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The pentagon of life

Chapter from Steven J. Gould's book "Since Darwin"

The cover of the book 'Since Darwin'
The cover of the book 'Since Darwin'

When I was 10 years old, I was startled by James Arens in the character of a giant carnivorous carrot in the movie "The Thing" (1951). A few months later, older, wiser, and a little bored, I watched the televised release of the film with a distinct sense of anger. I recognized the film as a political document, expressing America's worst feelings during the Cold War: the hero, a tough military man who only strives to completely destroy the enemy; the villain, an innocent liberal scientist seeking to learn more about the enemy; the carrot and the flying saucer, meant to represent the red menace; The famous last words of the film - an impassioned plea of ​​a journalist to "watch the skies" - a call to increase the horror and nationalism.

And in the midst of all this, she had a scientific thought about the way of comparison, and this article was born - the obfuscation of all the so-called absolute classification diagnoses. In the world, so we are told, live animals that have conceptual language (us) and those that lack it (everyone else). However, the chimpanzees have already learned to speak (see article 5). All the creatures are either plants or animals, but Mr. Arens looks quite human (if terrifying) in his role as a giant mobile plant.

plants or animals. The foundations of our view on the multiplicity of life are based on this diagnosis. But it represents nothing but a prejudice, born out of our status as large terrestrial animals. Although yes, the macroscopic organisms that surround us on land can be clearly classified, provided that we include the fungi with the plants, since they are rhizomes (even though the process of photosynthesis does not take place in them). But if we were floating as tiny creatures in the plankton of the ocean, we certainly wouldn't be able to make such a diagnosis. At the single-cell level, there is a lot of lack of clarity: mobile "animals" with functioning chloroplasts; Simple cells, like the bacteria, that have no clear connection to one of the groups.

Those who deal with classification exhausted our primitive opinion by recognizing only two kingdoms for all life forms - Planta (plants) and Animalia (animals). Surely there will be readers who will see the improper sorting as a trivial matter. After all, if we characterize organisms accurately, who cares if our basic divisions do not express the richness and complexity of life for the better? But classification is not a neutral shelf to hang hats on; The classification expresses a theory of interrelationships, which oversees our world of concepts. The bed of Sodom in which we put the plants and animals distorted our worldview and prevented us from understanding some important aspects of the history of life.

A few years ago, R.H. Whitaker, Cornell University ecologist, ordering life in a five-kingdom system (Science, January 10, 1969); A few years later, the program found an enthusiastic supporter - Lynn Margolis, a biologist from Boston University, who even expanded it (1974, Evolutionary Biology). The criticism that the two level at the traditional division begins among the single-celled organisms.

The view of man as the center of the world produced results in a wide and impressive field, starting with deep mining and ending with the slaughter of whales. By being integrated into the popular classification, it simply makes us make fine diagnoses between creatures that are close to us and very broad diagnoses between "simple" organisms, which are further away from us. Each new bump on a tooth defines a new type of mammal, but we tend to lump all single-celled creatures together as "primitive" organisms. However, the experts claim today that the most fundamental distinction between living things is not between plants and "superior" animals; This diagnosis actually divides between single-celled creatures - bacteria and blue-green algae on the one hand, and other groups of algae and single-celled organisms (amoebae, rhizomes, and so on) on the other. Whittaker and Margolis argue that the name plant or animal does not accurately define either group; Hence we must establish two new kingdoms for unicellular organisms.

Bacteria and blue-green algae lack the internal structures or "organelles" of more developed cells. They do not have a nucleus, chromosomes, chloroplasts or mitochondria (the "powerhouses" of more developed cells). Such simple cells are called prokaryotes (roughly, "before nuclei", from the Greek word karion, which means "carrier"). Cells with organelles are called "eukaryotes" (with true nuclei). Whittaker sees this diagnosis as "the clearest separation based on effective buffering between levels of organization in the life world". Three different arguments support this division:

1. The history of prokaryotes. The first evidence we have for the existence of life was found in rocks about three billion years old. From then until a period ending at least a billion years ago, all fossil evidence points to the existence of prokaryotic organisms only; Mats of blue-green algae were the most complex life forms on earth for two billion years. From then on, opinions are divided. third'. and. Schoff, the paleobotanist at the University of California, Los Angeles, thinks he has found evidence of eukaryotic algae in Australian rocks that are roughly a billion years old. Others argue that Schoff's organelles are nothing more than the post-mortem decomposition products of prokaryotic cells. If these critics are right, then we have no evidence of eukaryotic cells until the end of the Precambrian, just before the big "bang" of this era, which happened 600 million years ago (see articles 14 and 15). After all, prokaryotic organisms have held sole dominion over Earth for two-thirds to five-sixths of life's history. Undoubtedly, Schoff is fully justified in calling the Precambrian the "age of the blue algae".

2. A theory that explains the origin of the eukaryotic cell. In recent years, a lot of interest has arisen around the modern reasoning offered by Argolis to an old theory. The idea sounds horribly absurd at first, but it soon begins to attract attention, if not generate approval. I myself see her with great enthusiasm. Margolis claims that the eukaryotic cell was born as a colony of prokaryotic cells - that our nucleus and mitochondria, for example, come from independent prokaryotic organisms. Some modern prokaryotes can invade eukaryotic cells and live in them in symbiosis. The dimensions of most prokaryotic cells are about the same as eukaryotic organelles; There is a striking similarity between chloroplasts of photosynthetic eukaryotes and between whole cells of some blue-green algae. And last but not least, some organelles have private genes that replicate themselves, a remnant of their former independent status as complete organisms.

3. The evolutionary significance of the eukaryotic cell. Proponents of contraceptives stand on solid biological grounds when they claim that sex and reproduction are completely different matters. Reproduction continues the existence of the species (Species), and you have no more effective method of reproduction than the asexual methods - spawning and division - of the prokaryotic creatures. The biological function of sex (Sex) is expressed, on the other hand, in the mixing of the genes of two (or more) individuals to promote versatility (sexuality is usually combined with reproduction, because there is an advantage in that the mixing will take place in the offspring).

No major evolutionary change can take place except among organisms that possess a large pool of genetic diversity. The creative process of natural selection is based on the preservation of desirable genetic variants, which are selected from a large pool. The species can provide variation on such a scale, but effective sexual reproduction requires packaging of the genetic material in defined units (chromosomes). The gametes of eukaryotic organisms therefore contain only half the number of chromosomes of normal body cells. When two gametes join together to form an offspring, the original amount of genetic material is left intact. Sexual reproduction of prokaryotic organisms is, however, an infrequent and inefficient process. (This is a one-way process, involving the transfer of a few genes from a donor cell to a recipient cell.)

Asexual reproduction produces identical copies of progenitor cells, unless a new mutation appears that causes a small change. But a new mutation is a rare event, and asexual species do not develop sufficient versatility for significant evolutionary changes. Algal mats have remained kelp mats for two billion years. But the eukaryotic cell made sexuality a reality; Less than a billion years have passed and the land is teeming with creatures of every kind and species - humans, cockroaches, seahorses, petunias and snails.

In short, when we come to define the difference between prokaryotic and eukaryotic single-celled organisms we must use the highest classificatory diagnoses. This also means that two kingdoms of single-celled creatures must be established: Monera for prokaryotes (bacteria and blue-green algae); Protista for eukaryotes.

As for multicellular organisms, the two previous kingdoms - plants and animals - continue to retain their traditional meanings. So where did the fifth kingdom come from? Let's examine the mushrooms. Our previous division relegated them to the plant kingdom, probably because they take root in one place. But the similarity between them and real plants ends with this deceptive feature. Higher fungi develop tube systems, similar on the surface to those of plants; But the tubes of the plants conduct nutrients, while the tubes of the fungi transport protoplasm from place to place. Many fungi reproduce by combining the nuclei of several individuals into a multinucleated tissue, without any nuclear fusion. And so you can add to this list over and over, but all these details pale in comparison to one decisive fact: the fungi do not assimilate and do not produce food in the process of photosynthesis. They settle for the rest of their lives inside their food source and absorb everything they need from it (often after secreting enzymes for external digestion). The fungi are therefore the fifth and last kingdom.

Whittaker holds that the three kingdoms of multicellular life represent an ecological classification, not just a formal one. The plants (production), the fungi (decomposition), and the animals (consumption) well represent the three ways of livelihood in our world. And to put another nail in the coffin of our self-importance, I hasten to point out that the main life cycle runs between production and decomposition. The world would certainly be better off without its consumers.

I like the Five Kingdoms system because it presents a logical story of organic versatility. It conducts life at three levels of increasing complexity: the prokaryotic unicellular (Mona), the eukaryotic unicellular (Protista) and the eukaryotic multicellular (plants, fungi and animals). Furthermore, as we climb the stages, life goes on and becomes more diverse - as expected, since increasing complexity of planning gives rise to more opportunities for variations. The number of distinctly different forms of Protista is greater than the number of Monera. At the third level, the versatility is so great that we need three different kingdoms to encompass. Last-last, it is worth noting that the evolutionary transition from each level to the one after it occurred more than once; The benefits of increasing complexity are so great that many independent lineages converge on the few possible solutions. The thing that unites the sons of each kingdom is a common denominator, not necessarily a common origin. Whittaker claims that plants evolved at least four separate times from single-celled protist ancestors, fungi did so at least five times, and animals at least three times (the weird mesozoa, sponges, and all the rest).

At first glance, one might get the impression that the system of three levels and five kingdoms records an inevitable progress in the history of life. The ever-increasing variety and the multiple transitions from level to level apparently reflect a determined and tireless progress towards more developed forms. However, the paleontological record does not support any such interpretation. There was no steady progress in the supreme development of organic planning. Instead we had long periods of time marked by small changes, or a combination of small changes and one evolutionary burst that created the entire system. For two-thirds to five-sixths of life's history, only monar creatures inhabited the earth, and we have not found any traces of a stable progression from "primitive" prokaryotes to "superior" prokaryotes. Furthermore, since the Cambrian explosion filled our biosphere, no basic programs have been added (although there may be room to point to limited improvements within a few programs – for example, among vertebrates and vascular plants)1.

This means, the life system in its entirety was created during ten percent, approximately, of its history around the Cambrian explosion, about 600 million years ago. In what follows, I will define two main events: the evolution of the eukaryotic cell (which provided, by virtue of effective sexual reproduction, the same genetic diversity that enabled the development of additional complexity) and the filling of the ecological barrel with a multidirectional explosion of multicellular eukaryotes.

The world of life was quiet before and is still relatively quiet from then until now. The evolution of mind in recent times should be considered the most revolutionary occurrence since the Cambrian if only because of its geological and ecological history. First events above in evolution do not require the creation of new programs. The flexible eukaryotes will continue to produce innovations and diversity, provided that one of their latest products knows how to restrain itself enough to ensure that this world has a future.

1. Plants with a system of transport tubes (the translator).

The book was published by Sefrit Ma'ariv in 1995, translation: Naomi Carmel

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