Two questions will occupy us in this column, both of which concern the relationship between the whole and its parts. Let's start with Nir's philosophical questioning: in the animal world (fish, reptiles, mammals and birds) we know that all the organs of the body serve the brain, without a brain the rest of the body has no right to exist. But what happens in the plant world? On which part of the tree can it be said that all the other parts serve it?
Two questions will occupy us in this column, both of which concern the relationship between the whole and its parts. Let's start with Nir's philosophical questioning: in the animal world (fish, reptiles, mammals and birds) we know that all the organs of the body serve the brain, without a brain the rest of the body has no right to exist. But what happens in the plant world? On which part of the tree can it be said that all the other parts serve it?
More of the topic in Hayadan:
On the first failure in the question: identifying the "animal world" with the vertebrates (fish, reptiles, mammals) while ignoring a much larger variety of animals in which the nervous system is distributed and the brain is less vital We were already in another column. Another fallacy underlying the question is related to the imagery we use when we think of a complex system as a plant. Why is it obvious to Nir that there must be a part that is "the most important" in a living creature, similar to the shareholder or the CEO of a company or the captain of a ship?
In an attempt to understand a complex system, we are pushed almost involuntarily into metaphors. The primary reservoir for images and analogies includes our bodies and the family-social circle into which we grow. Society and its institutions have been treated since the dawn of human thought as analogous to a living body and biological systems as human societies. Thus the father was, until recently, the "head of the family" and leaders of states are "heads of state". The image of the head in which the sense organs and communication are concentrated for the leader seems to us self-evident and embedded in the language. But, when Aesop, the great Greek author of parables from the sixth century BC, asks for an image for the relationship between a commander and his soldiers, he places leadership precisely in the stomach. Perhaps the part of the body that is closest to the plate and receives the resources first is a more successful image for leadership than the thinking organ? Metaphor is an essential tool but used carelessly it is misleading and even dangerous. Advocates of eugenics, for example, often used the image of society as a living creature to point to disabled people as degenerate body parts, infections or tumors in the "social body". And to the point of the question: biologists see a living creature first of all as a machine for spreading its genes, therefore the most important part of the tree is the apple that fell from it. From a thermodynamic point of view, As we saw on another occasion, a living being is an "island" of order (or negative entropy) that exists from destroying order and increasing the entropy of the universe around it. As an "entropy increasing machine" the plant is a system in which every part is essential.
And on a similar note, "Reese" asks: Is each cell in a multicellular creature a living creature in itself? Am I a collection of lots of living things?
No, Reese, you are not "a collection of many living things" but one multicellular. The transition from unicellular to multicellular is a development that changed the raw material of evolution. Biologists know of only 5 transitions of this type in the history of the animal world: from molecules replicating in the ancient "stock" to the first cell, the joining of simple cells to form the developed cell (eukaryotic) containing cell organelles, the appearance of sexual reproduction, the joining of cells to form multicellular organisms and the last and most modern transition : Creating communities such as bee swarms or ant nests. These transitions are unique because the process of natural selection begins to act on a new unit: a mutation in a cell will die out or survive only according to the benefit or damage it brings to the entire multicellular creature, just as the fate of a change in the characteristics of a working bee depends only on its effect on the chances of the entire hive to provide queens for new hives. When it becomes part of a tissue in a multicellular organism, the cell gives up the basic function of any living organism: reproduction. We are used to thinking of the animal world in terms of a war for existence, but almost every cell in our bodies is an example of evolution's ability to create Cooperation and altruism. The single cell will not hesitate to commit suicide and display proteins on its envelope that will mark it to the immune system as a candidate for destruction. Even whole tissues and organs are willing to give their souls for the common good as the leaves of the trees in the fall season prove.
Fortunately for evolutionists, there are many intermediate stages between single cells and multicellular animals like us. There are families of amoebae and algae that include both distinct unicellulars and relatives that form clusters of cells that have a defined division of functions. There are even creatures that switch between a unicellular and multicellular life form depending on the environmental conditions. Dictyostelium discoideum It is a species of amoeba that lives in abundant conditions as a single cell for everything in the soil. In times of scarcity, the amoebas coalesce to produce "fruiting bodies" - a ball of amoeba that will produce spores for the next generation is carried on a "stem" made up of amoeba that sacrificed themselves to allow the survival of their sisters. This wealth of forms at the bottom of the evolutionary branch that leads to Rabbi Taiyim is a window that allows a glimpse into the motivation of our ancestors who joined together to create the first Rabbi Taiyim and the tools that led them to this creation. The multicellularity is not a one-time flash: it was invented at least 16 separate times in breakthroughs that led to the creation of the plants, animals, fungi and lineages of algae. Every reinvention of multicellularity required the development of different mechanisms of attachment and intercellular communication. Therefore, the way cells bind to tissue in an animal is different from the way cells bind to tissue in a plant, fungus, or algae. Thus, in animal cells, proteins that cross the cell membrane are responsible for the connection with their neighbors, in plants, the cells are attached with cellulose-based adhesives, and the brown algae useAlginate - A gel well known to molecular cooking enthusiasts.
The first herbivores were red algae that appeared about 1.2 billion years ago. And the first multicellular animals evolved a little more than 600 years ago. There are those who attribute this transition to the appearance of a new factor in the primitive ecology: the predators. Until their appearance, the main evolutionary pressure was directed to the competition for resources and their efficient utilization: in terms of efficiency and thrift, bacteria have no competitors. A single, small and adaptable cell is a creature that evolution loves and indeed 99% of the creatures around us are unicellular. With the appearance of predators in the environment, size became an advantage for the first time. It is difficult for microscopic hunters to deal with large prey, i.e. with a cluster of cells. The first step is simply creating clusters of cells, each of which maintains its uniqueness and is able to manage even on its own. Since single-celled cells reproduce by division resulting in two cells side by side, all that is required is not to move away immediately after division. Bacteria create a "biofilm", meaning a sticky layer that adheres to the substrate so that it is difficult for a predator to peel off and swallow a single bacterium. In an experiment in which a carnivorous amoeba was introduced into the liquid in which a unicellular algae lived, it became clear that after 100 generations the algae preferred to live their lives in groups of 8 cells. The evolutionary race also brought the carnivores to coalesce into groups when the same adhesive substances that were originally intended as a trap for bacteria were now used to attach the cells to each other. This is the presumed evolutionary origin of collagen is the cement that also holds our tissues together. When the monocells clung to each other, additional advantages to size were discovered, chief among them the possibility of division of labor. The same mechanism that allows the individual cell to move through Shotton or eyelashes (cilia) is also used in cell division and a single cell cannot sail and divide at the same time. When cells move in a group some cells can divide while their friends row. Already at this primitive stage, the problem of "cheating" arises: why would a member of the group choose to row with his neighbors when he can completely give up the expensive rod and spend his time dividing? Such altruistic behavior is possible when there is synergy, that is, when the benefit from cooperation exceeds the benefit that each cell derives from selfish behavior. A parasitic mutation will cause the creature to quickly lose the advantage of cooperation and therefore become unstable. A higher stage in multicellular evolution is differentiation - cells in a certain location specialize in one function such as movement, secretion, or sensing. This specialization comes at a heavy price: only a few cells get the job of creating reproductive cells, meaning that only they will pass their genetic material to the next generation. This concession is only possible if the cell is sure that the person who takes over the role will pass on the same genetic heritage, that is, only if all the cells are identical in terms of the hereditary material. The risk of a parasitic cell taking over the role of reproduction is the reason why every multicellular creature makes sure to go back, if only for a moment, hundreds of millions of years back and go through a unicellular stage (fertilized egg). Only cells that originate from the division of the same cell can trust their neighbors enough to give up reproduction. The need to prevent a hostile takeover of the reproductive process is strong enough to justify the heavy price of including such a sensitive and vulnerable stage in the life cycle. The sponges are the first multicellular, but they are not "true multicellular" (Eumetasoa) because the cells do not differentiate into tissues with a defined function. The sponges give birth to and live mobile lives in which it is possible to distinguish areas specialized in movement and those responsible for nutrition.
The origin of the evolved animals is perhaps an ancient spongebob that, instead of settling down as an adult in Bikini Bottom, continued to swim and cultivate this uniqueness. The first tissue that appeared was the intestine: cells that gave up the ability to move and hunt and specialized in secreting digestive enzymes into the internal space. But the tissue that turned us from a spongy collection of cells into a real animal is the one that produces sex cells. From the moment when the production of sperm and eggs became the special expertise of a separate group of cells, the genetic material of all other cells ceased to be used as raw material for the evolutionary process. Since reproduction has become the dedicated work of one tissue, there is no longer competition between the cells.
Since the cells of the body have lost the ability to distribute genes independently, we cannot be called "a collection of many living beings". The situation in which a cell in the body returns to being an individual that takes care of self-reproduction without considering its friends is called cancer and the immune system, unfortunately, does not always know how to treat it. Only one cell population in multicellularity remains competitive: the germ cells. The reduction division (meiosis) creates two gametes that differ in the genetic load they carry. The testicles house the only group of cells that have a conflict of interests and accordingly the wild competition of the sperm cells to the egg is not always sporty. Aggression (killing or paralysis) of a sperm cell against its race partners has been observed in a variety of animals from flies to mice. Competition may alternate with cooperation in animals where the sperm cells meet the sperm of other males in the same female. In such species, sperm cells have been observed that coalesce into groups that coordinately activate the whips to get the competitors. A special example of the egoism of reproductive cells was observed in plants where both male stamens and ovules are formed from the same tissue. Mitochondria, the energy organ of the cell, contains its own DNA and this hereditary load is passed only in the female cells. Mitochondria, itself a descendant of an ancient cell that assimilated into another cell at the dawn of evolution, has an interest in "its" cell becoming an egg and not a sperm. Indeed, mutations in which the mitochondria inhibit the creation of male gametes and encourage the production of eggs occur in such plants. The plants are saved from extinction thanks to competing mutations in the DNA in the nucleus because when the majority of reproductive cells are female, those who produce male gametes have a better chance.
Well Rees: the only cells in your body that are "lots of living things" are precisely the ones that will rush to leave it at the first opportunity.
Thanks to Dr. Rick Grosberg, Dr. Richard Strathmann, Dr. Elizabeth Ostrowski, Gad Shaulsky for their help
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Ahla wrote