating food on their own.

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    $\begingroup$ Brains are our largest metabolic cost, not growth. You're right that food would be a major issue, but I'd be shocked if development slowed down - if anything, I'd expect it to speed up so that the kids could fend for themselves sooner. Losing a few from the litter during lean years is the expectation in animals. $\endgroup$ – user121330 Jun 30 '17 at 0:52
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    $\begingroup$ @user121330 That is a good point. Developing multiple offspring is usually so that you can afford to lose a few. $\endgroup$ – kingledion Jun 30 '17 at 10:52
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    $\begingroup$ "Probably obvious, but you will need more than two breasts to feed more than two children." [citation needed] I'd say it is far from obvious, at leat to me (though I may be a little thick :P) Certainly you need more than two breasts to feed them at the same time, but is there a reason why they can't just take shifts? Or why it would be, beyond a shadow of doubt, an evolutionary advantage to feed all your offspring at the same time? $\endgroup$ – xDaizu Jun 30 '17 at 12:07
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    $\begingroup$ @kingledion I know that point. I also know that "correlation doesn't imply causation". Intelligence may have an impact. Most animals just tend to lay down and let the puppies grab a nipple and do their thing. There should be a nipple for every one or some will go undernurished. This happens often with large litters. An intelligent mother, with organizative skills, can make sure that every child has his fair share. That, united with "breasts are a heavy and uncomfortable waste of space" may make advantageous to lose them. Maybe I'm wrong, but I think it deserves more thought that "it's obvious" $\endgroup$ – xDaizu Jun 30 '17 at 12:24
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    $\begingroup$ @xDaizu: Serial (as opposed to parallel) feeding puts quite a bit of additional stress on the mother, as she will have less time to recover between feeding sessions. Also, breasts are large in humans (much larger than other mammals, other than maybe those that have been specifically bread to produce milk) due to being a secondary sex characteristic. So maybe there could be a bigger and a smaller pair of breats, or human females could have evolved larger ears. $\endgroup$ – Jonas Jun 30 '17 at 15:22
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r-K selection

We'd love our kids less. No, really, when animals have more babies, they invest less time in them, whereas when they have fewer babies, they invest more in each baby. See r/K-selection. The result is that we'd likely lose more kids (to disease, starvation, sibling murder, new parent murder, etc...). That may not be the worst thing though - the evolutionary pressure would keep the 'unfit' from surviving (note, I'd probably be unfit).

Baby size

Would start smaller.

Gestation period

The current hypothesis about why gestation takes 9 months is that Mom's metabolic rate when she gives birth is about double her non-pregnant rate, and if she holds on any longer, she won't be able to metabolize enough energy for both her and the fetus(es). There is no evidence that bipedal walking is affected by the width of the hips. Since fetuses are growing (basically) exponentially when they are born, that only means a one or two month change: the same as most triplet and quadruplet deliveries now.

Since you want the kids to be just as intelligent as current humans, infants will be helpless for a few months longer and children will be autonomous at a much younger age (so the parents can deal with the next litter).

The largest metabolic cost for humans is neither growth, nor activity: it's our brains. Any suggestion that we'd grow more slowly because Mom and Dad can't handle the caloric load ignores that fact. Evidence suggests we'd actually grow faster so that the kids could handle themselves earlier.

Anatomy

There is no reason to suspect that human anatomy would change beyond more mammary capacity (Mom's and maybe Dad's!) and fat stores. If you want something else to change, consider Pleiotropy - nobody will prove you wrong for at least 25 years.

Menopause

The only other animals who undergo menopause are killer and pilot whales, both of whom send daughters out to other "family" groups when they mature, and usually only produce one calf at a time. If the family structure doesn't involve sending women away when they become mature (for example, because the boys leave too) the twilight of female life may involve more babies.

Population Size

An 'r-type' omnivorous apex predator is terrifying. It's very unlikely that such a creature would ever evolve to be as intelligent as we are, but if it did, there would be a lot more internecine warfare because humans would be the only creatures applying downward pressure on the population.

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    $\begingroup$ "An 'r-type' omnivorous apex predator is terrifying. . . . there would be a lot more internecine warfare because humans would be the only creatures applying downward pressure on the population." Great observation. The comment on Menopause is interesting too although I'm not quite sure that I follow the chain of logic there. $\endgroup$ – ohwilleke Jun 30 '17 at 18:48
  • $\begingroup$ The logic chain is: menopause is biologically rare, humans share a few social and gestational similarities with the species that have it, and you're getting rid of one which would influence and maybe get rid of the other. It's a regression to the mean, at the very least. $\endgroup$ – user121330 Jun 30 '17 at 19:54
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More like kangaroos

A species designed to have more babies would have smaller babies. Kangaroos are known for their newborns being the size of a baked bean. They develop outside the body.

The reason human babies take so long to mature is the hips are too narrow to admit a fully developed brain. Triple the number of children and the starting brain becomes even more basic. They know how to suckle and hold on and nothing else.

Perhaps your humans used to have one or more pouches like a marsupial but these dwindled as artificial alternatives took over.

The first thing these joey-humans learn to do is eat. To lower dependency on the mother's milk. She cannot reliably produce milk for three normal size babies. She does not have the stomach space to produce that much milk.

Then the babies can be fed prechewed food by any member of he family -- in particular their dozens of brothers and sisters. Only then do they start the process of crawling towards sapience.

One could imagine the mother would be less important to the children's development once they learn to eat, and this might make fathers more likely to act as caregiver. But I think the presence of much larger families will have a greater cultural impact than that.

There will be a stage in the development of these babies -- after they learn to eat and move but before their brains start growing in earnest -- where they are closer to animals than people. I imagine they would be treated as such, in a manner real people would find horrific. For real humans that animal stage occurs when the child is still helpless and so is hidden thereby.

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  • $\begingroup$ A like the analysis and thoughtful and creative approach you take to this answer. $\endgroup$ – ohwilleke Jun 29 '17 at 23:02
  • $\begingroup$ Babies don't have fully developed brains at birth, and there is no evidence that Mom's hips are to blame. $\endgroup$ – user121330 Jun 30 '17 at 0:01
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    $\begingroup$ You should be careful when the article mentions a scientific study but neglects to give the name of the publication. $\endgroup$ – Daron Jun 30 '17 at 11:08
  • $\begingroup$ @Daron If you can't argue the facts, attack the source? Link 1 Link 2 $\endgroup$ – user121330 Jun 30 '17 at 20:11
  • $\begingroup$ Careful now. . . . $\endgroup$ – Daron Jun 30 '17 at 22:38
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Just a quick shot out of my head:

  • 2 pairs of mammal glandes. One cannot feed more than 2 babies at once with just 2 breasts. Taking turns in feeding won't supply enough energy to the growing babies.
  • farewell biped walking. Standing comes with narrower pelvis, which in turn makes delivery more difficult. And homo sapiens is maybe the only mammal to suffer delivery death.
  • smaller babies or, less likely, larger uterus. With the present homo sapiens multibabies pregnancy are troublesome for both the babies and the mother.
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    $\begingroup$ "And homo sapiens is maybe the only mammal to suffer delivery death." This is not true. Delivery deaths are a serious and common concern for horses, cows and many other forms of mammal livestock. $\endgroup$ – ohwilleke Jun 29 '17 at 21:44
  • $\begingroup$ This answer covers something the other answers haven't mentioned: difficulty of walking. The human pelvis is a compromise between giving birth to large babies and being able to remain upright for considerable periods of time. Given that human newborns are already somewhat underdeveloped, they probably can't be shrunken down much without spiking infant mortality (which is already high in nature). Mothers have a known risk of death in childbirth, and this is likely to become worse with triplet pregnancies unless the pelvis widens. This will compromise walking upright in general. $\endgroup$ – Palarran Jun 29 '17 at 23:12
  • $\begingroup$ @Palarran you and Dutch are wrong about Mom's pelvis. $\endgroup$ – user121330 Jun 29 '17 at 23:55
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The question is probably nonsense from an evolutionary perspective

It's not just humans who typically have a single child, it's the whole evolutionary branch leading to us. The primate branch that includes children produces one child at a time going back for tens of millions of years. The evolution of human-level intelligence thus takes place in the context of organisms that have very high individual investment in their offspring. Given the extraordinarily high levels of investment that are necessary to raise human offspring to adulthood - and even chimp offspring to adulthood - it is likely that these could not have evolved in a species which favours multiple-offspring-at-a-time.

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  • $\begingroup$ I agree that finding a scenario that would lead to this state of affairs as an alternative history against the backdrop of evolution as we know it would be challenging (perhaps it might have been due to horizontal gene transfer via retrovirus from another mammal species with that size litters early in human evolution ca. 200 kya, or perhaps people genetically engineered them as colonists on some new planet much like ours due to terraforming and they forgot their origins). But, for the purposes of this question, it doesn't really matter how it came to be. $\endgroup$ – ohwilleke Jun 30 '17 at 18:40
  • $\begingroup$ Of course, one of the main reasons that it doesn't matter is that, like most speculative fiction, the real purpose of exploring an idea like this is to gain a better understanding of cause and effect relationships and interconnectedness of different aspects of our biology and culture in the real world but using thought experiments to consider alternatives that illustrate the relationships more vividly and obviously. $\endgroup$ – ohwilleke Jun 30 '17 at 18:42

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