One Finite Planet

One Finite Planet

the surprises hiding in life expectancy numbers.

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Optimum population of humans: Ideally, how many people can, or should, the Earth support?

It can seem like the human population can grow forever, but analysis makes it clear growth must stop eventually. The question becomes at what level should it stop?

Do we go for the maximum possible people before everything collapses, even if average living standards could be far better with a smaller population? Is it like a chicken farm in an egg farm, where having less chickens is seen as preferable if it means chickens get better living conditions? What population strikes the right balance for humans?

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Why Population growth even before the explosion?

Throughout history, although no other species on Earth has experienced such long term overall population growth, even before the recent population explosion, the human population kept slowly growing.

Yes, we recently had an unprecedented population explosion, driven by is hidden by by the near elimination of previously tragic infant mortality, but against the background of long term growth, many of us never didn’t even realise their was an explosion.

But what drove population growth even before the explosion? What will now happen as the explosion ends?

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Optimum Population Instinct: nature’s birth control.

Human birth rates are falling at a rate that has some fearing population collapse, but could this be a natural instinctive biological repose to threats of overpopulation, rather than any cause for alarm?

This raises the question as to what controls population in other species and, why is overpopulation rare? Would any species just multiply like bacteria in petri dish whenever possible until resources and the ecosystem collapses? Or do species, and even potentially humans, have instinctive mechanisms to constrain population at a more optimum level for long term survival? In practice, resource constraint and predation alone as population controls would for many species would result in repeated huge population swings, so logically, there must be more.

Analysing population mechanisms in other species may provide some interesting insights and possible answers to at least a large part of what is happing with birth-rates that will determine whether our future is population is one of: continued growth, collapse, or stability, on a planet where population of all life is not growing.

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Population on a Finite World: No Vacancies.

The total biosphere of the planet is slowly contracting. Every niche on Earth where life exists, is filled by species to maximum capacity. Except for brief periods following Earth changing events, or other change of circumstances, there have been no new ‘vacant’ liveable environments on Earth for over 500 million years. Any increase in population of one species, requires a reduction in population of other species, in a process Darwin declared “survival of the fittest”.

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The main surprise is that life expectancy reveals very little about how long people typically live. From the 1500s till around 1800, life expectancy throughout Europe hovered between 30 and 40 years of age, and it was only as recent as the 20th century, that life expectancy rose from 49.2 to 80.3 years. Yet famous historical figures from 2,000 years ago, typically lived to around 70 or longer.

the surprises hiding in life expectancy numbers.

The main surprise is that life expectancy reveals very little about how long people typically live. From the 1500s till around 1800, life expectancy throughout Europe hovered between 30 and 40 years of age, and it was only as recent as the 20th century, that life expectancy rose from 49.2 to 80.3 years. Yet famous historical figures from 2,000 years ago, typically lived to around 70 or longer.

Looking far back as we can know, a full lifespan has always been around 70 years or longer. The biggest change has not the length of a human lifespan, but instead, the dramatic increase in percentage of people who get to enjoy that lifespan. How ancient people lived was nothing like life expectancy suggests, and we have not yet extended lifespans to the extent you may think.

During the 20th century, life expectancy rose from 49.2 to 80.3 years

average lifespan vs life expectancy vs full lifespan.

Average lifespan for a population is calculated by recording the age at death of each individual, and once the last person has died, calculating the average. Consider a small group of 10, where 1 died as an infant (before the age of 1) another as a 10 year old, another as an 18 year old teenager, another at 40, another at 50 and remaining 5 lived into retirement and died on average that age of 70. It could be said that for this group:

  • average lifespan = 0+10+18+40+50+ (5×70) = 46
  • full lifespan = 70
  • life expectancy was 46 years

Average lifespan is simple, add up individual lifespans and divide by population size. The problem is, you need to wait until everyone in that population dies to have a statistic.

Life expectancy requires a larger population, but gives a continuous figure without waiting for everyone to die, by looking at the parentage of the population in every age group who to calculate the risk of dying at each age, in order to project, based on current calculations, statistically how long a person will live.

‘Full lifespan’, or just ‘lifespan’ is calculated by considering only the people deemed to die of age related causes or ‘old age’. This excludes the baby, the 10 year old and 18 year old, as well as those who would be considered to have ‘died young’. In a sense, it is the age a person will live if it could be said “nothing goes wrong”. Unlike average lifespan, with a simple formula, and life expectancy with a more complex formula, a value judgement is required to in borderline cases arbitrarily allocate deaths as ‘age related’ or not, making ‘lifespan’ an inexact value. As a concept I feel it is very important, but there is no scientific exact number.

A trap is that we often think ‘life expectancy’ as reflecting ‘lifespan’, but the two numbers will only very close when only very few people ‘die young’. Confusing these two can lead to surprising differences between reality, and what we would think is/was happing in a society from misreading life expectancy.

lifespans in the past were almost as long as lifespans today.

I watched a discussion on the prospect of research enabling people to live longer, and I was disappointed the discussion was somewhat derailed by myths and of misunderstandings of history, from misinterpreting ‘life expectancy’ as representing ‘full lifespan’. But it was not until I took a deeper look, that it became clear how different the past was from what I imaged.

A did a search on random figures from the past, and found: Galileo born in 1654, died at 77, Newton born in 1727 died at 84 and Darwin born after 1800 in 1809 died just a little younger at 73.

Yes Mozart famously died at 30, but was poisoned. What about ancient Greeks? Pythagoras lived to around age 75, back 500 years BCE! Socrates? Lived to around 71. Plato? 80. Aristotle? 60..61! Finally someone died a little younger, but it becomes very clear that historically even 2,000 years ago when life expectancies were so much shorter, that did not mean that it was typical to die as a young adult. Many people lived to an age not that much less than we consider a typical life span today.

Further evidence comes from societies long isolated from the rest of the world, such as the indigenous Australians. It is clear that prior to contact with Europeans, tribal elders could also life to an age of 70, and even an age of 100.

When you think about it, despite the life expectancy in early 20th century being little over 50, we have people born at that time over the age of 110. Indigenous Australians are genetically very similar to Europeans and evidence is that genetics have not changed much in at least 100,000 years. This means humans even 100,000 years ago were genetically capable of living until at least around 70, even if the harsh living conditions at that time made such a full lifespan uncommon.

Reduced life expectancy is mostly due to child mortality.

Looking at historical data, where, despite the average life expectancy being around 30, Plato lived until 80 and almost everyone who was famous who was not assassinated or something lived until at least 60, it at first looks like the numbers just do not add up.

But then you realise, historically, at least 2 thirds of children born died before reaching adulthood. Dying in childhood would most often mean the person never became famous, especially considering that, of those who died, a significant proportion died as infants in their first year.

Throughout human history up until the 20th century, the age with of people at the greatest risk of death for a human being was 0 years. There is still at 10% chance of death in the first year for those in Afghanistan today, highlighting the importance of medical care in humans surviving even to their first birthday.

Clearly many of those who do not live to have their own children would would have died as infants, and the average age if child deaths would be less than 10 years, but even assuming that average age of those who died as children to be 10 years, if two thirds of a population dies on average by the age of 10, then even if the remaining 1 in 3 all live to an average age of 70 years, the average life expectancy at birth would be (70 + 10 + 10) ÷ 3 = 30 years.

It becomes clear that in a time without modern medicine, statistical life expectancy would be well below the age that represented a full life.

In most countries today, life expectancy today is close what people would consider a full lifespan, which means if we hear a country has a life expectancy of 80, we think of people typically living until their death at around 80 years of age. However historically, it was not like this at all. When life expectancy was around 30,years, almost no one died a the age of 30, and this would have been the age at which people were close to their lowest risk of dying. Like the colours of squares on a chess board where the average colour would be grey, even though there are no grey squares, with all being black or white. Almost no one died at the average age, but instead mostly either died very young as children, or lived to a relatively ‘ripe old age’.

No, humans never lived lives limited to just 30 years of age.

Well….yes, they only lived until 30 average, but just as the average roll of a dice is 3.5 but you could still roll a six, the average outcome from the subset who survived childhood and lived to become adults, would have been more like living until 50 or more. Consider chimpanzees who have a life expectancy in the wild of just 15 years, but those who live past 12, typically live then until close to age 30. Just take those numbers and approximately double them, and you get the picture for ancient humans,

I sometimes encounter statements of how primitive humans had a life of expectancy of only 30 years, so they had to have children very young, in order to be able to raise them before they died. These statements are misguided. By the time one of these paleolithic humans was a parent, they had already survived their young period of danger, and were very like to live until at the very least around 50 years of age.

If people were equally likely to die at all ages, then if the average was dying at 30, there would be an even spread of people dying at ever age from 0 as at 60. Reality is, humans are most vulnerable as infants and when older, so most people historically either died as a young child or a grandparent, unless they met with misadventure in battle or were otherwise unlucky.

Our genes have not changed, and ancient people tended to get exercise and stay clear of junk foods excessive alcohol and smoking. Even if the lack of any medical care meant most died as children, once clear of childhood, there was a reasonable lifespan ahead, just as there is in indigenous groups still living traditional lifestyles today.

Remember, that in 1920 the live expectancy was just 56.4, now in 2022, there are millions of people now over the age of 102, who were all born 102 years ago in 1920, and almost doubled that life expectancy.

How much longer can life expectancy get?

We are close to the limit on eliminating early deaths.

Although some people do live until and even beyond the age of 100, we currently lose health rapidly from around 70 years of age and the qualify of life diminishes from that point. Yet many in many countries, life expectancy already exceeds 80 years. There are still, tragically, many early deaths the we would hope medicine will in future be able to prevent, but life expectancy is getting close to the level of a full lifespan under current conditions.

Despite wishful claims that the ’40 is the new 30′, and ’50 is the new 40′, there has not really been a breakthrough in people aging slower, beyond that of undoing the harm from recent backward steps in terms of diet, smoking and lack of exercise. A person does not suffer accident or disease, now about as long as our rate of aging allows. There seems little point in being kept alive much longer if health continues to decline through aging.

But have not really started on ending ageing.

The next step, and one that has not really advanced much yet, is to tackle aging itself.

Living in a world where everyone ages, aging seems both natural and unavoidable, but this is not necessarily the case. Consider:

  • Single cell organisms never age.
  • It is also possible for multicellular plants and animals to live forever.
  • Every human ever, over hundreds of thousands of years, is made from cells made by previous humans, resulting in a long chain of effective immortality.
  • The cells in our bodies range from a few hours old through to decades, meaning we are constantly being renewed, and can be much older than our own cells.
  • Our DNA effectively runs a timer, deliberately limiting our lifespan.

Search “aging as a disease” and you will yield multiple results confirming that aging could be seen as a potentially curable disease. Or a least a disease that where the effects could be delayed.


There are a few key points:

  • Improvements in life expectancy so far have predominantly resulted from eliminating causes of early deaths rather than extending the overall biological clock.
  • Shorter life expectancies as low as 30 years from the past, result from the average age being much shorter, mostly due to infant and child mortality resulting in many very short lives significantly lowering the overall average.
  • The human biological clock still runs at very much the same speed it always has, and life expectancy is now close to the limits of that clock.
  • Further significant increases in life expectancy can only result from slowing the biological clock, which does appear, at least in theory, possible.


  • 2022 May 20: Started discussion on further increase in life expectancy.

Planned: More discussion is needed on extending life expectancy.