The systematic relationship between animals and plants and their historical origins

According to the five-kingdom classification system of organisms, plants and animals, as two high-level classification levels of organisms, evolved from another level - some different categories of the kingdom of protists, and are parallel to the latter. Relationship.

Such a classification system highlights the hierarchical relationships between the major levels of living things from simple to complex, from low-level to high-level.

​However, it also has shortcomings. In particular, it does not reflect the systematic relationship and historical origins of animals and plants, the two most basic and progressive groups of modern organisms.

In fact, not only can the ancestral types of plants and animals be found in protists, but their ancestral types in primitive organisms even have a certain identity. This identity can still be found in a modern primitive organism - Euglena.

Euglena or Euglena

Euglena is a single-celled protist living in water. The body is long fusiform or cylindrical, with a notch at the front end from which a flagellum protrudes. The reaction force generated by its swing in the water can promote body movement; there is a red eyespot with photoreceptor function below the notch. (The Euglena gets its name from its eyespots). If they are placed in water containing organic matter, Euglena can absorb the organic matter "food" in the water through its cell membrane and live an animal-like heterotrophic life. These properties lead zoologists to believe that Euglena is a "protozoa." But at the same time, the cells of Euglena have chloroplasts containing chlorophyll, which can carry out photosynthesis and produce their own nutrients. Therefore, botanists believe that it is a "native plant"; because there is no cell wall outside its cells, botanists gave it another name - Euglena.

This "animal and plant duality" of Euglena has led many scientists to believe that animals and plants have a common ancestor - it is likely to be some kind of single-celled protist that lived in ancient waters similar to Euglena. In the long process of evolution, some of their molecules have strengthened the structure and function of movement and feeding along with changes in the genome. At the same time, they have gradually "lost" the structure and function of photosynthesis. Eventually, their lifestyles have transformed into completely alien ones. autotrophic; other molecules change to a completely autotrophic mode along with other changes in the genome. The former way represents the emergence of the earliest animals, and the latter way represents the emergence of the earliest plants.

Primitive protozoa and protozoa were both single-celled at the beginning of their differentiation. Later, they developed into multi-cells.

In the paleontological community, the understanding of the divergence time of plants and animals is constantly updated with the continuous discovery and accumulation of fossils, the advancement of new research ideas, research methods, and the interaction between disciplines.

As early as when Darwin published "The Origin of Species" in 1859, he also put forward the difficulties in the theory of evolution at that time with a realistic and scientific attitude. One of them was the famous "Cambrian Explosion."

The so-called Cambrian explosion refers to the seemingly sudden appearance of numerous fossil animal groups discovered by scientists in the strata from 570 million to 500 million years ago, including sponges, coelenterates, and annelids. animals, molluscs, arthropods, brachiopods, echinoderms, and primitive chordates. Based on these findings, some scientists believe that these animals suddenly appeared on the earth in an explosive process. At the same time, the explosion of this animal also illustrates the beginning of the separation of animals and plants. Therefore, they speculate that animals and plants The time of divergence was close to the beginning of the Cambrian period 600 million years ago.

In 1949, paleontologist Sparig discovered a large number of skeletonless marine invertebrate fossils in the Ediacara area north of the Adelaide Mountains in southern Australia, and determined the age of this fossil fauna. It is the early Cambrian period. But 10 years later, paleontologist Glasnan came to three extraordinary conclusions after careful and detailed research on this animal group: (1) Whether it is coelenterates, annelids or arthropods in this fossil animal group, , no Cambrian genera and species have been found among them; (2) the composition of microfossils in this fossil group is completely different from that of the Cape of Good Hope; (3) the Bunt rock formation in which this fossil fauna is buried It is 1,000 meters thick and is not continuous with the Cambrian strata covering it (this is called an unconformity contact in stratigraphy). Therefore, the Bunt rock formation should belong to a different geological age than the Cambrian. At the International Geological Congress held in 1960, the scientific community officially named this fossil fauna the Ediacaran Fauna. Absolute age determination by various methods since then has shown that the age of the Ediacaran fauna is 680 million to 620 million years ago. In 1974, the International Union of Geological Sciences identified the Ediacaran fauna as late Precambrian. At this point, paleontologists have pushed back the time when invertebrates appeared on a large scale on the earth, so the time when animals and plants diverged is also considered to be older than 600 million years ago.

At the same time that the Ediacaran fauna was correctly re-understood, there was also a breakthrough in the discovery and research of plant fossils, mainly plants formed by eukaryotes. In 1969, Claude discovered single-cell green bacteria and golden algae fossils that lived 1.3 billion years ago in the Baker Spring Formation in eastern California, USA; in 1971, Scheff and his colleagues discovered in the Bitter Springs Formation in Australia. Found some plant fossils belonging to dinoflagellates, red algae and green algae that lived 900 million years ago.

Since the mid-1970s, more and more fossils of animals and plants from the Precambrian period have been discovered. On the one hand, the fossil groups that have been discovered have increased in number and location. For example, the Ediacaran fauna has now been found in the Precold Prevail of Southwest Africa, North America, the United Kingdom, Scandinavia, the Soviet Union, China, etc. They were successively discovered in the strata of the late Bian Period, and the members of the fossil group developed from the initial 5 genera to 19 genera. By the early 1980s, 56 genera had been identified. On the other hand, new fossil groups are discovered from time to time. For example, Chinese scholars discovered abundant baldbrachial and annelid fossils in the late Precambrian strata in the Huainan area of Anhui Province. Their specimens and internal structures are clear and intact. It is very rare at home and abroad, and seven genera of 740-million-year-old annelid fossils and 840-million-year-old annelid fossils have been identified.

Both brachiates and annelids are higher invertebrates. Therefore, scholars believe that animals had a long history of development before this. Therefore, they believe that the distinction between animals and plants began more than 1 billion years ago.

Since the 1970s, some scholars have studied the molecular structure of proteins. Proteins are compounds composed of amino acids. Many amino acid molecules are linked together in chain-like polypeptides to form proteins. A protein molecule consists of one or more polypeptide chains. The sequence of amino acids in the polypeptide chain constitutes the primary structure of the protein molecule. The primary structure of proteins - the sequence of amino acids not only determines the secondary, tertiary, and even quaternary structures of protein molecules, but also the differences in primary structures can reflect the genetic differences between different species and the kinship between species relation.

So far, scientists have clarified the primary structures of hundreds of protein molecules, and the established molecular evolution system is basically consistent with the traditional classification system. This shows that the molecular evolution and morphological evolution of biological species are basically consistent. In 1982, molecular biologist Liusnikau selected hemoglobin and myoglobin, which are present in most invertebrates and all vertebrates, for research. The results showed that invertebrates had appeared at least 1 billion years ago. . Other scholars' research on cytochrome C shows that the divergence time of animals and plants on the earth was even as far back as 1.3 to 1.2 billion years ago.

animal tags： Euglena