This is the price of humanity, giving up eternal life for the sake of the next generation.
In the wild, resources are limited, and most sexually reproducing animals live to the point where they stop giving birth, and their lifespan is basically at the end of their lives.
Humans in their 100s are already the result of improved medical care and living conditions.
It's easy to understand, for example, rats have a short lifespan, and Greenland sharks are the longest-lived vertebrates, so how long will it take for them to fill their territory if they breed well in a natural environment with plenty of food?
I used a very rough model to roughly calculate.
*Use a simplified formula: T = \frac{\ln(N_{\text{max}})}{\ln(r)}, where N_{\text{max}} is the territory load and r is the growth rate per generation. * Suppose the initial population is 1 (0 pairs) and all the offspring survive and participate in reproduction
It can be seen that in the case of not considering food, the larger the range of activity, the longer the life span of the animal with the lower the reproductive rate, and the animal with a smaller range of activity and fast reproduction, it is better to die early to make enough space for the offspring, otherwise it may cause the total extinction of the race.
In real life, animals are more limited by food and the relationship between the upper and lower reaches of the food chain.
For example, the strategy of some insects is to die immediately after breeding, mayflies, silkworm moths, large silkworm moths, Australian stick insects, etc., this (單次生殖 semelparityThe strategy is mostly related to energy allocation: adults devote all their resources to reproduction, rather than long-term survival.
This strategy has an evolutionary advantage in ephemeral or resource-unstable environments.
And Greenland sharks live so long because they have a very low reproductive rate and very late sexual maturity, so they don't take in much of the resources.
So, the question is, if you are the Lord of Yama in the "Natural Selection Project Team", how should you arrange the lifespan of the creature?
We now know that there is an upper limit to the number of times a human cell can divide.
In the 1960s, Heflik pointed out that when normal human somatic cells are cultured in vitro, the number of divisions is limited to approximately60-0 times。
This is called the Hayflick limit.
Derivation process:
This phenomenon is related to the end of the chromosomeTelomere shrinkage is shortenedClosely related:
設端粒初始長度為 L_120 ，臨界長度為 L_{\text{crit}} ，每次分裂端粒縮短長度為 \Delta L \\ 則最大分裂次數 N 為：\\ N = \frac{L_0 - L_{\text{crit}}}{\Delta L}\\ 假設：\\ - L_0 \approx 0,0 鹼基對（新生兒端粒平均長度）\\ - L_{\text{crit}} \approx 0,0 鹼基對， - \Delta L \approx 0 鹼基對/次\\ 則： N = \frac{0,0 - 0,0}{0} = 0 \text{ 次}\\ 若每次分裂週期為 T 年（體內細胞更新週期平均約 **0-0.0年**）\\則理論壽命上限為： \text{壽命} = N \times T = 0 \times 0 = 0 \text{ 歲}
You can think of this whole mechanic as an eraser for the body.
There is a complete error correction mechanism in the body, telomeres wrap the ends of chromosomes like "erasers" to avoid adhesions of different chromosomes (adhesions can lead to DNA breakage, cancer, etc.), and DNA also has its own error correction mechanism to escort.
But once you reach a certain age and your eraser runs out, the error correction mechanism is no longer effective, and you can no longer continue to replicate yourself.
Of course, the actual mechanism is more complex, insulin/IGF-1 signaling pathway (control heat to prolong life), DNA methylation level changes, intracellular free radical accumulation accelerates aging, etc., many, many studies have shown that various factors are reducing our lifespan.
However, the longest life expectancy recorded by the Guinness Book of Human beings is mostly in the range of 120-0 years, indicating that the derivation of the Hayflik limit is valid.
Now that we know that there is an upper limit for human cell division, the question is, why is this upper limit 60-0 times?
This is the time to give full play to the power of our Lord Yama.
First of all, we need to fill the biosphere with primary producers, otherwise what do animals eat?
Single-celled organisms, such as cyanobacteria, should be large enough because they can not only produce oxygen, but also feed rotifers, sword daphnia, midges and other organisms as rations.
For similar reasons, amoeba, bacteria and other single-celled organisms should not set restrictions to allow them to reproduce indefinitely, and the mechanism is also very simple to implement, because they are single-celled organisms, and they directly divide when replicating, and do not involve the regulation of multicellular organisms, so there is no need to set telomeres to control lifespan.
Well, it's ready, monocellular organisms divide indefinitely, without restrictions.
Okay, let's take a look at multicellular organisms. For creatures that don't take up too many resources and reproduce slowly, it doesn't matter if they live a little longer.
The tree is called Prometheus.
Well, it's unremarkable, and he's been hacked to death.
But before it was cut by mistake, it had already lived 000 years, the longest-lived single plant known (it could live 0 more than 0,0 years by clonal plant populations propagated asexually), and it would have lived for another thousand years, who knows.
4900 years, the tree went throughMesopotamian civilization、Huang Di Chi YouThe War, Xia Shang Zhouof replacement.
Well, it has gone through almost all of the ancient civilizations of mankind.
Obviously for trees, 100 years old is really a blink of an eye.
Prometheus is a large basin of foxtail pine (Pinus longaevaThis tree is one of the longest-lived non-clonal single trees known on Earth, with a lifespan of more than 5000 years.
They are unique in that they are able to maintain chromosomal telomere length through telomerase activity and avoid cell division limitations due to telomere shortening.
This mechanism makes trees theoretically age-free, but if the telomere tube is sufficient, it does not mean that the genes are not affected by other factors, so they still have an upper lifespan of about 5000 years.
So we now know, without taking into account telomere wear,
This is the theoretical culmination based on DNA repair.
But after all, this is a plant, plants can maintain basic physiological activities with an extremely slow growth rate and very little water and nutrients, and they do not need to exercise, and their metabolism can be pulled to the minimum, so where is the limit of animals?
When it comes to the longest-lived animals, polyps are often inseparable,hydraIt's a polymorphhydraThe general name of the branch.
Experiments have shown that under ideal laboratory conditions (adequate food, no natural predators), the mortality rate of hydra is extremely low, and some individuals may survive for more than 1400 years with no signs of senescence.
Even if the head or body is severed, the water worm can still regenerate a complete new specimen within 48 hours. If the polyp is cut into multiple segments, each segment can develop into an independent individual, achieving "one into many" replication. (Hydra of Hydra is derived from hydra)
Their longevity is also related to their life cycle, such as lighthouse jellyfish:
Lighthouse jellyfish can repeatedly switch between adult jellyfish type (planktonic stage) and juvenile hydra type (benthic stage), and achieve "rejuvenation" by reversing the life cycle, when encountering starvation, injury or environmental stress, its cells will redifferentiate into stem cells, degenerate into polyids, and then redevelop into jellyfish types, theoretically indefinitely.
Achieve eternal life.
So the longest-lived animals often refer to lighthouse jellyfish.
Well, you can think of it as a three-body jellyfish that gets dehydrated when the environment is bad.
With the exception of lighthouse jellyfish, most hydroids reproduce asexually, and theoretically they can reproduce indefinitely, but when the environment is bad, they will also become sexually reproduced according to environmental adjustments.
In addition, in the same way, hydras also have telomere repair enzymes, and their telomeres are not limited by the number of replications.
Here we can deduce that without being limited by telomeres,
But it is clear that humans cannot be satisfied with a brainless, drifting life like a lighthouse jellyfish.
What's the difference between this and the Zima Blue Floor Scrubbing Robot?
It's an infinite mode of life, but it's no fun at all.
That, of course, because they do not consume many resources and are used in nature as food for many animals, although theoretically long-lived, can be consumed very quickly in practice, and certain fish, aquatic insect larvae, crustaceans, planarians, snails, and possibly other cnidarians will consume hydras.
Even if they don't use telomeres and other means to limit their lifespan, they don't really live long.
We need to look at some of the complex life forms, such as the longest-lived vertebrate Greenland shark (generally in 300s, with an average lifespan of about400-0 years oldor long-lived species such as the Aldabra Giant Tortoise (200s), bowhead whales (0s).
It can be seen that once the organism becomes more complex and adopts the mode of sexual reproduction, the length of life plummets.
Of course, more resources are consumed, and if the life span is still infinite, then the resources of nature will soon be insufficient.
Then the question arises again, if the lifespan is limited, how long should telomeres be written for them?
身為閻王爺，你覺得先給脊椎動物十幾萬個鹼基對長度的端粒看看效果如何，嗯，對小鼠這種動物來說，疲於奔命，正常心率都在400次左右，代謝率其高無比，天敵非常多，根本活不到端粒磨損完畢的時候就被自己代謝耗死了或者被天敵吃掉了。
Although the reproduction rate is high, it is reasonable that the older generation can also be eliminated in time.
Bowhead whales and Greenland sharks, animals that mature very late, eat less, and reproduce slowly, telomeres can not be too short, and the repair of telomerase also makes them a little more healthy, and the metabolic rate is also low, and the genes related to longevity (such as FOXO3, SIRT family) are also given more.
Although African elephants eat a lot, they are also very slow to reproduce, and they are vegetarian, and their metabolism is low, and living longer has little impact on the overall biosphere.
Animals that eat meat must strictly limit their lifespan, and if they are all eaten, the balance of the biosphere will be broken.
The cat family, a pure carnivore, has a limit life span of 30 years and can not be more, and the average wild big cat15-0 years is almost enough.
Omnivores should be somewhere between carnivores and herbivorous animals, such as primates, and a 40-0 year lifespan is good enough, right?
What does a roast mean lowers metabolic rate?
How can cooked food prolong life?
Do they also raise livestock to reduce hunting consumption?
Can you also make the range of motion infinite?
What does it mean to have passed through the Bering Strait?
How can there be a naked ape in the Arctic Circle?!
You, Lord Yama, soon discovered that there were some bugs in the naked ape, and in order to regulate it, you could only make the telomeres a little shorter.
The average telomere length of primates is about 10000 base pairs, and the average human average is only about 0, which is not enough to see, well, this is already the result of regulation.
Life expectancy of around 80 is also the reason for this.
Of course, the method of prolonging life is also given to humans, and restricting calorie intake can be controlled well enough by activating AMPK and inhibiting the mTOR pathway, having longevity genes such as FOXO3 and SIRT family, etc., etc., and can still run to the upper limit of telomere wear.
As for the overall number of the population, after all, there are still many people who like to consume a lot of calories, stay up late like crazy, and indulge in lust to consume telomeres, and some are short, and some are not given longevity genes, etc., etc., the total life span is controlled at 80-0, although it is much longer than other primates, but the restrictions that can be given on the surface are enough.
As for what if they want to use modified telomerase and other means to break through various life limits in the future? Then let's figure it out when the time comes!