Science

Why did life evolve the death function?

Survive or perish? This is not only a philosophical question, but also a real and ultimate question in biological evolution. Life and death exist dialectically and uniformly at both ends of an individual's life, and it is the important mission of reproduction that links the life and death of an individual and even the survival of a species.

For individual human beings, death often brings misfortune and suffering, and human beings in ancient and modern times have always held the dream of immortality. But from a broader timeline, individual aging and death are as important and valuable to the reproduction of the entire species as new life.

For the longevity of the species, the cruel race of nature has taught the creatures of the earth two vital laws of survival.

1. Biological evolution is based on two things: a sufficient number of individuals to sustain a population and genetic diversity to satisfy adaptation to environmental change.

2. To complete the generation change in a reasonable time and form in a limited resource environment, which is beneficial to the development of the whole species.

Let's follow the evolutionary trajectory and see how these two laws work.

Genetic diversity: A secret weapon against competition

Paleontologists have found that as early as 3.5 billion years ago, microorganisms such as photosynthetic bacteria that can use sunlight for energy production and archaea that can produce methane already existed on Earth. These ancient microorganisms once made an indelible contribution to the evolution of the Earth's environment. It is worth mentioning the cyanobacteria, which were born about 1 billion years ago, and its appearance made the atmosphere, which was originally full of carbon dioxide, methane and ammonia, start to have oxygen. Anaerobic organisms that could not adapt to the increase in oxygen would die or even become extinct. In order to survive and reproduce, all kinds of life had to evolve their own skills to adapt to environmental changes, the curtain of species competition has been opened from ancient times.

Species evolve from two main sources: genetic mutations in individuals and natural selection. The difference between these two is that mutations can produce changes for better or worse or can exist silently, while natural selection almost unilaterally removes the parts that are not good for survival. Increased genetic diversity means that there are more strategies and possibilities to cope with natural selection when it arrives. One of the most recent natural selection events observed in humans occurred in August and September 2017, when Harvard University scientist Colin Donihue had just completed work on a small population of Anolis scriptus (Anolis spp.) in the West Indies when Hurricane Irma made a surprise visit a few weeks later, and in the aftermath of the hurricane, scientists caught 100 lizards along the same path and found that their average front and rear toe plate areas were 9.2% and 6.1% larger, respectively, and that the hind legs were longer and the front legs shorter, making it easier for such lizards to survive the hurricane by holding onto tree branches [1]. It is the presence of genetic diversity that has prevented all lizards of this genus from becoming extinct after a single hurricane.

There is an inextricable link between maintaining the number of individuals in a population and genetic diversity. Since the 1960s, the International Union for Conservation of Nature and Natural Resources (IUCN) has published the Red Data Book of Threatened Species, which classifies species as endangered according to their threat level and risk of extinction, and one of the most important criteria for defining the different levels is the number of reproductive individuals in the population. The fewer reproductive individuals there are in a population, the higher the probability of extinction. We know that the offspring of consanguineous marriages are likely to have certain genetic diseases, and this is true in other organisms as well. The fewer the number of individuals in a population, the more genetically similar the offspring will be, which on the one hand is not conducive to genetic diversity, and on the other hand, the higher the risk of those offspring suffering from common genetic diseases, which will lead to a vicious circle in which the entire population will be in a state of decline leading to extinction.

In the history of the Earth, organisms have experienced five natural mass extinctions, and we are now in the process of the sixth mass extinction of species. With the intensification of human activities, the generation of more and more extreme weather and changes in the living environment, have caused more frequent and dramatic impact on other species than natural disasters, humans have an inescapable responsibility for this mass extinction of species. Those creatures that survived had to learn how to stand in the city, and of course there were some species that adapted to human activities and benefited from them. The law of survival of the fittest is like a big knife, chasing every creature forward. Survival is not easy, and live and cherish.

Sexual reproduction: the key to unlocking genetic diversity

Reproduction is fundamental to the existence of species and is a basic phenomenon shared by all life. The continuation of any species is inseparable from reproduction, and their respective reproduction methods are very different, if we want to talk about this one in detail can not finish. Existence is reasonable, no matter which direction of evolution, as long as not submerged in the history of the river is still alive on this earth, is successful.

In the early stages of evolution, bacteria, viruses, etc. usually use asexual reproduction. Bacteria reproduce by asexual bifurcation, which simply means that they replicate all of their material in two and then split it in two. The self-replication of viruses is even more simple and brutal, it even gives up the cell structure, only the outer shell and a section of genetic material, through the parasitic way to use the host, tirelessly doing only self-replication this one thing, can simply be said to be the standard bearer of the reproduction world. In some primitive animals, a new form of reproduction - solitary reproduction - has slowly evolved. Without the presence of an individual male, a single female can reproduce by copying her own DNA. Unlike asexual reproduction, where there is no reproductive system but the mature cells themselves replicate and divide, solitary females have a reproductive system and the egg cells divide meiotically, meeting the definition of "female".

However, asexual or solitary reproduction is, after all, a replication of the same set of maternal DNA, and its offspring are much less likely to have mutations than sexual reproduction. Sexual reproduction is the combination of germ cells of both sexes (e.g., sperm and egg cells) produced by the parents to form a fertilized egg, which then develops into a new individual. In sexual reproduction the offspring receive genes from both parents, and the process of fusion greatly enriches genetic diversity. In the process of evolution, the higher the organism, the more it tends to reproduce sexually in order to obtain more genetic mutations that facilitate adaptation to the environment.

There are always organisms in the transition zone of evolution, such as fungi that can reproduce asexually through germination and sexually after maturation, and jellyfish that have both asexual and sexual reproductive generations, as will be mentioned later. The vast majority of Cryptobacterium histolytica are hermaphroditic in nature, possessing a pair of sex chromosomes that allow them to fertilize their own eggs with sperm produced by their own spermatophore. However, under natural conditions there is a 5 in 10,000 chance of losing one of the sex chromosomes and producing a male offspring, and this "one in 10,000" male can produce more offspring by mating than by self-fertilization, and the hermaphrodite will preferentially use the male's sperm. Interestingly, heat stimulation of pre-sexually mature hermaphrodites is often used in the laboratory to induce the loss of sex chromosomes and thus increase the chance of male production. Perhaps this is also true in nature, where under normal conditions Cryptobacterium histolytica produces numerically and genetically stable offspring by self-fertilization, with only a very small number of males serving to enrich genetic diversity; when faced with unfavorable conditions, more offspring are produced by increasing the number of males produced to improve the chances of survival of the entire population.

Why did life evolve the death function?

Aging and death in higher organisms, like apoptosis, are the result of some programmed mechanism, and the switch that turns on the aging and death patterns is written into our genes, encoded in our DNA step by step with evolution.

The survival, reproduction and death of lower organisms, whether they are bacteria, fungi or viruses, are not actively regulated choices, but are determined by the external environment: if the environment is suitable, they can exist endlessly, and if the environment deteriorates and they cannot continue to reproduce, they will die.
A bacterial colony can multiply indefinitely as long as it has enough nutrients and space, once the nutrients are lacking, the whole colony may go to death. In this case, bacteria respond by taking up more nutrients to speed up replication without giving each other a chance to grow, while different types of bacteria can coexist in harmony for mutual benefit. The human intestinal flora is a good example of a variety of bacteria that help digest and break down different foods to maintain the overall balance.

Viruses are parasitic on their hosts and cannot survive if the host dies, so viruses have evolved their own strategies: the infamous Ebola virus can kill in as little as two weeks; HIV can destroy the body's immune system after years of incubation; and flu viruses often change "vests" and rely on constantly changing The influenza virus often changes "vests" and fights the immune system by constantly changing surface antigens. These strategies are all designed to infect new hosts before they die.

Fungi take a different path and can reproduce their offspring and expand their populations by germination before maturity; after maturity, they form sporophytes and spread out through media such as wind media so that this colony can find new ground to continue to develop before the parent body is depleted of nutrients. Some fungi have also evolved the ability to kill other species in the matter of territory occupation, and penicillin was discovered because Penicillium dissolved a staphylococcal colony on the same petri dish.

Why did life evolve the death function?

As evolution continued to move forward, plants and some lower animals evolved the ability to control the rhythm of development. When the environment is unsuitable for growth, plant seeds can enter a dormant state and temporarily turn off the germination switch. In Cryptobacterium hidrophilum, scientists Sherwood discovered the existence of developmental checkpoins before reaching sexual maturity to assess whether current conditions are suitable for continued development, and if conditions such as food shortages occur, development stops at this point and a larval form called a dauer is formed. The dauer survives for months longer than the average lifespan of three weeks and does not senesce until conditions are suitable to continue development. However, this ability disappears after sexual maturity, meaning that once sexually mature, the nematode is on its way to death [2]. Subsequent studies have confirmed that the maturation of the nematode reproductive system provides an important signal for the opening of the death switch. This genetic switch acts on the heat shock response mechanism of the cell, which responds to external stress by stimulating a cascade of signaling channels that enable the cell to counteract adverse external stimuli and maintain it in an excellent state. However, the protective heat shock response is turned off 8 hours after nematode sexual maturation, and the cells lose this protection and slowly age, leading to senescence and death of the nematode [3]. The turning on of the death switch is largely related to development and sexual maturation.

Although we admire the ability of Cryptobacterium histolytica to maintain itself as a "juvenile", unfortunately this regulatory mechanism has not been written into the mammalian genes with evolution. Evolution has given mammals a thermostatic system for regulating body temperature, a fetal mode of birth with a higher chance of survival, and a range of abilities that are sufficient to cope with changes in the external environment and no longer require them to regulate their development according to their environment. This has allowed us to move from birth to death moment by moment.

The turning on of the death switch is largely related to development and sexual maturity, which is actually a natural consequence of natural selection and evolution. Individuals cannot exist without the resources on which they depend, and as the number of individuals in a population increases, interspecific competition and survival of the fittest will be introduced. As mentioned earlier, the longevity of a species depends on the number of reproducible individuals and genetic diversity within the population. The development of the species as a whole is facilitated by a reasonable time and form of generation change within a limited resource environment. This phenomenon is most pronounced in single reproducing insects and some fish, where once the reproductive task is completed these individuals die quickly to complete the generational transition, perhaps most notably the "live and die" mayfly, and annual herbaceous plants follow the same logic.

Few creatures can escape the curse of death, and the long-established lighthouse jellyfish is the only known species that can return to the juvenile stage again after sexual maturity, an ability known as diversionary transfer that theoretically allows them to live indefinitely. The phylum Stinging Cnidaria, which includes the lighthouse jellyfish, has alternating life histories between the polyp and medusa types. In 1996, Piraino et al. conducted a transformation induction test on 4,000 medusa-type lighthouse jellyfish at different stages of development by artificially changing their environment, including starvation, sudden changes in water temperature, and lowering salinity. sudden change of water temperature, reduction of salinity and mechanical damage. The results showed that all the lighthouse jellyfish at different developmental stages were transformed from jellyfish type to hydroid type, which can be called "rejuvenation". Other species in the phylum Acanthozoa were not so fortunate, and the jellyfish died once sexual reproduction ended. [1]

And for species like us humans who have multiple opportunities to reproduce, the moment we lose the ability to reproduce is also the moment we turn on aging and death mode - it's time to leave enough resources for future generations. As to why we mammals don't go into death right away like mayflies, but instead set aside a process of aging, I think elephants may give us a clue: the experience and wisdom of the old can bring certain benefits to the population.

Why did life evolve the death function?

Is immortality really worth it?

Tolkien once wrote in The Lord of the Rings that elves have immortal life, and death is a gift from the Creator to mankind. In Tolkien's novel, it is because of the short life of human beings bursting like fireworks that a brilliant and glorious chapter of history was written. And those who are envied by the human elves, even with the incomparable ability, after a long period of time, but the wood, slowly fade out of history.

What will the world become after immortality? The movie also gives a lot of possibilities. Time Planning Bureau" and "Elysium" have depicted that even if immortality, as long as the gap between rich and poor is still there, human nature is still greedy, the world after immortality will not become better, only more endless oppression. What's more, with the current number of human beings, the Earth is already overwhelmed, if they are immortalized that is not far from the destruction of the Earth. You may say we still have the starry sea ah, unfortunately, the current level of human technology simply can not do interstellar migration. We have only the Earth, we can leave future generations the best gift is not wealth and status, but a better and healthier Earth, and no longer need to sacrifice the Earth's resources more green and environmentally developed technology.

Finally, I would like to say one thing: putting aside the life and death of an individual, a species can exist and reproduce for a long time, in a sense, is immortal.

References:

[1] Donihue, C.M., et al., Hurricane-induced selection on the morphology of an island lizard. Nature, 2018. 560(7716): p. 88-91.

[2]Schindler, A.J., L.R. Baugh, and D.R. Sherwood, Identification of late larval stage developmental checkpoints in Caenorhabditis elegans regulated by insulin/IGF and steroid hormone signaling pathways. PLoS Genet, 2014. 10(6): p. e1004426. [3]Labbadia, J. and R.I. Morimoto, Repression of the Heat Shock Response Is a Programmed Event at the Onset of Reproduction. Mol Cell, 2015. 59(4): p. 639-50.