With the development of science, the boundaries of biology are deepening. In the Linnaeus era, creatures were distinguished mainly by the characteristics that could be observed by the naked eye. Carl von Linné (1735) clearly put forward two world systems, Animália and botane, according to whether living things can move or not, which were still adopted by most textbooks until 1950s. After the widespread use of microscopes, it is found that many single-celled organisms have the characteristics of both animals and plants (such as euglena, etc.). ). This intermediate type of organism is evidence of evolution, but it is a difficult classification problem. Therefore, Hogg (J.Hogg, 1860) and Heckel (E.H.Haeckel, 1866) classified protozoa (including bacteria and bacteria).
With the development of electron microscope technology, biologists may reveal the fine structure of bacteria and cyanobacteria cells and find that they are significantly different from other organisms, so they put forward the concepts of prokaryote and eukaryote. H.F.Copeland (1938) established prokaryotes as another realm, and proposed a four-realm system, namely Monera realm, protozoa realm (including unicellular algae, simple multicellular algae, myxomycetes, fungi and protozoa), metaphase cells and metaphase cells. With the improvement and wide application of electron microscope technology and the accumulation of biochemical knowledge, the idea of making prokaryotes a world has been widely accepted and has become the basis of modern biological system classification. In 1969, R.H.Whittaker put forward a five-realm system according to the complexity of cell structure and nutritional mode. He divided fungi from the plant kingdom into other fields, namely, fungi, protozoa, fungi, plants and animals. This system is gradually being widely used, and some textbooks are still in use until the 1990s (diagram-1, 2, 3).
The evolutionary history of life has gone through several important stages. The initial life is cell-free, that is, cell-free stage. From non-cell to cell is the second stage of biological development. The original cells are prokaryotic cells, and organisms composed of prokaryotic cells are called prokaryotes (bacteria and cyanobacteria). From prokaryote to eukaryote is the third stage of biological development, and from unicellular eukaryote to multicellular eukaryote is the fourth stage of biological development. The five-realm system reflects three stages of biological evolution and three branches of multicellular biological stage, that is, prokaryotes represent the primary stage of cells, protozoa represent the single-cell stage of eukaryotes (advanced stage of cell structure), and then evolve to the multicellular stage of eukaryotes, that is, plant kingdom, fungi kingdom and animal kingdom. Plants, fungi and animals represent three directions of evolution, namely autotrophic, saprophytic and heterotrophic.
The system of five realms does not reflect the stage of non-cellular biology. Chen (1979), a famous entomologist in China, put forward a system of three universes and six universes, namely, a cell-free universe (including viruses), prokaryotes (including bacteria and cyanobacteria) and eukaryotes (including plants, fungi and animals) (list- 1). Some scholars think it is not necessary to establish a kingdom of protozoa, and classify algae and protozoa into plant kingdom and animal kingdom respectively, thus becoming a relatively compact four-realm system. Other scholars advocate expanding the protozoan world and bringing fungi into another four-domain system. Because the virus is a cell-free organism, whether it is primitive or secondary is still inconclusive. Therefore, it is still controversial to list viruses as the boundary of primitive life types.
In recent years, some scholars have proposed a six-realm system (such as R.C.Brusca et al., 1990), which divides archaea into monera realm and archaea (also translated as protobacteria, including anaerobic methanogens, etc.). ), protozoa, fungi, plants and animals. Some scholars (T. CA-valier-Smith, 1989) put forward an eight-boundary system, which divides prokaryotes into archaea and eubacteria, and eukaryotes into two superboundaries, namely archaea and metacells. The pre-hyperboundary contains only one boundary, namely archaea. The latter includes protozoa, algae (including Cryptophyta and Chromite), plants, fungi and animals. Some scholars believe that this demarcation system is more reasonable and clear.
To sum up, we can see that there is no unified opinion on the demarcation of biology at present. But in any case, judging from the fossil record of paleontology for 3 billion years or the present situation of living creatures on earth; From morphological comparison and physiological and biochemical examples, the evolutionary direction of organisms from prokaryotic to eukaryotic, from simple to complex, from low to high is revealed. Biological boundary shows the development process of life history.
The relationship between creatures is very complicated, but the basic requirement for their survival is nothing more than eating food to gain energy, occupying a certain space and reproducing. Organisms have various ways to solve these problems. In terms of obtaining nutrition, those who can use carbon dioxide, inorganic salts and energy to synthesize the food they need are called autotrophs, while green plants and purple bacteria are autotrophs. Therefore, plants are producers of food, and the food connection between organisms begins. Animals must get nutrition from autotrophs. Plants are eaten by herbivores, which are the food of carnivores. Therefore, animals are predatory heterotrophic and food consumers in the biological world. Fungi have the ability of catabolism and absorption, and are in the position of reducing agent. All these show that these three kinds of organisms are the most basic, and their nutrition in evolution and development, as well as the coordination of organisms in the ecosystem and the role they play in the process of material cycle and energy cycle are all complete and systematic.