Spoilers for third section of Seveneves:

Moira is able to synthesize a Y chromosome to create boys for the second generation of infants on the Cradle. And yet, there is no explanation of what prevented racial mixing between the grandchildren of different Eves. If each Eve and her offspring were physically separated, this might make sense. But look at the (later) attraction of Kathtwo and Beled, and imagine a world where frequent sexual reproduction and avoiding inbreeding were absolutely essential to the survival of the human species.

In such a scenario, wouldn't the Eves's grandchildren be getting it on constantly? In such a scenario, I can't understand how racial purity would have been preserved, and certainly not how it would continue for 5000 years. For example, look how much black blood has intermixed with whites (and that's just over a few hundred years, not 5000). Or, for examples of enemies's children getting together, read Romeo and Juliet.

In story, I feel like Stephenson needed some sort of absolute prohibition on sexual intercourse, so that all babies would be generated via IVF (maybe to combat solar radiation?) in order to hold out the possibility that racial purity could be maintained.

Out of universe, I think that Stephenson just enjoyed his different archetypes of women enough that he figured it would be more fun to project them forward 5000 years, even if there's no possible way that such racial separation could have been achieved.

Is there any in-universe explanation which would justify the extended racial separation of the Seveneve's bloodlines?

  • This doesn't really seem a question? It's just a rant about a plot point that doesn't make a lot of sense.
    – Theik
    Commented Sep 25, 2015 at 7:57
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    I'm no good at editing, so I'll leave it to the author or someone else to figure out where to put in the actual question: What prevented the racial mixing between the grandchildren of the different Eves? Commented Sep 25, 2015 at 8:48
  • Thanks for the edit @gowenfawr. You crystallized my question.
    – Dan Kohn
    Commented Sep 25, 2015 at 10:58
  • There were definitely people of mixed race in the book. They tended to collect at different points around the ring. Commented Sep 25, 2015 at 16:31
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    @ScottWhitlock Mixed race people were a tiny minority. I'm wondering how any single race individuals could have remained after 5000 years, without complete separation or some sort of extreme taboos, none of which were discussed.
    – Dan Kohn
    Commented Sep 25, 2015 at 19:00

5 Answers 5


Part 3 lists several stages in the development of the races.

Following the establishment of the seven races, there is a period of weeding out the worst genetic errors:

“Correction” was the name given to the phase that had begun after the first round of Gestations, when Eve Moira had fixed errors that had led to several nonviable infants

Following this there is a period of mixing in the single Cradle habitat:

Stabilization, which lasted through the following ten generations or so as Y chromosomes were patched up, lingering genetic mistakes were fixed, and members of different Strains began to interbreed to produce hybrids within their own racial groups

At this point the decision is made not to revert back to the original human genetic template:

To undo those decisions by reverting to the “rootstock” human race was viewed almost as a kind of auto-genocide.

As the population multiplied, in the period known as Propagation, separate racial groups tended to form isolated colonies across Cleft, and further afield, leading into an era where the races diverged again:

Isolation: the formation of racially “pure” populations. Isolation led to Caricaturization: breeding, pursued consciously in some cases and unconsciously in others, that had the effect over many generations of intensifying racial differences.


In some cases, they had been used for Enhancement, which meant deliberate genetic manipulation for the purpose of rendering racial characteristics more pronounced...

So the canonical answer is each of the races kept to their own habitats during the 4,000 year period before the habit ring was constructed, and thus there was little opportunity for the groups to mix, as travel was difficult in this era. Additionally, this physical isolation encouraged a culture where some actively worked to maintain separate racial identities.


There's definitely racial mixing in the community. To begin with, there had to be mixing at the start when the population was small. At the time of part three in the book, the chart on the plate at page 601 (HarperCollins first edition) maps the population in terms of its "predominant ancestry" and includes the categories "mixed" and "none." There's no mention of prohibitions against unions between people of different ancestry groups.

My guess would be that at the very beginning, when the Eves themselves were still having children, racial lines were reasonably clear. Then when the population started reproducing on its own, racial lines were blurred, but because they were always watching The Epic (video of their history), people always had a clear image of the seven ancestral types. When the population got still bigger, it began to differentiate again as people started to select their mates for showing certain archetypal characteristics.

However, I don't know how realistic this redifferentiation scenario is. It could only happen if people chose mates who they perceived to be of the same ancestral type. Without going into gory detail about Mendelian inheritance, traits aren't really blended: the genes are just shuffled. Skin colour, for example, is determined by several genes. Two people of intermediate skin colour will have mostly kids with intermediate skin colour, but a few offspring can have quite dark or quite light skin. If those unusually dark- or light-skinned people selected each other, you could have a reemergence of a group of dark- or light-skinned people, given enough generations and a big enough population. Same for height, eye and hair colour, etc.

As you mention in the question, the book mentions that certain cross-racial relationships are common (Moirans and Teklans). This seems like it would have a blending effect, but I think it's conceivable that the kids would tend to fall into one type or the other, so that the racial types could be preserved, depending on what the genetic differences between Moirans and Tecklans are and how they are inherited. It would be interesting to hear from a geneticist.

  • So genetic traits are one thing, but would the different greeting gestures used by each type survive?
    – Pixel
    Commented Sep 25, 2015 at 19:13
  • The greeting gestures seem easy, since culturally all hipsters, and all jocks, and all sci-fi geeks act similarly.
    – Dan Kohn
    Commented Sep 25, 2015 at 19:38
  • I was under the impression that each of the Seven is purebred from their Eve, not that they have dominant racial characteristics.
    – Dan Kohn
    Commented Sep 25, 2015 at 20:09

I'm in the process of reading the book now and I'm wondering the same damn thing. It's driving me nuts.

Here's a theory I came up with, though maybe it will be jossed as I read on:

The "races" aren't races in the traditional sense. The seven Eve's descendants are mating with each other all the time, but a baby will always randomly come out as one of the seven "races," because of some genetic programing the seven women put into their offspring to ensure that the trait they wanted to "fix" in humanity lived on. So a Julian and a Moiran could have three kids, who might randomly be a Julian, a Teklan, and an Aidan.

Like I said, I'm still reading the book, so I don't know if my hypothesis can hold up, or not.


This was also driving me crazy. One idea that I had was that NS could have just included one line that says that Moira developed some kind of genetic coding that ensured the babies would carry the genetic traits of the mother always. So even when there was mixing, the seven lines would continue as they started. Otherwise, this whole story, while great, makes absolutely no sense.

  • Is this your own speculation on the matter or is it actually a line in the books? If the latter could you edit in the relevant quote? If the former then it doesn't really answer the question as it's more of what you wanted to happen/speculation whereas the question appears to be looking for a canon answer.
    – TheLethalCarrot
    Commented Jun 21, 2019 at 12:57

Reading the book now and have the same concerns. My conclusion is that this is a “hard science fiction” book from the astrophysics perspective (expertise of the author) but completely magical realism regarding genetics. It also annoyed me that they pretended early in the book (before the hard rain) that frozen embryos could be obtained from a pregnant woman, which is not how it works at all, and seemed to be an oversight rather than intentional plot point.

  • 2
    Hi, welcome to SF&F. This appears to be commentary, not an actual answer to the question. Do you have an in-universe explanation for how the races remained separate for thousands of years? Please don't post comments as answers. When you earn some reputation you'll be able to leave comments; in the meantime you might want to take the tour.
    – DavidW
    Commented Jul 27, 2022 at 22:36

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