Characteristics of life are controlled by genes, which are passed from parents to offspring, and are located on chromosomes, like the ones shown here, that are found in every cell. This describes a fundamental idea of gene theory, one of the four unifying principles of biology.
Four unifying principles form the basis of biology. Whether biologists are interested in ancient life, the life of bacteria, or how humans could live on the moon, they base their overall understanding of biology on these four principles:
According to the cell theory, all organisms are made up of one or more cells, all the life functions of an organism occur within cells, and all cells come from preexisting cells. In fact, each living thing begins life as a single cell. Some living things, such as bacteria, remain single-celled. Other living things, including plants and animals, grow and develop into many cells. Your own body is made up of an amazing 100 trillion cells! But even you—like all other living things—began life as a single cell.
The gene theory is the idea that the characteristics of living organisms are controlled by genes, which are passed from parents to their offspring. A gene is a segment of DNA that has the instructions to encode a protein. Genes are located on larger structures, called chromosomes, that are found inside every cell. Chromosomes, in turn, contain large molecules known as DNA (deoxyribonucleic acid). Molecules of DNA are encoded with instructions that tell cells what to do.
Homeostasis, which is maintaining a stable internal environment or keeping things constant, is not just a characteristic of living things. It also applies to nature as a whole. Consider the concentration of oxygen in Earth’s atmosphere. Oxygen makes up 21% of the atmosphere, and this concentration is fairly constant. What keeps the concentration of oxygen constant? The answer is living things. Most living things need oxygen to survive, and when they breathe, they remove oxygen from the atmosphere. On the other hand, many living things, including plants, give off oxygen when they make food, and this adds oxygen to the atmosphere. The concentration of oxygen in the atmosphere is maintained mainly by the balance between these two processes.
Evolution is a change in the characteristics of living things over time. Evolution occurs by a process called natural selection. In natural selection, some living things produce more offspring than others, so they pass more genes to the next generation than others do. Over many generations, this can lead to major changes in the characteristics of living things. Evolution explains how living things are changing today and how modern living things have descended from ancient life forms that no longer exist on Earth.
As living things evolve, they generally become better suited for their environment. This is because they evolve adaptations. An adaptation is a characteristic that helps a living thing survive and reproduce in a given environment. Look at the mole in Figure below. It has tentacles around its nose that it uses to sense things by touch. The mole lives underground in the soil, where it is always dark. However, by using its touch organ, it can detect even tiny food items in the soil in total darkness. The touch organ is an adaptation because it helps the mole survive in its dark, underground environment.
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