Foundations of modern biology
There are five unifying principles of biology

Cell theory. All living organisms are made of one or more cells, the basic living unit of function in organisms. All cells come from preexisting cells that multiply through cell division.
Evolution. Through natural selection and genetic drift, a population's inherited traits change from generation to generation.
Genes. A living organism's traits are encoded in DNA. Segments of DNA that, taken as a whole, specify a trait are known as genes. In addition, traits are passed on from one generation to the next by way of these genes. All information transfers from the genotype, the unobservable genetic traits, to the phenotype, the observable physical or biochemical characteristics of the organism. Although the phenotype expressed by the gene may adapt to the environment of the organism, that information is not transferred back to the genes. Only through the process of evolution do genes change in response to the environment.
Homeostasis. The physiological processes that allow an organism to maintain its internal environment notwithstanding its external environment.
Energy. The attribute of any living organism that is essential for its state. (e.g. required for metabolism)

Cell theory

Cell theory states that

The cell is the fundamental unit of life.
All living things are composed of one or more cells or the secreted products of those cells, such as shells.
Cells arise from other cells through cell division
In multicellular organisms, every cell in the organism's body is produced from a single cell in a fertilized egg.
The cell is considered to be the basic part of the pathological processes of an organism

Evolution
Main article: Evolution
A central organizing concept in biology is that life changes and develops through evolution and that all life-forms known have a common origin. Introduced into the scientific lexicon by Jean-Baptiste de Lamarck in 1809, Charles Darwin established evolution fifty years later as a viable theory by articulating its driving force: natural selection (Alfred Russel Wallace is recognized as the co-discoverer of this concept as he helped research and experiment with the concept of evolution).

Darwin theorized that species and breeds developed through the processes of natural selection and artificial selection or selective breeding.[4] Genetic drift was embraced as an additional mechanism of evolutionary development in the modern synthesis of the theory.

The evolutionary history of the species— which describes the characteristics of the various species from which it descended— together with its genealogical relationship to every other species is called its phylogeny. Widely varied approaches to biology generate information about phylogeny. These include the comparisons of DNA sequences conducted within molecular biology or genomics, and comparisons of fossils or other records of ancient organisms in paleontology.

Biologists organize and analyze evolutionary relationships through various methods, including phylogenetics, phenetics, and cladistics. For a summary of major events in the evolution of life as currently understood by biologists, see evolutionary timeline.

Up into the 19th century, spontaneous generation, the belief that life forms could appear spontaneously under certain conditions, was widely believed. This misconception was challenged by William Harvey's diction that "all life

Genetics

Genes are the primary units of inheritance in all organisms and are made of DNA. Widely different organisms, including bacteria, plants, animals, and fungi, share the same basic machinery that copies and transcribes DNA into proteins. For example, bacteria with inserted human DNA may, under the right circumstances, manufacture a human protein.

All the genes in an organism or cell are together known as the genome, which–in eukaryotes but not bacteria–is stored on one or more chromosomes. A chromosome is an organized structure consisting of DNA and protein. Cells transcribe a DNA gene into an RNA version of the gene, and a ribosome then translates the RNA into a protein.

Homeostasis
Homeostasis is the ability of an open system to regulate its internal environment to maintain a stable condition by means of multiple dynamic equilibrium adjustments controlled by interrelated regulation mechanisms. All living organisms, whether unicellular or multicellular, exhibit homeostasis.

Homeostasis exists at the cellular level, for example, cells maintain a stable internal acidity (pH); and at the level of the organism, for example, warm-blooded animals maintain a constant internal body temperature. Homeostasis is a term that is also used in association with ecosystems. For example, the roots of plants help prevent soil from eroding, which helps to maintain the ecosystem. Tissues and organs can also maintain homeostasis. It is also the maintenance of stability of numbers of individuals within a population.

Energy
The survival of a living organism depends on the continuous input of energy. Chemical reactions that are responsible for its structure and function are tuned to extract energy from substances that act as its food and transform them to form new cells and sustain them. In this process, molecules of chemical substances that constitute food play two roles; first, they contain energy that can be transformed for biological chemical reactions; and also develop molecular structures made up of biomolecules.

Nearly all of the energy needed for life processes originates from the Sun, which plants and other autotrophs convert into chemical energy (organic molecules) via photosynthesis in the presence of water and minerals. A few ecosystems, however, depend entirely on energy extracted from methane, sulfides, or other inorganic molecules by chemosynthetic microorganisms.

Some of the captured energy is used to produce biomass to sustain life and provide energy for its growth and development. A part of this energy is lost as heat and waste molecules. The common processes for converting energy in chemical substances into energy useful to sustain life are metabolism[8] and respiration.