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(Note: I am not yet sure about the population numbers involved here, but if it is a very crucial aspect of the answer, somewhere between 1 million to 100 million at the start is almost certainly correct. The tech-level when magic appears is late bronze/iron age, and technological progress is similar in order to our world, but about ~30% slower.)

A group of people (a few dozen people) in my world were given the ability to access magic. The only way (as of now; I'll have to add other ways if this method is too slow) to gain the ability to access magic is if one of your parents also had that ability.

This "gene" is guaranteed to pass to one's children, and doesn't get "weaker" or "diluted" if only one parent has it vs both parents had it.

This "magic" is not strong but IS desirable; individual strengths vary considerably but almost all magic users can access this magic to temporarily become stronger, faster or use magic tools (though magic tools are a much later invention; they are very hard to invent)

Some other points:

  1. The group that gains magic originally as a founding principle wants to spread magic all over the continent. This sentiment may not be shared by those who they give it to, however.
  2. One of the major religions is against this magic, but soon enough it splinters off into more moderate sects who are not. The priests and hardcore followers of this group aren't counted in "everybody" for the purposes of this question.

It originally seemed to me that this method would not work and "magic" would remain confined to the nobility. But after reading that pretty much all of humanity is a descendant of Charlemagne, I think it might be possible.

Is it feasible that in around 2000 years, every human (on this continent; there's nothing else that's relevant) has this "gene?" If not, is there a number of years when this is nearly guaranteed? If yes, what would be a reasonable estimate for that number of years?

A clear timeline of when which group (ie nobility, then maybe rich commoners, then later peasants) gain this ability would be very helpful. This might be too story-based, though, so the main question is just the upper one. Insights would be appreciated.

EDIT 1: Since someone asked, I'm providing some clarifications of the most common ways magic can be used. Not everyone can use all of these, but most users can use most of these.

  1. Increase muscular strength
  2. Increase the speed of perception. Essentially makes one see the world as slow-motion
  3. Detect other magic users
  4. Produce body heat (More of a side-effect. All magic use produces body heat but this can be exploited by intermediately skilled users to pretty much never freeze as long as they aren't too tired to do magic)
  5. Produce flame. This is pretty hard as it involves controlling your body's rate of magic "combustion" to overheat a very small part, like your fingertip, then use it to heat something flammable. If done badly (or deliberately uncontrolled), a user can burst into flames or even explode.
  6. Most other uses require specially designed tools. They really didn't come into play until much later, some 1500 years down the line and remained primitive for thousands of years more (mostly due to extinction level events that reduced knowledge. However, these events happened after everyone already was able to do magic)

The previous mention of "extinction level events": Possibly it can be said that only people with magic survived those events. But this is not true as per my current iteration, unless no other possibility can be.

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    $\begingroup$ IMHO too social-based for a strict answer, and since the gene described is autosomal dominant in spreading, only society stands between it spreading to the entire humanity over time, considering it gives a MAJOR adaptability boon to its wielder. Yet since magic might be harmful to the wielder, there's no guarantee for the humanity to transcend because first generation could just burn themselves up in attempts to employ magic in their infancy out of competent control. $\endgroup$
    – Vesper
    May 3, 2023 at 11:39
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    $\begingroup$ Some ten or twelve thousand years ago, one or more people in Europe acquired a mutation which provided for the persistence of lactase into adulthood, meaning that they, and (some of) their descendants could drink milk as adults without getting sick. Ten or twelve thousand years later, this clearly advantageous mutation has spread to about half the (indigenous) people of Europe. The rest of the world, not so much. $\endgroup$
    – AlexP
    May 3, 2023 at 12:08
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    $\begingroup$ How an allele spreads depends heavily on factors like how advantageous or otherwise it is to reproduction. This is a complex factor that depends on environmental pressures (for example an ability to conjure clean drinking water can be very advantageous where clean water is not readily available), so these factors you really have to work out yourself before the question can be answered. $\endgroup$
    – biziclop
    May 3, 2023 at 13:01
  • $\begingroup$ How quickly are they killing off those without the mutation, that is the major factor, there are many isolated populations on earth. If nothing is killing those without the mutation you are looking at hundreds of thousands of years. $\endgroup$
    – John
    May 3, 2023 at 21:23
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    $\begingroup$ Even if the gene is originally confined to the nobility, it'll probably escape pretty quickly in the form of a bastard child (assuming similar societal trends as ours). Then you've just got the same setup as before, minus the nobility restrictions and only delayed by a few decades. $\endgroup$ May 4, 2023 at 20:10

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2000 is plausible.

It is not a precise science, and with initial population being in few dozens luck would play a big role, especially in the beginning. But it is not impossible.

"A clear timeline of when which group (ie nobility, then maybe rich commoners, then later peasants)" - this would require some heavy assumptions about your story, you should have put better time frame and cultural background, like "similar to Europe, years 1 to 2000 with development similar to 1 to 1400".

"would remain confined to the nobility" - if at least one of these people was noble or rich at the beginning, it may make it much easier for them. For the most of the period in question there was slavery, your magic cult could use it to dramatically accelerate the process.

But even without it it is plausible. Especially if people consider magic desirable. With over 90% of the population working in agriculture, strengthening would have clear benefits for them. The same for warriors of all kinds, and most artisans like blacksmiths or carpenters. For nobility, a military aristocracy, it would be pretty much a must-have upgrade, much more valuable than other benefits they can get from marriage. And they would need it only once in a bloodline. The only notable class without clear benefits would be clerics.

One more thing you could throw in - a better immunity for mages. Child mortality was horrific for the most of the history, even minor improvement would help a lot.

I would even upgrade it to "very likely". https://en.wikipedia.org/wiki/Jesus_bloodline: Ultimately, the notion that a person living millennia ago has a small number of descendants living today is statistically improbable. Steve Olson, author of Mapping Human History: Genes, Race, and Our Common Origins, published an article in Nature demonstrating that, as a matter of statistical probability:

If anyone living today is descended from Jesus, so are most of us on the planet.

And it was just a one guy without any special magic benefits.

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    $\begingroup$ It's been discovered that 44 of the 45 individuals who have been President of the United States are all descendants of King John of England (Most famous for a reign that resulted in the Magna Carta... and being the primary antagonist to Robin Hood). The one outlier was Martin van Buren. On top of that all of the U.S. presidents are descendants of William the Conqueror of England. $\endgroup$
    – hszmv
    May 3, 2023 at 15:15
  • $\begingroup$ @hszmv. Good example. King John was born in 1166 and there is no known historical direct link between him and all the presidents. William the Conqueror was born in 1028, he was John'n ancestor and his genealogical tree was wide enough to include Martin van Buren. People often underestimate exponential growth. With 2 parents, 4 grandparents, etc. the number of ancestors each of us potentially can have in a thousand years is unbelievable. $\endgroup$
    – D'Monlord
    May 4, 2023 at 4:11
  • $\begingroup$ How would slavery improve the spread of this gene? Slavery IS present in some societies in my world, but why would the owners want the slaves to have magic? It does make them better workers but also better fighters and harder to contain. They can also cause a lot more collateral damage if they have magic, such as setting the owner's house on fire at the cost of their own life $\endgroup$ May 5, 2023 at 10:36
  • $\begingroup$ @TheInfiniteOne I believe the implication is that slave holders would coercively reproduce with their slaves without too much thought to the consequences (an exceedingly common practice historically). Further slave trading would promote more human migration. Some slaves were for combat historically as well (see Ghilmān and Mamluks in the medieval Islamic world) $\endgroup$
    – user71781
    May 5, 2023 at 23:54
  • $\begingroup$ @TheInfiniteOne According to your settings "group ... originally as a founding principle wants to spread magic all over the continent". If that's their primary objective, containing slaves is obviously less important. A trivial solution - brainwash them into the "spread magic" cult and set them free. Also, it takes time for kid to grow up and become dangerous, by that time he/she can be sold far away and become someone else's problem. The clear benefit for the group is to quickly jump from few dozen to hundreds or even thousands circumventing typical social limitations like monogamy. $\endgroup$
    – D'Monlord
    May 19, 2023 at 10:00
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The mutation would spread though heredity, so that every person who has the mutation would have to be descended from one or more of the original mutants.

Jani Miettinen's answer mentions Y chromosome "Adam" that every human in descended in the male line from, and Mitocondrial "Eve" that every human is descended in the female line only. Considering how long ago those persons are calculated to have lived, Nuettinen concluded it would take hundreds of thousands of years for someone to become the ancestor all living persons.

But the number of ancestors someone has X generations previously will be 2 to the X power, while only one of those ancestors will be in the pure male line and only one will be in the pure female line. Someone will have 1,024 ancestors in the 10th generation back, which will included 1,022 in mixed male and female lines of descent.

Similarly, after many generations, the vast majority of a person's descendants will be in mixed male and female lines of descent.

The age of the MRCA of all living humans is unknown. It is necessarily younger than the age of either the matrilinear or the patrilinear MRCA, both of which have an estimated age of between roughly 100,000 and 200,000 years ago.[14]

A study by mathematicians Joseph T. Chang, Douglas Rohde and Steve Olson used a theoretical model to calculate that the MRCA may have lived remarkably recently, possibly as recently as 2,000 years ago. It concludes that the MRCA of all humans probably lived in East Asia, which would have given them key access to extremely isolated populations in Australia and the Americas. Possible locations for the MRCA include places such as the Chuckchi and Kamchatka Peninsulas that are close to Alaska, places such as Indonesia and Malaysia that are close to Australia or a place such as Taiwan or Japan that is more intermediate to Australia and the Americas. European colonization of the Americas and Australia was found by Chang to be too recent to have had a substantial impact on the age of the MRCA. In fact, if the Americas and Australia had never been discovered by Europeans, the MRCA would only be about 2.3% further back in the past than it is.[15][16] [17]

https://en.wikipedia.org/wiki/Most_recent_common_ancestor

Thus it should take only about one to three thousand years, I guess, depending on various factors, for every person on your planet to be descended from a mutant if the mutation appeared in one person. If the mutation appeared in several persons about the same time, then the descent from mutants should spread a little faster.

In real life some descendants of mutants would not inherit the mutated gene, but the question specifies that all children of people with the mutated gene inherit, and so every descendant of the first group of mutants will have the mutant gene.

Thus the period, about one to three thousand years, required for everyone to be descended from one or more of the original mutants would be the period for everyone to to acquire the mutant gene.

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This is a bit of a social issue. In 1500, the population of Europe was around 61 million. If everyone who had the gene only married those who didn't have the gene, then you could convert the entire population in about 26 generations; less than 500 years. You could do it in less if the magi had a lower infant mortality rate, if they were selected preferentially for important jobs, etc.

The problem is that humans are inherently tribal and insular. Prior to the enlightenment, this ability would have been hounded by religious leaders, who would find it threatening, and therefor against God. Anyone who married into such a group could be excommunicated or killed. It would probably start holy wars.

And that's just Europe. insemination to geographic areas like East Asia and the Americas would be a lot slower, for obvious reasons.

The reality is that it depends on how striking the features of someone who has the power are. Special eye color? Skin tone? Shape of the ears? Any of those would be targeted by haters, and the features would be more obvious with geographical distance.

In reality, "all" people is probably "never." There are still some areas of the world that refuse to even allow visitors. You're never going to get interbreeding into those areas. There will be purist cults who try to keep it out of their blood line, right up to the point of the last untainted person.

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It's all about the math...

If a magic user sires (or bears) six children with non-magic users, and all those descendants do the same, then there will be just under 1.7 million descendants after eight generations. When "a few dozen" people do that, then there will be just over 60 million magic users after 8 generations: 36 x 6^8 = 60,466,176.

Problem solved!!!

But no, since messy practicalities always stand in the way:

  • The men could certainly do that (cuckolding, etc); would the women do it?
  • Will all the children live long enough to reproduce instead of dying too young to have all six children?
  • Will the rest of the societies in the world let that happen? (Magic or not, "quantity as a quality all it's own": jihads, assassinations, etc could decimate the magic population.)
  • Planets are BIG, and full of mountains, deserts, jungles, oceans with fierce storms, and far away continents & islands. People live in all those places.

How are your bronze-age magic workers in (for example) Mesopotamia or the Ural mountains going to even know that they have to get to the Andes mountains, the Amazon jungles, Australia, the North American Great Plains and Nunavut?

Thus, given how long it took "us" to get from the Late Bronze Age to Magellan's circumnavigation (about 2,600 years) and assuming that the magic users were intrepid explorers ("Are there other people Beyond The Ocean?") then a reasonable answer would be 3,000+ years.

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  • $\begingroup$ In the question, I DID say that there's only one continent (which is relevant to this question). There's still remote places, of course... $\endgroup$ May 4, 2023 at 17:09
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This basically follows the inheritance mechanics of a gene drive. A gene drive manipulates cellular machinery to bypass the normal recombination process and force all progeny to carry the gene.

Early on, it will spread roughly twice as fast, because all progeny will carry it even when only one parent has it. The "natural" form will also rapidly go extinct once the majority of a population has the gene, since it will become increasingly difficult to find mates that lack it. So you will end up with populations where magic ability is universal, and any outsiders who join those populations will have children who can use magic.

This still will take many generations to take over an entire population if selection is random, it might be strongly desirable to have one's children be able to use magic. If so, those without magic will strongly favor mates that do have it.

This could be tweaked to favor spread even further, if children with only one magic-using parent were stronger than other magic users: then those who do have magic would have reason to prefer mates that do not have it. Such pairings will be the norm at first, but will become increasingly rare as the magic-using gene takes over.

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Basically you are asking how long it takes that there are no longer children who lack that mutation. In other words, everyone is descendant of the original group with the mutation.

It is assumed that all men share a common male ancestor from whom the Y chromosome came. He is called Y chromosome Adam, and he lived probably 200,000 - 300,000 years ago. There is also a woman, mitochondrial Eve. She is the ancestor of the mitochondrial line our cells have. She lived 180,000 - 580,000 years ago.

So, concluding from those two, it takes a very long time for a trait to dominate a whole species, and it is not even proven that something that will ever happen again. There might not be a genetic Merlin at all, and if there is, it probably takes hundreds of millennia for the gene to get spread like that.

It can happen in a subpopulation in a reasonable time, but not the whole species. 2000 years is definitely not enough for it.

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    $\begingroup$ "concluding from those two, it takes a very long time for a trait to dominate" - your conclusion is unrelated to your data. Y-chromosomal Adam is the most recent male from whom all living humans are descended through an unbroken line of their male ancestors. "have unbroken line of male ancestors to one person" is a much stronger requirement than "have at least one common ancestor with group of dozens of people over 2000 years". $\endgroup$
    – D'Monlord
    May 3, 2023 at 13:48
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    $\begingroup$ If we look at the number of men descended from a Khan just a few centuries ago, it takes a lot less time than you think. There is a man who has provided the sperm to produce over 500 children who was recently ordered to stop. bbc.com/news/world-europe-65429936 We only need to look at how certain haplogroups make up large percentages in an area such as Haplogroup R-DF27 which is supposed to have started 4,200 years ago. en.wikipedia.org/wiki/Haplogroup_R-DF27 $\endgroup$
    – David R
    May 3, 2023 at 14:26
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    $\begingroup$ This is at best a very wide upper limit. The existence of a Y-MRCA for today's population that existed 200k years ago does not indicate that there was no Y-MRCA until the year 2023. We can observe today that all men are descended from the Y-MRCA, but that doesn't mean it took 200k years to achieve. It's possible that all men 190k years ago were descended from the Y-MRCA as well, and still have no more recent common ancestor for everyone 190k years later. $\endgroup$ May 3, 2023 at 14:52
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    $\begingroup$ @Jani Miettinen - simply put, you don't understand what you data means and make completely unrelated conclusion based on it. Steve Olson, author of Mapping Human History: Genes, Race, and Our Common Origins, published an article in Nature demonstrating that, as a matter of statistical probability: "If anyone living today is descended from Jesus, so are most of us on the planet". Jesus presumably lived 2000 years ago and he was a one guy. If there were few dozens of people, it wouldn't be "most", it would be "all". And it doesn't even take into account clear advantage of marrying a mage. $\endgroup$
    – D'Monlord
    May 3, 2023 at 15:43
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    $\begingroup$ @JaniMiettinen That the Y-MRCA is 200k years old does not imply that it took 200k years until all men had one male common ancestor. Your statement that "it has taken more than 100,000 years for something to get passed to all humans" simply cannot be concluded from the fact that the Y-MRCA is 200k years old. Suppose the Y-MRCA is the only man in the world, he passes his genes to the entire rest of the population in a single generation. But as long as his descendants continue to procreate, 200k years later, Y-MRCA is still the only common ancestor among everyone. $\endgroup$ May 3, 2023 at 17:18

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