What traits distinguish animals from other creatures? The question seems simple enough, but the answer requires an understanding of some of the more obscure characteristics of organisms, including multicellularity, heterotrophy, motility, and other complex concepts used by biologists. What signs of living things are common, at least for most animals, from snails and zebras to mongooses and sea anemones?
Multicellularity
If you are trying to distinguish a true animal from, say, paramecium or amoeba, it is not very difficult: animals, by definition, are multicellular creatures, although the number of cells varies greatly by species. For example, a roundworm, which is widely used in biology experiments, consists of exactly 1,031 cells, no more and no less, while a person literally consists of trillions of cells. However, it is important to note the fact that animals are not the only multicellular organisms, they also include plants, fungi, and even some types of algae.
Eukaryotic cell structure
Signs of living things include the eukaryotic cell structure. Perhaps the most important schism in the history of life on Earth is what happens between their two vast species. Prokaryotic organisms do not have membrane-limited nuclei and other organelles and are exclusively unicellular. These include, for example, all bacteria.
In eukaryotic cells, in contrast, there are clearly defined nuclei and internal organelles (such as mitochondria) that can group together to form multicellular organisms. Although all animals are euacaryotes, not all eukaryotes are animals: this extremely diverse family also includes plants, fungi, and tiny marine proto-animals known as protists.
Specialized fabrics
Another sign of living things is the presence of a wide variety of tissues. One of the most remarkable features of animals is how specialized their cells are. As these organisms develop, seemingly ordinary stem cells are actually not so simple. There are four broad biological categories: nerve, connective, muscle, and epithelial tissues (which build organs and blood vessels).
More advanced organisms exhibit even more specific levels of differentiation. For example, various organs of the human body are made up of liver cells, pancreas, and dozens of other species. The exception is sponges, which are technically animals, but practically have no differentiated cells.
Sexual reproduction
Another sign of living things is that most animals engage in sexual reproduction, combining their genetic information and producing offspring that carry the DNA of both parents. But here it is not without exception. Some animals, including some species of sharks, are able to reproduce their own kind asexually.
The benefits of sexual reproduction are huge from an evolutionary point of view. The ability to test different combinations of genomes allows animals to quickly adapt to new ecosystems. Again, sexual reproduction is not limited to animals: this system is also used by various plants, fungi, and even some very promising bacteria.
Blastula stage
This is one of the most difficult to perceive signs of living things. When a male sperm meets a female egg, the result is a single cell called a zygote. After the zygote passes several rounds of division, it gets the name - morula. Only real animals reach the next stage - the formation of a blastula, a hollow ball of several cells. Only in this case can they differentiate into different types of tissues.
Movement (animals)
It is very difficult to show the first five signs of living creatures using funny drawings. They are really quite complicated if you explain them, for example, to a child. What can not be said about the following sign. Fish are swimming, birds are flying, wolves are running, snails and snakes are crawling - all animals are able to move at a certain stage in their life cycle. One of the studied attributes of living creatures of the surrounding world in grade 3 is movement. If there is movement, then the body is alive.
This evolutionary innovation allows creatures to more easily find new ecological niches, pursue prey and avoid predators. Some animals, such as sponges and corals, are almost immobile in the adult stage, but their larvae are able to move before they take root on the seabed. This is one of the key features that distinguishes animals from plants and fungi. There is controversial opinion about some exceptions to the rules, including carnivorous flycatchers and fast-growing bamboo trees.
Metabolism
Among the signs of living creatures of the world that are studied by children (in the 3rd grade, this sign will be called "nutrition"), it is worth noting the metabolism and energy. All organisms need organic carbon to support the basic processes of life, including growth, development and reproduction. The ability to sample food, in scientific terms, is called heterotrophy. There are two ways to produce carbon: from the environment (in the form of carbon dioxide, a freely available gas in the atmosphere) or by eating other organisms rich in carbon.
Living organisms that get carbon from the environment like plants are called autotrophs, and living organisms that get carbon by ingesting other living organisms, such as animals, are called heterotrophs. However, animals are not the only heterotrophs in the world. All fungi, many bacteria, and even some plants are at least partially heterotrophic. The use of external sources of energy in the form of food, light, and so on, is an important sign of living organisms.
Advanced nervous systems
This is another sign of living things. Organisms, in particular animals, have advanced nervous systems. It is rather difficult to judge the intellectual level of plants and mushrooms. Of all the organisms on Earth, only mammals are so developed that they have more or less acute senses of vision, hearing, taste and touch (not to mention the echolocation of dolphins and bats, or the ability of some fish and sharks to sense magnetic vibrations in water).
Of course, these feelings entail the existence of at least a rudimentary nervous system, both in insects and starfish, and in the most advanced animals. Fully developed brains are perhaps one of the key features that truly distinguishes animals from the rest of nature.
Growth and development
Among the attributes of living creatures of the surrounding world studied in grade 3 there is such a point as growth. This property, which means an increase in size and mass while maintaining the general features of the structure, is accompanied by such a complex process as development.
Unified chemical composition
Living creatures are composed of the same chemical elements that include objects of inanimate nature. The difference is in an unequal proportional ratio. All life on Earth is 98 percent composed of elements such as carbon, oxygen, nitrogen and hydrogen.
Irritability
An integral feature that is inherent in all living organisms is the ability to respond to irritation. This trait is expressed in the way creatures react to external sources of influence.
Discreteness
A common property of living matter is its discreteness. This means that any biological system includes separate interacting elements that together make up a single structural and functional organization.
On planet Earth, life has been around for four billion years. The living creatures that live on our planet are incredibly diverse: from unicellular and invisible to the naked eye tiny organisms to giant trees, up to 90 meters, and massive animals weighing up to 150 tons. Despite all the biological diversity, there are a number of signs that help distinguish them from bodies of inanimate nature.