Why did life evolve the function of death?

Survival or destruction? This is not only a philosophical question, but also the ultimate question of the real existence of biological evolution. Life and death exist dialectically and uniformly at both ends of individual life. What connects the life and death of individuals and even the survival of species is the important mission of reproduction.

For human individuals, death often brings misfortune and suffering. Humans throughout history have always dreamed of immortality. However, from a broader timeline, the aging and death of individuals, like rebirth, are extremely important and valuable to the reproduction of the entire species.

In order to ensure the longevity of the species, the cruel natural selection has taught the creatures on Earth two vital laws of survival:

1. There are two foundations for biological evolution: a sufficient number of individuals to maintain the population and genetic diversity to adapt to environmental changes.

2. In an environment with limited resources, completing generational changes in a reasonable time and form is beneficial to the development of the entire species.

Let us follow the trajectory of evolution and see how these two laws work.

Part 1

Genetic diversity: the secret weapon against competition

Paleontologists have discovered that as early as 3.5 billion years ago, there were photosynthetic bacteria that could use sunlight to generate energy, as well as microorganisms such as archaea that could produce methane. These ancient microorganisms have made indelible contributions to the evolution of the earth's environment. It is worth mentioning that cyanobacteria were born about 1 billion years ago. Its appearance made the atmosphere, which was originally full of carbon dioxide, methane and ammonia, begin to have oxygen. Anaerobic organisms will die or even become extinct if they cannot adapt to the increase in oxygen. In order to survive and reproduce, various life forms have to evolve their own abilities to adapt to environmental changes. The curtain of species competition has been opened since ancient times.

The evolution of species mainly comes from two aspects: individual gene mutations and natural selection. The difference between the two is that the changes caused by gene mutations can be good or bad or exist silently, while natural selection almost overwhelmingly eliminates those that are not conducive to survival. The increase in genetic diversity means more strategies and possibilities for coping when natural selection comes. The most recent natural selection event observed by humans occurred in August and September 2017. Colin Donihue, a scientist at Harvard University in the United States, had just completed research on the small Anolis scriptus population in the West Indies. A few weeks later, Hurricane Irma suddenly visited. After the hurricane, scientists caught 100 lizards along the same path and found that the average area of ​​their front and rear toe plates was 9.2% and 6.1% larger than before, respectively. In addition, the hind legs were longer and the front legs were shorter. Such lizards were more likely to hold on to branches and survive the hurricane [1]. It is precisely because of the existence of genetic diversity that this genus of lizards has not been completely extinct after a hurricane.

There is an inextricable connection between the number of individuals that maintain a population and genetic diversity. The International Union for Conservation of Nature and Natural Resources (IUCN) has been publishing the Red Data Book of Threatened Species since the 1960s, which classifies species into different endangered levels according to the degree of threat and risk of extinction. One of the most important criteria for defining different levels is the number of reproductive individuals in the population. The fewer reproductive individuals in a population, the higher the possibility of extinction. We know that the offspring of close relatives are likely to have certain genetic diseases, and this is also true in other organisms. The fewer individuals in a population, the more similar the genes of their offspring are. On the one hand, this is not conducive to genetic diversity, and on the other hand, the risk of those offspring suffering from common genetic diseases is also higher. Once a vicious cycle falls, the entire population will be in a state of decline and eventually lead 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. With the intensification of human activities, more and more extreme weather and changes in the living environment have caused more frequent and severe impacts on other species than natural disasters. Humans have an unshirkable responsibility for this mass extinction. Those creatures that survived had to learn how to gain a foothold in the city. Of course, some species have adapted to human activities and benefited from it. The law of survival of the fittest is like a big knife, chasing every creature forward. Survival is not easy, so cherish it while you are alive.

Part 2

Sexual reproduction: the key to unlocking genetic diversity

Reproduction is the basis of species existence and a basic phenomenon of all life. The continuation of any species is inseparable from reproduction, but their respective reproduction methods are very different. If we were to talk about them in detail, this article would not be enough. Existence is reasonable. No matter which direction of evolution it is, as long as it has not been submerged in the long river of history and still exists on this earth, it is successful.

In the early stages of evolution, bacteria and viruses usually reproduce asexually. Bacteria reproduce by asexual binary fission, which simply means that all their substances are copied into two and then split in two. The self-replication of viruses is even simpler and cruder. They even give up the cell structure, only have a shell and a piece of genetic material, and use the host in a parasitic way. They tirelessly do only one thing, which can be said to be a model in the field of reproduction.

In some primitive animals, a new mode of reproduction has gradually evolved - parthenogenesis. Females can reproduce by replicating their own DNA without the presence of male individuals. The difference from asexual reproduction is that asexual reproduction does not have a reproductive system but is self-replicated and divided by mature cells themselves; while parthenogenetic animals have a reproductive system, and the egg cells exist in the form of meiosis, which meets the definition of "female".

However, asexual reproduction or parthenogenesis still replicates the same set of maternal DNA, and the probability of mutation in the offspring is far less than that of sexual reproduction. Sexual reproduction is a reproductive method in which the reproductive cells of both sexes (such as sperm and egg cells) produced by the parents combine to form a fertilized egg, which then develops into a new individual. In sexual reproduction, the offspring receives genes from both parents, and the genetic diversity is greatly enriched in the process of fusion. In the process of evolution, in order to obtain more genetic mutations that are conducive to adapting to the environment, the more advanced organisms tend to reproduce sexually.

There are always organisms in transitional zones in evolution, such as fungi that can reproduce asexually through budding and reproduce sexually after maturity, and jellyfish that have asexual and sexual generations, which will be mentioned later. Caenorhabditis elegans is in the transition zone between parthenogenetic reproduction and sexual reproduction. In nature, most Caenorhabditis elegans are hermaphrodites. They have a pair of sex chromosomes and can fertilize their eggs with sperm produced by their own spermatophores. However, under natural conditions, there is a 5 in 10,000 probability of losing one of the sex chromosomes and producing male offspring. This "one in a million" male can mate to allow hermaphrodites to produce more offspring than self-fertilization, and hermaphrodites will give priority to using male sperm. Interestingly, in the laboratory, heat is often used to stimulate hermaphrodites in the early stages of sexual maturity to cause the loss of their sex chromosomes, thereby increasing the probability of male production. Perhaps this is also true in nature. Under normal conditions, Caenorhabditis elegans produces offspring with stable numbers and genes through self-pollination, with only a very small number of male worms playing a role in enriching genetic diversity. When encountering an unfavorable environment, it will produce more offspring by increasing the number of male worms to increase the chances of survival of the entire population.

Adult Caenorhabditis elegans. The one in the upper picture is a hermaphrodite, and the one with the "little hook" in the lower picture is a male.

Image source: www.wormbook.org

Part 3

Active regulation: a giant step forward for evolution

The aging and death of higher organisms, like cell apoptosis, are the result of the operation of a certain programmed mechanism. The switch that turns on the aging and death mode is encoded in DNA and written into our genes step by step as evolution progresses.

The survival, reproduction and death of lower organisms, whether 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; if the environment deteriorates and they can no longer reproduce, they will die.

A bacterial colony can reproduce indefinitely as long as it has enough nutrition and space. Once nutrition is lacking, the entire colony may die. In this case, the bacteria's way of dealing with it is: for bacteria of the same type, they take up more nutrition to speed up replication and do not give the other party a chance to grow, and for bacteria of different types, they can coexist harmoniously and benefit each other. The human intestinal flora is a good example. Various bacteria help digest and decompose different foods to maintain overall balance.

Viruses parasitize hosts, and if the host dies, the virus cannot survive, so viruses have also evolved their own strategies: the notorious Ebola virus can kill people in just two weeks; HIV can destroy the human immune system after lurking in the body for many years; influenza viruses often change their "vests" and rely on constantly changing surface antigens to fight the immune system. These strategies are all to infect new hosts before the host dies.

Fungi take a different path. Before maturity, they can reproduce by budding to produce offspring and expand the population. After maturity, they form spores and spread through media such as wind, so that the colony can find new territory to continue to develop before the mother's nutrition is exhausted. Some fungi have also evolved the ability to kill other species in the process of occupying territory. Penicillin was discovered because Penicillium dissolved the Staphylococcus aureus colony on the same culture dish.

Penicillium under a high-power microscope Image source: Internet

As evolution continues, plants and some lower animals have evolved the ability to control the rhythm of development. When the environment is not suitable for growth, plant seeds can enter a dormant state and temporarily turn off the germination switch. In Caenorhabditis elegans, scientist Sherwood discovered that there are developmental checkpoints before reaching sexual maturity, which are used to assess whether the current conditions are suitable for continued development. If there is a shortage of food, development will stop at this point and form a larval form called dauer. Dauer can survive for several months, which is much longer than the normal average lifespan of three weeks, and will not age until conditions are suitable for continued development. However, this ability of Caenorhabditis elegans disappears after sexual maturity, which means that once sexual maturity is reached, the nematode is on the road to death and will not turn back [2]. Subsequent studies have confirmed that the maturity of the nematode reproductive system provides an important signal for the opening of the death switch. This gene switch acts on the cell's heat shock response mechanism. It responds to external stress by stimulating cascade signal channels, enabling cells to resist adverse external stimuli and maintain cells in an excellent state. However, the protective heat shock response is completely shut down 8 hours after the nematode reaches sexual maturity. The cells lose this protection and slowly age, leading to aging and death of the nematode[3]. The activation of the death switch is largely related to development and sexual maturity.

Although I envy the ability of C. elegans to stay in its "juvenile" state, it is a pity that this regulatory mechanism has not been written into the genes of mammals with evolution. Evolution has endowed mammals with a thermostatic system to regulate body temperature, a viviparous method with a higher chance of survival, and a series of abilities to cope with changes in the external environment, so they no longer need to adjust their own development according to the environment. This also makes us move towards death without stopping from birth.

Part 4

Generational change: Death is inevitable

The activation of the death switch is largely related to development and sexual maturity, which is actually the inevitable result of natural selection and evolution. The existence of individuals is inseparable from the resources they rely on for survival. As the number of individuals in the population increases, inter-species competition and the survival of the fittest will inevitably be introduced. As mentioned earlier: the longevity of a species depends on the number of reproductive individuals in the population and genetic diversity. In a limited resource environment, completing the generational change in a reasonable time and form is conducive to the development of the entire species. This phenomenon is most significant in single-reproducing insects and some fish. Once the reproduction task is completed, these individuals will quickly die to complete the generational change. The most famous may be the mayfly that "lives in the morning and dies in the evening". Annual herbaceous plants also follow this logic.

Very few creatures can escape the curse of death. The famous Turritopsis dohrnii is the only known species that can return to the larval stage after sexual maturity. This ability, called differentiation transfer, theoretically allows them to have an unlimited lifespan. Cnidaria, including the Turritopsis dohrnii, has a life history of alternating generations, namely polyp and medusa. The polyp generation reproduces asexually, producing many jellyfish buds through budding. This generation does not age or die; once sexually mature, it enters the medusa generation and reproduces offspring through sexual reproduction. In 1996, Italian researchers Piraino and others conducted a transformation induction experiment on 4,000 medusa-type Turritopsis dohrnii at different developmental stages by artificially changing the environment, including starvation, sudden changes in water temperature, reduced salinity, and mechanical damage. The results showed that Turritopsis dohrnii at different developmental stages all showed the phenomenon of transformation from jellyfish to polyp, which can be called "rejuvenation." Other species in the phylum Cnidaria are not so lucky. Once sexual reproduction ends, the medusa will die. [1]

For species like us humans who have multiple opportunities to reproduce, the moment we lose our reproductive ability is the moment we start aging and dying - it's time to leave enough resources for our offspring. As for why we mammals don't die immediately like mayflies, but reserve an aging process, I think elephants may give us a revelation: the experience and wisdom of the elderly can bring certain benefits to the population.

Elephant groups are usually led by an experienced female elephant. Image source: Internet

Part.5

Is immortality really worth it?

Tolkien once wrote in The Lord of the Rings: Elves have immortal life, and death is a gift from the Creator to humans. In Tolkien's novels, it is precisely because of the short life of humans that bursts out like fireworks that a brilliant and glorious chapter of history is written. However, the elves, who are envied by humans, even though they have unparalleled abilities, are dying after a long time and slowly fade out of the stage of history.

What will the world be like after immortality? Movies also give many possibilities. Both "In Time" and "Elysium" depict that even if immortality is achieved, as long as the gap between the rich and the poor remains and human nature is still greedy, the world after immortality will not become a better place, but only endless exploitation. Moreover, with the current number of humans, the earth is already overwhelmed. If everyone is immortal, the destruction of the earth will not be far away. You may say that we still have the stars and the sea, but unfortunately, the current level of human technology is simply not enough to achieve interstellar migration. All we have is the earth, and the best gift we can leave to future generations is not wealth and status, but a better and healthier earth, and greener and more environmentally friendly advanced technology that no longer requires sacrificing earth resources.

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

References:

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

[2]Schindler, AJ, LR Baugh, and DR 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 RI Morimoto, Repression of the Heat Shock Response Is a Programmed Event Response at the Onset of Reproduction. Mol Cell, 2015. 59(4): p. 639-50.

Author: Song Mengjiao

Author unit: Center for Excellence in Molecular Cell Science/Institute of Biochemistry and Cell Biology, Chinese Academy of Sciences

Produced by: China Science Expo

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