How can evolutionary pressures lead to a decrease in modern day human lifespan?

Question

I, Charlemagne Darwinius, am a great scientist who has devoted his life to the study of the evolution of different species. Priests speak of man being created by a supernatural being called God, who became increasingly angry with them due to their descent into wickedness. To punish man, he sent a great flood to wipe all life from the earth, sparing only a few humans and animals who took shelter in a great ark. For 40 days and nights, the flood drowned all living things on the planet, until the waters receded. The survivors on the ark were able to carry on their existence, and today's humans are the descendants of those survivors. However, as a way to humble humanity, God decreased their lifespans going forward. When once humans would live hundreds of years in good health, now they would be lucky to live a few decades. As they aged, their bodies would grow old and decrepit until dying, as a way to remind humanity of who was really in charge.

Recent archeological evidence has revealed that their actually had been a great flood at one point which encompassed the entire world, leading to an extinction level event. Other evidence has emerged which suggests that human civilizations existed before this flood, with numerous empires and cultures inhabiting the earth. This gives credence to the stories told in these parables, which has forced me to update my theories. If the evidence is true, then humans had lived much longer, healthier lives in the past, and were far more able than we are today. For reasons left unexplained, our lifespans had shortened over millennia, to the point where those who get past a century is incredibly rare. This is despite the fact that we are much more technologically advanced than our ancestors, with access to better medicine and care.

My phrase "Survival of the fittest" is a way of describing the mechanism of natural selection. The biological concept of fitness is defined as reproductive success. It is best understood as survival of the form that will leave the most copies of itself in successive generations. Shorter lived humans have outcompeted those with longer lifespans, appearing to make us less fit. What explanation could be given for this occurrence?

Answer 1

There are several benefits to humans living ~80 years compared to living hundreds of years:

Population Growth

If humans die faster, then they need to breed more to compensate. This would lead to an explosion of reproduction and human populations, as where before a human would maybe have one or two kids over the course of 400 years, they are suddenly having 4 or more within the first 40 years of their life. This sudden increase leads to greater amounts of manpower required for many things and helps cement the human status of a global super-apex species.

Geographical Expansion

With so many more humans walking around, it's natural for them to expand their influence at a comparable rate. More people means more settlements, more people exploring unknown territories, and more people to form armies with which to tame either the wild or their neighbors. As a result, where people once confined themselves to small pockets scattered throughout the world, suddenly there are fledgeling kingdoms and civilizations popping up everywhere. After tens of thousands of years of human progress more or less stagnating, it only takes a couple thousand years for entire countries to start springing up like weeds.

Record Keeping

Before, every village would have one or more elders whose job was to retain the knowledge and history of their clan. This knowledge is built up over centuries and is passed to the individual who would become the next elder. However, if humans no longer live as long, that knowledge would get passed more often, and more and more knowledge would get lost in the repeated retellings. As such, humans needed a way to store their knowledge in forms that would be immune to the failings of the mind and the ravages of time, leading to rapid development and widespread use of writing systems.

Technological Innovation

As a consequence of an increase in record keeping, knowledge suddenly becomes a lot more accessible to the general public; no longer needing to receive a sermon from an elder who may take days retelling something and details getting lost or mutated over time, the average person can just go to the local library or record house and read whatever they needed to know. The records can also be duplicated by scribes and taken all over the world, allowing a vast sharing and mingling of different cultural and scientific wisdom. With so many more eyes and minds viewing the same information, technological breakthroughs are guaranteed to happen and at a much faster rate.

Agriculture

More people means more ability to claim and protect land, which means large-scale agriculture finally becomes a feasible option. Before, it was neither necessary nor practical to create farms larger than what would feed half a dozen families, as there simply weren't enough hands to harvest the crops nor eyes to watch for animals, pests, and the inevitable march of nature. But with more people to help manage the farm, you could have farmland that spanned miles of countryside, leading to an abundance of food and overall prosperity.

Genetics

Now that humans are breeding more often, new generations are being created more rapidly. This results in a higher rate of beneficial genetic code getting integrated into the overall gene pool, as well as the rate of potential breakthrough mutations. And once tribes get to the point where they are large enough to start interacting on a grand level, there will be a much larger and more diverse gene pool leading to a more stable and successful place in the biological scale.

Answer 2

Shorter lived humans have outcompeted those with longer lifespans, appearing to make us less fit. What explanation could be given for this occurrence?

Shorter lead time of new features and consequent faster adaptation.

The math is easy: if a species takes X years to be able to reproduce, no mutation will spread with a pace faster than once every X years. Therefore a species reproducing faster than that will take less time to adapt to a changing environment.

That's one of the reason why bacteria develop resistance to antibiotics very quickly.

Answer 3

Resources

Survival of the fittest doesn't mean having the most copies of that species/lifeforms. It is having the most stable way to keep this subset of lifeforms to stay alive. This is important. Humans for example used to get a lot of children. This is a strategy in times with high child mortality. With many children you have more that might make it, as well as a bigger community that can help raising children.

Yet when we look at countries that have plenty of food and good medicine, we suddenly stop expanding so rapidly. We devote more resources to a few or even singular children. Despite having enough resources and low mortality, we don't explode into populations that are unsustainable.

This is important. Expanding rapidly normally means that resources will get tight. Many species will then meet with rapid decline of the species if resources dwindle, leaving them less strong over time. Although not practiced by all species, expansion in good times is best slowed.

Here you can see something interesting as well. In humans we keep our elderly and infirm, even though they might not contribute or be able to reproduce anymore. Yet for many species this isn't a given. If an individual becomes old or weak, it can be more advantageous if it dies. It'll not eat up more resources, allowing for more new children to be raised. Strong new generations that can keep the species alive and stable.

A short life span is then ideal. This is practiced by so many species it's hard to keep track. For example, fruit flies could in theory live much longer and reproduce more frequently. Yet they live short bursts of life, dying just as quickly as they started. Leaving a nice niche for the new generations to fill.

Even in humans we can see a supposed forced shorter live span. Although not proven, in some Scandinavian tribes it's suggested they practiced Ättestupa, suicide or murder of the elderly. To prevent being a resource drain, they threw themselves (or were thrown) off a cliff.

Things you do for your children, right?

Answer 4

Lack of selective pressure for longevity

Shorter lived humans have outcompeted those with longer lifespans, appearing to make us less fit. What explanation could be given for this occurrence?

This is a misconception, although one that has been quite pervasive in ageing studies. There is no evidence that humans have a genetic "kill switch" that can be modulated by evolution: genetic determinants of human lifespan are, basically, determinants of health, because health failures are the principal age-linked reason of death.

Perhaps more surprisingly, the same is true of other animals (with possibly the exception of some eusocial species like honeybees). All known animals basically live until something in their body gives way, even when there is no evolutionary advantage in doing so (e.g. because you're no longer able to produce offspring and you don't contribute to the fitness of the next generation). This extends to stereotypically short-lived insects (fruit flies can live up to 3 months if treated well, by the way). A dogmatic understanding of evolution might make you think that evolution would yield some pressure to get rid of "dead wood" but this is not actually what happens, either because evolution has absolutely no problem with random junk hanging around, or because there is no mechanism to directly shorten lifespan without also affecting health (including, usually, reproductive output).

So why doesn't every animal live indefinitely long lives? Because their body does give way, and it gives way with a probability that increases with age. Now we all have repair mechanisms to work against the forces of biological entropy, but they suffer from time-dependent decay as well, and in any case repairs are inferior to the original, and these suboptimal patches accumulate over time (as an aside, the reason the axolotl is able to regrow body parts so efficiently is probably because it makes much better repairs than other animals: if wounded, it doesn't scar but rebuilds the same skin as an embryo would).

So if you want an evolutionary explanation to your scenario, you would have a situation where the repair mechanisms have become less effective. This can be due to active selection, for example because a disease arose that used cells or mechanisms of the wound repair machinery to attack its host; or it could simply be passive drift if repair is not actually selected for. This could happen if, for a sufficient number of generations, almost all humans died relatively young of non-aging reasons.

For a real-world example, we all harbour the prion protein in our brain. The progress of "regular" prion disease (not the cow-derived "infectious" kind) is estimated to lead to symptoms of CJD around the age of 200. So we don't have any mechanisms to protect against it, because it would bring no selective advantage. Imagine a version of humanity where almost everyone dies of infectious disease, environmental toxicity or violence around the age of 30: they may not have any pressure to retain mechanisms against stroke, heart attacks or any of the other late-middle-age disease. They may develop a prion variant that kills by the age of 60 and it would go unnoticed because nobody makes it to 60 anyway. When, later, this cause of early deaths disappears, their health-preserving machinery is permanently degraded, and they never recover their original longevity.

Answer 5

Lifespan was not the trait driving selection.

There were other traits that gave our ancestors a selective advantage over more long lived forebearers. Smarts, disease resistance, efficient metabolism, patience with kids, better alcohol tolerance might be some. These fit mutants happened to be short lived. But they left more children whom themselves lived to reproductive age.

Answer 6

MORE RAPID POPULATION GROWTH

Shorter-lived species typically reach maturity faster. The earlier age a female can give birth, the shorter the time between generations will be. If, say, a short-lived female gives birth to 4 children, of which 2 are females, at an average age of 12 rather than 24 for a longer-lived race, the population will double in half the time, all else being equal - and multiply a thousandfold while the longer-lived would only multiply 32 times.

This means that the shorter-lived species would recover much faster from any decimating event, whether war, epidemic or natural disaster - a huge evolutionary advantage.

Of course, the advantage would be even greater if the species remains long-lived, but matures at a younger age. This would increase the ratio between mature (productive) and immature (unproductive) members of the species,

Answer 7

Evolution doesn't select for creatures most likely to live longer; it selects for creatures most likely to reproduce, which necessitates surviving to reproductive age but not necessarily much further.

Creatures can still benefit their species' evolutionary fitness by helping to raise children even after they cannot produce children themselves. On the other hand, they continue to consume resources, and once they reach a certain age they require care from the younger generation themselves, instead of being able to contribute it. Our morality dictates that we do spend our resources caring for the elderly, but natural selection might favour a species which doesn't need to do this.

Answer 8

Deny the parasites easy hosts

In general, a trait which exhibits itself after a creature has preocreated, cannot be selected by evolution. The genes which cause it have already been passed on, whether they are beneficial or deleterious for the old age of that creature.

There are exceptions. Where the children learn from their parents into adulthood, or where grandparents and further extended family play an important role in nurturing children, genes that reduce life expectancy will also impinge on the survival prospects of the children, and will tend to be selected out. Hence social species tend to be longer-lived than similar but asocial ones.

What about the reverse? One theory for the evolution of sexual reproduction is that it developed to shuffle the genes of the next generation, so that parasites (which reproduce many time faster) which have become more evolutionarily fit for infecting a parent, acquire no advantage with respect to infecting the parent's descendants.

But the child does share half its genes with each parent, so perhaps those parasites are able to acquire some degree of advantage if the parent persists to an older age than is required for raising sufficient children to adulthood? In which case, all things being equal, evolution will select genes which shorten life expectancy, rather than genes which lengthen it.

So the answer might be, a deadly endemic parasite with a short life cycle and an unusually high ability to adapt to its host's genetics by evolution.

(Closely related, are killer genes such as Thalassemia and Sickle-cell. Why have they not evolved out of the human race, since they often kill before puberty? Because the disease is caused by inheriting two copies of a recessive gene, one from each parent. Inheriting just one copy is beneficial. It gives the individual considerable immunity against the malaria parasite which also causes a killer disease.)

Answer 9

The modern economy

Consider the modern economy and retirement. Up to the time you retire, you produce wealth for yourself and your family. As soon as you retire, you begin to drain that accumulated wealth. In most societies, even if the elderly is no longer physically productive, they still provide a net benefit to the family via childcare, advice, governance, etc.

However, consider what happens if more importance is placed on financial resources. As a very particular example, if we look at the current trend in U.S. medicine, birth mortality is actually increasing, and the cost of medical care continues to climb. If this trend continues, we could reach a point where childbirth has a substantially higher chance of survival for those able to afford the absolute best care, which would require basically the life savings of a middle class family. You could imagine this trend intensifying for various reasons, e.g. moving to a more automated society, where the few jobs still requiring human workers become astronomically expensive and the competition for acquiring limited human services is fierce.

The families most able to afford this staggering cost would be those where the older members die soon after retirement, and in particular die quickly so that their deaths don't incur substantial medical bills. If every generation dies shortly before their grandchildren would be born, that inheritance can be spent on better quality childbirth and early childhood care, leading to significantly higher successful childbirth rates.

Answer 10

Allow this Charlemagne Darwinius a few knowledge ahead of his real-life counterpart:

1. Genetic bottleneck:

In your question, you state:

Recent archeological evidence has revealed that there actually had been a great flood at one point which encompassed the entire world, leading to an extinction-level event

Needless to say, such an event would not have affected everywhere equally, leading to some parts of ancient civilization being affected much more than others. Couple that with an understanding that an organism cannot just "make" a new trait, but adapt an already existing part of its biology to express differently, you can argue that the flood had caused all the longer-living people to die off, leaving the shorter-living people.

For extra credibility, you can add actual evidence of evolution from our world: Convergent Evolution. The ancient long-living superhuman from the past have died from the flood but as technology, medicine, and safety of your narrative world improved, people start to become longer-living, more capable (through nutrition, education, etc.) once more, thought through a different mechanism.

2. Shorter life is not always a negative trait

Most of the answers to this question have stated this already, so I will not add more, but a shorter life cycle means faster adaptation, biologically or culturally ("Meme! the DNA of the soul!"). Furthermore, a shorter life cycle tends to indicate a more r-selected gene, and thus faster to replenish their population in response to environmental changes (like say, a flood, for example)

Answer 11

The shorter lifespan would have to lead to more offspring. Two possible routes:

• The elderly take up resources that could be used to have more children without any corresponding benefit.
• The presence of the elderly discourages reproduction. Youngsters feel more free without the eye of the elders, or the reminder of mortality in their parents' death encourages them to reproduce as a means of immortality.

Answer 12

There's only so much an individual can do

Successful evolution in the face of shorter lifespans means a penchant for planning with a view beyond the needs of one member of the species or even one generation. Thus, the shorter life span would inspire or pressure the species to work together for the survival of the many, not just the survival of the one.

Answer 13

Faster evolution

If you reproduce faster and have more generations in a shorter span of time, then you generate and select for desirable traits faster, thus giving you an adaptative edge. Take bacterias for instance, they can overcome in days antibiotics in petry dishes just by reproducing and dying a lot.

Answer 14

If this takes place in a world where rapid degradation of living creatures is the norm (for instance, a world with a very high background radiation which damages living creatures, or a world filled with monsters or diseases which tend to cause early injury or death, etc.) than it may be more advantageous for the genome to let existing damaged creatures die relatively quickly and focus instead on creating new "fresh" undamaged creatures which will reproduce rapidly before they too become damaged. Many species follow this path by focusing on rapid reproduction rather than hardy individuals that will likely live a long life.

Quicker mutation can also be a benefit in scenarios like these (having the species evolve more rapidly to deal with specific types of monsters, etc.).

You may find the biological distinction between "k strategist" species versus "r strategist" species to be illuminating. The comparison table at the following website in particular is worth a look:

https://www.cs.montana.edu/webworks/projects/stevesbook/contents/chapters/chapter002/section004/blue/page003.html

Answer 15

Inbreeding.

With so few humans on the ark, their genetic diversity was lost. This has caused health problems to build up which resulted in this accelerated aging and shorter lifespans.

Answer 16

Why Do Humans Live Long In The First Place?

You've got some good answers here, but the easiest way to think of this is: why do humans live as long as they do now? Adding pressure for shorter lives is unlikely to do too terribly much if there's still pressure pushing for longer lifespans.

Intelligence = Slow Maturation

Humans have a relatively lengthy gestational period (~9 months) and a truly glacial maturation rate (~25 years for full maturation). This is almost completely due to humans having such advanced brains, with evolutionary pressure pushing toward intelligence.

Shorter lifespans is going to necessitate a quicker maturation process. To that end, you will either need to a) select against intelligence (unlikely), or b) find a way to develop brain matter more quickly (this could be as simple as a major dietary change - perhaps a massively increased protein intake?). Perhaps these older humans matured much more slower due to an inability to process proteins efficiently, leading to longer childhoods and slower brain maturation.

Caregiving

Humans are well-primed toward civilization. We're pretty excellent caregivers. We take care of children. We take care of grandchildren. We take care of other children. We take care of the sick. We put ourselves in danger to save others routinely, and we glorify people who do so. The social contract of responsibility we have is very much built into us.

Perhaps these older humans were even more charitable, with people hundreds of years old still contributing to society and those around them. Take away those positive contributions and you remove a pressure for older ages.

Cancer

This doesn't fit with my general theme, but I wanted to point it out anyway: absent other causes, cancer tends to put a time limit on our lives, even if it might be centuries out. A handful of humans on this planet seem to have genes that defy this, but perhaps that could be a remnant of those older humans.

Answer 17

Most obviously, evolutionary pressures cannot lead to a decrease in lifespan, be it modern-day human or what.

If you care, can you explain what led you to think they might?

Answer 18

Well, if your god drowned all life on earth, he did a poor job since meermaids ruled over everything for 40 years :D

Joke aside, many bacterial and fungal infections (especially after a flood) could force the need to further mutate the genetic code. Like the information theorem: if you happen to have only windows computers in the whole world, nothing would stop a worm of eating them all up in a relative short time. There could be a "coronarious" virus going on for ages, with many mutations, so humans need to adapt and mutate their genome in response. More children == more different dna.

Some animals have such a short livespan that they literally only live to reproduce, then die. This is more commonly found in insects. The reason for this? Well, probably they concentrate on the most important thing for their species to survive. Naturally, this would be bad for more "intelligent" live forms who need a lot of time to actually learn motoric and social intelligence. So why only live short? It could be that the time between generations is very huge. Imagine a planet that rotates in such a matter that you practically have like 30 years of ice or storms, then a short period of 20 years in which live can bloom. Naturally this would imply the humans had found some way to hibernate or the likes (magic, tecnology, just laying eggs instead? )

Also, you can always trade a long live for some pretty benefits. Like dinosaurs, they grew to an absurd size, on the other hand that would pretty much limit their livespan notably. Or dogs, better example since it happened through artificial evolution that some are big and some are small, and the small ones live longer