Worldbuilding Immortals in Science Fiction

worldbuilding immortals

What would be the implications of an ageless, or biologically immortal, human or alien race? Or even a race that lived much longer than humans? Worldbuilding immortals is certainly a fascinating exercise. Eternal youth has always fascinated, enticed, and mesmerized people; the Epic of Gilgamesh, one of the earliest stories written down by anyone that survives to our time, features a quest by the titular hero to discover the secret of eternal youth. The Fountain of Youth is a similar concept in Western culture, dating back at least, as with so much else, to classical Greece. Science fiction, space opera, fantasy, and other forms of speculative fiction provide fertile grounds for exploring this concept.

Biological immortality, or agelessness, also known more obscurely but perhaps more technically as “negligible senescence”, has an air of the impossible or the fantastical about it, yet it is perfectly feasible in principle. There is no law of physics that says life-forms have to undergo the process of aging. Indeed, we know that quite a few species don’t.

Worldbuilding Immortals among Humans

Leaving aside the question of worldbuilding immortal aliens for the moment, what would immortal humans look like? Growth stops, and aging begins, in the teens and twenties, so humans in such a society who were not children would all look like young adults do today. An interesting implication of this development would be a population explosion. Mortality, though diminished in the past few centuries, is one of the most powerful factors constraining population growth. The end of mortality from old age means death rates would plummet, causing populations to rise even with what would now be below-replacement birth rates.

How much would it rise? Consider that over the past 50,000 years it is estimated that a cumulative number of 108 billion “modern humans” have lived. If you include the archaic humans from Homo erectus onward as “people”, as I would, then the number would be considerably greater than 108 billion, by dint of people being born over the course of 2 million years instead of 50,000.

Nevertheless, I find it interesting that that 108 billion figure isn’t all that much higher than the current population, amounting to around one order of magnitude greater. If no one ever died over the course of 50,000 years that would be the population. Some versions of “immortality” have complete immunity from death, but mere agelessness is more interesting since people would still die, but not from old age.

Agelessness, not Deathlessness

This means the mortality rate will still be greater than zero. In the United States between birth and the age of 25, as reasonable cutoff as any before aging begins even if only modestly, 2% of the population dies according to the Social Security Administration’s life table. 0.6% of Americans die in the first year after birth. The chance of dying within a year then declines to a trough of 0.009% or so at age 10, and then steadily increases with age. Except in the very oldest age brackets, beyond 100 or so, the mortality rate for men is much higher than women. 0.16% of 25-year-old men die within a year compared to only 0.06% of 25-year-old women.

If people only live less than a century anyway this 2% attrition per quarter century isn’t very significant, but if people don’t die of old age it adds up over time. A “decay rate” of 2% every 25 years, as reasonable an estimate of mortality for ageless humans as any, amounts to a “half life” of 858 years.

Life Expectancy

What this means for worldbuilding immortals that even if old age is “cured”, the life expectancy at birth would still “only” be eight centuries or so, not infinite. Half of the population will die from disease, suicide, homicide, or accidents within that time. Still, it would represent by far the greatest-ever gain in human life expectancy. Hygiene and antibiotics led to perhaps 40 years greater life expectancy; the conquest of aging would represent 20 times that gain.

It’s worth noting that the human life span has remained the same for thousands if not millions of years; super-centenarians, those who live beyond 110, are attested in ancient times. Life expectancy at birth was 20-40 before hygiene and antibiotics in agricultural societies (interestingly, in hunter-gatherer societies it was substantially higher), but that doesn’t mean people died of old age at 30. Infant and childhood mortality were very high, which drug down the average. If half the population dies in infancy and the other half lives to 100, the life expectancy at birth is 50 despite no one dying at that age. “Life expectancy at birth” is influenced by mortality rate throughout life much more than it is by when people die of old age.

Now, today life expectancy at birth is pretty close to the age at which people die of being elderly, but in a future where agelessness has been achieved these two numbers will once again decouple.

Even a life expectancy of eight centuries would be very different from what we have today. Imagine if we still had people around who remembered the thirteenth century and were as fit as a young adult. Now worldbuilding immortals is starting to seem even more interesting. Of course half of the population from then wouldn’t still be alive now because mortality rates were much higher back then; nevertheless it would be a significant number. How many?

Well, using an estimate of Britain’s numbers from 1750 for the pre-industrial norm, which is likely too low an estimate, yields an additional 1% per year mortality in general. Assuming this also holds true for newborns to 25-year-olds, the “attrition rate” would amount to 1.1% per year, yielding a half-life of 68 years.

This means that almost twelve half-lives would have elapsed since the thirteenth century, leaving 0.03% of the original population still alive today. Over the course of 1000 years, four out of a hundred thousand of the population would survive. Over the course of 2000 years only one in a billion would survive. There weren’t a billion people around back then, but there were well more than a billion born before that.

So it’s quite possible we’d have people around who remembered ancient Greece and Rome but they would number very few indeed. It’s worth noting that the risk of death is not evenly distributed; those who have much stronger immune systems and happened to practice good hygiene over all that time would have much lower risk of death from infectious disease.

Given modern American mortality rates, with the 858 year half-life, 20% of the population survives 2000 years. 310 out of 1,000,000 people, a small but significant number, would survive 10,000 years. Out of 100 billion people 3 would survive 30,000 years. It seems unlikely that anyone, except a few outliers here and there, would live for longer than that.

Given a life expectancy of 850 years on average, that is about 10 times as long as people live now. The birth rate per year needed to replace the population would plummet. It is important to note, however, that the total number of children per lifetime that would be needed would be unchanged at 2.1, since childhood mortality would stay the same.

If people still have 1.5 children every 20 years after adulthood, as is the average now, a rate of one child every 15 years, then the average number per lifetime adds up to 53. Every generation would be 26 times larger than the preceding one! However, due to the long length of time involved this only amounts to a population growth rate of 0.4% per year.

Low population growth, however modest, might be the norm in such a society. To actually shrink the population over the long term less than two children per eight centuries would need to be born to each woman. This would require a rate of less than one child every four centuries. If mortal humans bred at that rate they would have a total fertility rate of only 0.06 children per woman. Even the most sterile parts of urban East Asia have birth rates ten times higher than that.

This of course assumes that mortality rates stay at modern American levels forever. They could easily be much lower in the future, or even much higher! There’s no way to tell for sure.

As an aside, it is interesting that there isn’t more research and resources devoted to eliminating aging than there actually is. Although the chances of success with our science and technology are very slim, the potential gains are very large; there is no other line of medical research that has even the theoretical potential to add centuries of life expectancy at a stroke. A variety of estimates have suggested that without the elimination of old age human life expectancy at birth will never be much higher than 100 even if all diseases are cured.

Worldbuilding Immortals among Aliens

Where this intersects with worldbuilding is that an alien race, unlike humans, may be naturally immortal. Therefore their birth rates will have evolved to match their mortality rate in the wild. If they have a similar mortality rate in the wild as humans in developed countries do, then they could easily have evolved to bear a child on average once every four centuries.

The human menstrual cycle provides a pathway toward this; human women as it is are only fertile for 3 days per month. We could easily posit a life-form that is only fertile for 3 days per year, with a correspondingly lengthened cycle. It’s not unusual for humans to take a year to conceive a child, or around 12 menstrual cycles; this species would often take a decade to conceive. A tenth as many births balances out a tenth as many deaths, and the population would be stable.

A life-form with a much lower mortality rate might live on average 10,000 years instead of 800 years. In that case the replacement rate would be one child every 5000 years. Their menstrual cycle might be something like 200 months, or 16 years long. It wouldn’t be unusual for them to take a century to conceive.

Similarly, an average life-span of 100,000 years implies a 160 year menstrual cycle, and a million year life-span implies a 1,600 year cycle. The case of a million-year life-form is interesting, because a half-life of 1,000,000 years implies that only 6 out of every 10 million of this species would die in a given year.

Worldbuilding Immortals with Low Death Rates

In a population of 7 billion, what we have today, only 4,200 people would die every year worldwide, or 11 per day. In pretty much every country every single death would be a rare enough event to easily make national headlines if the species had the psychological disposition to give attention to it. If they didn’t give it a lot of social attention an alien might think at first glance that they were truly immortal, never dying of anything, until they looked very closely.

Similarly, a half-life of 10 million years leads to 420 people dying every year, or just over one per day worldwide. 100 million years leads to 42 dying every year, and a billion years leads to just 4 per year worldwide. A half-life of ten billion years would yield a global death toll of one person every two and a half years.

Such a long life-span is hard to imagine. If this species were around back then substantial numbers of them would still be around who lived through the earliest history of our universe; the Big Bang, after all, happened 13 billion years ago.

It’s certainly not impossible when one is worldbuilding immortals to imagine species in a science fiction, fantasy, or space opera setting who are more or less like humans but are extremely long-lived. Simply making them far tougher against disease and accidents, less violent toward their own kind, less prone to suicide, and drastically lengthening their menstrual cycle would do the job.

Truly Alien Immortal Life Cycles

More exotic possibilities are out there. There are some insects who spend the vast majority of their lives in what we would call middle childhood, the late stage of childhood before adulthood, and only become sexually mature for a brief period before dying. This is usually associated with a drive to mate during this time for obvious reasons.

While this is not agelessness, since they age and die, when worldbuilding immortals who are a bit further away from the human range we could take inspiration from this. Instead of lengthening the menstrual cycle so that there is a regular days-long period of fertility throughout adult life, the entire period of sexual maturity could be fertile but very brief.

In short, you could posit when worldbuilding immortals a species that lives for a long time, thousands if not millions of years, in a late childhood state but then at the end of that time becomes sexually mature and frantically mates, producing and raising a replacement number of children during that time.

Worldbuilding Immortals with Reversible Life Cycles

Instead of dying at the end, the species in question could revert to an earlier stage and grow up anew. This sounds bizarre but this is the life cycle of Turritopsis dohrnii, the so-called immortal jellyfish, who after reproducing reverts to a polyp stage, their equivalent to childhood. As far as anyone knows this can be done any number of times, making the jellyfish ageless.

So instead of dying after reproducing once every million years or whatever number you want it to be, each adult could revert to childhood and spend another million years living in sexual immaturity before doing it all over again.

In science fiction this is sometimes referred to as regeneration; the Time Lords in Doctor Who can regenerate. In the case of the Time Lords, their physical appearance may be completely altered in this process, along with their personality. This opens up some possibilities when it comes to worldbuilding immortals.

Marry, bear Children, Regenerate, Repeat?

In a space opera setting in particular, limiting breeding to a particular season of life, where a large number of children may be born but only in widely-separated periods, helps to prevent the immortals from dominating the cosmos’s population over time.

If, for example, there is a decade of frantic childbearing every thousand years or so, one percent of the length of each “cycle” of life would be devoted to mating. At any given time around one in a hundred of this species would be in a sexual frenzy, which is a very significant number of people in a setting like a space opera galaxy. It might prove to be a very interesting premise.

Depending on the species’s psychology, the children of these unions might remain with their parents for a very long time. Compared to even a thousand year life-span, let alone a million years, they might spend only a small fraction of their lifetimes as dependents of their parents, even if the absolute time spent is longer than humans. Still, the benefit of parental guidance given their possibly very long life experience coupled with the fact that they won’t be having children of their own for a long time might compel older children who are more or less independent to stay with their parents until their own sexual frenzy starts.

There is also the possibility, of course, of meeting someone and forming a relationship before sexual maturity, only consummating the union and bearing children after “puberty”. Evolution might even push such a species in this direction, perhaps with sexual intimacy occurring long before fertility sets in.

A wilder possibility for worldbuilding immortals is that a form of “imprinting” through physical and sexual intimacy over a very long period of time with the same person might be biologically required to become fertile and bear children. This might even trigger the attainment of “adulthood” in such a species, and start a biological clock ticking before regeneration and reversion to “childhood” begins.

In any case these regenerative species probably wouldn’t revert themselves into children until their children are more or less independent, even if they do usually stay with their parents until their own libidos kick in. So we could have a species where after the first regeneration is complete whole families are essentially in middle childhood, but the parents got there from adulthood and the children got there from early childhood.

What would happen with the couple after reversion to childhood is yet another interesting question. They might stay together as long as both them live, in which case another “biological clock” would tick and prevent them from reproducing again for a long period of time even if they are already imprinted. Put another way, the imprinting process might reset after they have their first family together.

Alternatively, they might part ways never to meet again, and a new mate may be acquired every “season”. This would be especially likely in species where the children go off on their own long before sexual maturity or libidinous desire sets in. In species where children stay with the parents parental separation might be a problem; either all the children would stay with the father or the mother or perhaps they would each stay with the parent of their choice.

Worldbuilding Immortals with high Death Rates

All the foregoing assumes that a species where most members live long lives is the idea. Another possibility is to have a species that is ageless but also has a high mortality rate. A half-life of 50 years or so, 1.4% attrition per year, means that human-like demographics and lifespan expectations would come into play even if the species were ageless.

This figure, you might notice, isn’t too different from pre-modern human mortality rates. A species that retains such high mortality into a modern era might not care about or fear death as much as we do, or they might face risks unknown to us that even technologies typical of science fiction cannot mitigate; fantastical diseases could certainly fulfill that tole.

Worldbuilding Immortals as Aggressive Reckless Risk-Takers

As a more interesting possibility for worldbuilding immortals, their own psychology might drive them to take great risks with their own lives because they find it thrilling, leading to a high rate of accidental deaths no matter what sort of technology they had. This faculty might have actually evolved, since an immortal with human-like psychology might think the risk of losing so many years of life is so high as to not make any risk worth it.

If groups who were more pre-disposed to take risks were more successful over the long term, as human history tells us they in fact are, then the psychology of immortal aliens refined by natural selection over geologically significant lengths of time might seem to us to be extremely reckless and impulsive. Far from being calm, collected, and experienced, they might in fact tend to be hotheads.

If 1.4% die in a given year, then one in a thousand will survive five hundred years, one in a million will survive a thousand years, one in a billion will survive 1500 years, and so on. It stands to reason that the most conservative members of this species will survive the longest, and thus there might be an inverse effect of more conservatism appearing among the longest-lived members who have undergone more reproduction cycles, which will find an equilibrium with the “risk-taker’s advantage” effect.

Worldbuilding immortals harbors many possibilities. It’s possible that the number of cycles one can go through and remain fertile is biologically limited for just this reason, to prevent conservative people from out-breeding aggressive people. A simpler possibility would be for the length of time between each bout of fertility to grow longer with each cycle, perhaps much longer.

As yet another alternative or supplement to this, perhaps the aggressive people simply have more children. The whole social system of this species might be designed by nature to reward aggressive people with more children, perhaps by making aggressiveness extremely attractive and conservativeness repulsive, so conservatives have a harder time finding a mate, balancing out the greater number of opportunities they have to mate.

High Sexual Dimorphism?

Yet another possibility for worldbuilding immortals, or any other race for that matter, is a partition of the species into conservative and aggressive segments. That way the benefits of both strategies could be realized. Vigen Geodakyan’s Evolutionary Theory of Sex proposes that sexual dimorphism itself evolved for similar reasons, that the division of species into a male sex that expands new traits and a female sex that conserves existing good traits provided a decisive advantage over everyone in the species pursuing one or the other strategy, thus explaining why sexual reproduction arose despite its inherent disadvantage of less genetic diversity in offspring.

In the case of worldbuilding immortals, males could take extremely aggressive risks and have short life expectancy and females could be very conservative and take very few risks, thus lengthening their life expectancy. For this to work aggressive risk-taking would need to be extremely attractive to the females.

In fact, if mating is limited to a certain season males might compete to see who can take the most risk to attract a female when in a sexual frenzy; most of the mortality that leads to the 50 year half-life might occur in this period. Alternatively, perhaps the menstrual cycle is just extremely long and males take a high level of risk permanently. There are all sorts of possibilities.

Living Memory Bank Matriarchs?

This would also imply that the population would be mostly female, unless far more males are born than females to balance it out. A significantly greater number of males than females are born as it is among humans, probably for just that reason: higher male mortality.

Another implication is that virtually all of the oldest members of such a species will be females, and they will on average have far more life experience. This might be enough of an advantage to lead to a matriarchy, where social power and authority resides with females. On the other hand, the very hotheadedness and strength of the males might lead to a patriarchy but with a revolving door of patriarchs.

Females would have more time to accumulate resources, but inheritances might pass from father to son while passing over the mother. Then again, with so long to plan and scheme it is realistic to suppose that females would not eventually gain the upper hand in controlling the resources?

I would think, however, that egalitarianism with strong sex roles is the most likely result of such strong sexual dimorphism. This is more or less the natural condition of humans, and as far as I know monogamous species in general tend to be egalitarian.


Worldbuilding immortals is fascinating stuff, and there are many possibilities. All of these ideas could even be used in the same setting! After all, there’s no rule of worldbuilding that says that you have to use just one idea in each world. In a space opera setting in particular, there could easily be many species that don’t age, with a wide range of evolutionary pathways that have dictated their psychology and lifestyle.

The idea of having a living link, even if it’s just a few females past reproductive age that serve as living memory banks for the younger members of the species and perhaps all other species in the galaxy as well, to events thousands or millions of years in the past is fascinating from both a worldbuilding and storytelling perspective. I’m certainly considering the use of it in my own setting, perhaps even incorporating some of these ideas into my Thalassa project, and I encourage any science fiction, speculative fiction, space opera, or fantasy worldbuilders to do the same.

3 Replies to “Worldbuilding Immortals in Science Fiction”

  1. Immortality doesn’t add up as a natural result of evolution. Take your half-life concept. If, for example, 10 out of a 100,000,000 live two hundred years, natural selection will ignore or re-purpose the longevity genes. These genes don’t “pay off” their energy cost and are, on the whole, detrimental to the species’ fitness.

    Various extant trees do live for thousands of years. Age increases size, which increases reproduction, given that every branch sheds cones or acorns. In this case, longevity is adaptive but even redwoods aren’t immortal.

    It is difficult to imagine an environment which selects for immortality.

    Psychologically, men and women have very likely adapted to the mortal lifespan. I suspect the long term effects of immortality would be crippling. An author might invent psychological adjustments but that would be a species both externally and internally different. Not much sense in calling it human.

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