If you are interested in the natural world around you and have started studying different living organisms and how they are classified, you will probably know that all living beings are divided into kingdoms. Every living organism can be filed into a kingdom, which is a group that unites them on the basis of common features. And although there are obvious differences between you and the weevil beetle, in terms of kingdom, you are two of a kind. This topic will give you a brief guide to the kingdoms of living things on planet Earth, including their main differences and common features. The division into kingdoms is a somewhat archaic classification system for living organisms, and new methods and discoveries dispute this classification. However, for general understanding, it is important and provides a good system.
LUCA
Life did not come out of nowhere, and it is logical to assume that at some point there lived a common ancestor of all living organisms. This ancestor is called LUCA — the last universal common ancestor. It was single-celled and had a very simple structure. It is impossible to say exactly when our great-great lived, but most likely it was about 4 billion years ago. LUCA lived in hydrothermal vents, where it was able to use the plentiful metals and soluble molecules that exist in those environments. All living kingdoms that exist now — from unicellular bacteria to multicellular animals and plants — originate from LUCA.
Thanks to modern methods of bioinformatics, scientists have been able to propose the genetic composition of LUCA — which genes it had and which it did not. It is clear that, most likely, it was not a single LUCA cell, but a population of cells.
Biodiversity
Kingdoms can be divided into two types depending on whether their representatives have discrete internal compartments — prokaryotes do not have these compartments, or organelles, but eukaryotes do. Prokaryotes are simpler and they are always unicellular. Eukaryotes are more complex and many of them are multicellular (although not all of them!). Prokaryotes include Archaea and Bacteria, and eukaryotes include Protozoa, Plants, Fungi, and Animals.
Archean kingdom
Let's start from the smallest to the largest, from the simple to the complex. The most ancient and most simply arranged (but not the least interesting!) is the Archean kingdom. Archaea is an amazing kingdom because despite the fact that archaea are very ancient, they have common features with both bacteria and eukaryotes. How can this be? Archaea were probably a field of experiments for evolution — as a result, this branch turned out to be an evolutionary dead end, but it allowed many interesting things to be "run into."
Archaea are single-celled and, as prokaryotes, do not have a nucleus. Their DNA is twisted into a ring that hangs out in the cytoplasm. Although they are prokaryotes, they have many features in common with eukaryotes, such as the structures of some protein complexes and some of the characteristics of their DNA. Archaea also have properties that are uniquely theirs, such as distinct structures of some organelles.
Bacteria
Bacteria is the second kingdom of prokaryotes. Bacteria are single-celled, their DNA is twisted into a ring in the center of the cell, and they are extremely diverse. There are bacteria that "eat" methane, and there are those that prefer sulfur. There are those who synthesize organics — for example, cyanobacteria — and those who decompose organics, for example, E. coli. The shapes of bacteria also vary — sticks, spirals, balls.
An interesting feature of bacteria is the presence of plasmids — circular DNA in addition to their genome, in which genes useful for the bacterium are "stored" (for example, the gene for resistance to certain antibiotics). Bacteria can exchange plasmids in a process called conjugation, which is a special case of horizontal gene transfer and helps spread beneficial genes within a population. Horizontal gene transfer (as opposed to "normal" or vertical gene transfer — from parent to offspring) allows the exchange of genetic information between unrelated cells.
Protozoa
Protozoa are the first real eukaryotes — despite the fact that many protozoa are unicellular and really simple, they have a nucleus with a nuclear envelope that preserves DNA and separates it from the rest of the cell contents. You probably remember examples of Protozoa from biology lessons — such as amoeba and slipper infusoria (Paramecium caudatum). In the Protozoan kingdom, there are no multicellular organisms, although there are species that form large colonies.
Protozoa are distinguished from the prokaryotes, which are also single-celled, because they have complex, membrane-bound organelles — mitochondria and plastids (in photosynthetic species). Both of these organelles are interesting in that they are, in fact, derived from separate species captured by an ancient host cell ancestor of the modern eukaryotes. The ancestors of mitochondria and plastids were bacteria, and when they were captured by eukaryotic cells, for some reason they were not digested, but took root. Therefore, they are unique from all other organelles in that they have two membranes — their own membrane and the membrane of the host vacuole. Mitochondria were the first to be captured, which is why all members of eukaryotic kingdoms have them. Plastids were captured later in the evolutionary process, so only some protozoa and plants have them.
Fungi
Mushrooms or Fungi are a kingdom that you are definitely already familiar with. Perhaps you even know that the stem and caps you see in the forest and the grocery store are not actually the whole mushroom, but only a fruit body. The fungus is made up of many thin threads called hyphae that are woven into a large body — the mycelium. Often the cells of the fungus fuse to form a large multinucleated megacell. Mushrooms are very diverse: they include edible mushrooms, yeast, and mold on bread. Two characteristics that unite all fungi are: reproduction by spores and the presence of a cell wall (often made up of chitin). All fungi are "saprophytes," meaning that they decompose organic matter, although some may exist in symbiotic relationships with plants or algae. Interestingly, the largest organism in the world is a fungus — Armillaria ostoyae from the United States has been living for 2400 years and covers an area of 800 hectares. Fungi have similarities both to plants and animals: the similarity with plants lies in the presence of a carbohydrate cell wall, and the similarity with animals in the storage of energy in glycogen instead of starch like plants.
Plants
Early in the evolutionary process, the ancestor of members of the Plant kingdom acquired plastids, and hence has the ability to photosynthesize — the process of creating oxygen and energy from carbon dioxide, light, and water. Plastids are organelles used for photosynthesis, and the abundance of them in plant cells and the chlorophyll they contain make plants green. The process of photosynthesis explains many of the characteristics of plants: they don't move to preserve energy, they're green, and they have a lot of surface area to absorb the ingredients of photosynthesis. Plants, unlike animals, can grow continuously throughout their lives — which is why giant sequoias exist.
Animals
And last but not least are the Animals. This is the diverse group about which you know a lot because you belong to it yourself. Animals are divided into two sub-kingdoms. The first — the most primitive — is the sub-kingdom of the prometazoans, which include motionless sponges. The second is the eumetazoans, which include all other animals. A major grouping among them is the bilaterally symmetrical animals like protostomes (worms, molluscs, and so on) and deuterostomes (echinoderms and chordates). Chordates are named so because during a certain period of life they have a chord, a long elastic tube. This idea of a long tube running along the body should sound familiar, because in a subgroup of chordates called vertebrates, the chordis is replaced by the spinal cord in the process of development. Vertebrates include several subtypes of relatively ancient creatures — for example, cartilaginous and actinopterygians fish, and also tetrapods. Tetrapoda includes our cousins — reptiles and birds, and also you and your cat, the mammals.
Common characteristics among all animals are that they can't photosynthesize and they consume organic matter, eating either those who can photosynthesize or other animals.
Conclusion
So, we found out that a long time ago in a galaxy not far, far away we all had a common ancestor — LUCA. His descendants became very different — some received a nucleus, some remained non-nuclear. Some eukaryotes remained single-celled protozoa, some became multicellular complex organisms, which became more and more complicated until you appeared. And your cat, of course, as the crowning achievement of this evolution.