# Biology

Animals: Hoverfly on a Compositae inflorescence
Plants: Wheat
Fungi: *Morchella esculenta*
Virus: Gamma bacteriophage
**Biology** is the science that studies life, living things, and the evolution of life. Living things include animals, plants, fungi (such as mushrooms), and microorganisms such as bacteria and archaea.

The term 'biology' is relatively modern. It was introduced in 1799 by a physician, Thomas Beddoes. It comes from the Greek word *βίος* (*bios*), "life", and the suffix *-λογία* (*logia*), "study of".

People who study biology are called biologists. Biology looks at how animals and other living things behave and work, and what they are like. Biology also studies how organisms react with each other and the environment. It has existed as a science for about 200 years, and before that it was called "natural history". Biology has many research fields and branches. Like all sciences, biology uses the scientific method. This means that biologists must be able to show evidence for their ideas and that other biologists must be able to test the ideas for themselves.

Biology attempts to answer questions such as:

* "What are the characteristics of this living thing?" (comparative anatomy)
* "How do the parts work?" (physiology)
* "How should we group living things?" (classification, taxonomy)
* "What does this living thing do?" (behaviour, growth)
* "How does inheritance work?" (genetics)
* "What is the history of life?" (palaeontology)
* "How do living things relate to their environment?" (ecology)

Modern biology is influenced by evolution, which answers the question: "How has the living world come to be as it is?"

## History

The earliest roots of science, including medicine, go back to ancient Egypt and Mesopotamia around 3000 to 1200 BCE. Their ideas influenced ancient Greek natural philosophy. Greek thinkers like Aristotle (384–322 BCE) added a lot to early biology. He studied causes in nature and the variety of living things. His student, Theophrastus, began the scientific study of plants.

In the medieval Islamic world, several scholars wrote about biology. Al-Jahiz (781–869) and Al-Dīnawarī (828–896) wrote about animals and plants. Rhazes (865–925) studied the body and how it works. Islamic scholars, working from Greek ideas, studied medicine deeply. They also followed Aristotle’s ideas about nature.

Biology grew quickly when Anton van Leeuwenhoek greatly improved the microscope. Scientists then discovered sperm cells, bacteria, and many tiny living things. Jan Swammerdam’s work led to interest in insects and new methods of studying tiny organisms. Better microscopes strongly changed how people thought about life.

In the early 1800s, scientists saw that cells were central to life. In 1838, Schleiden and Schwann said that (1) all living things are made of cells and (2) each cell shows the signs of life. They did not believe that (3) all cells come from other cells; they still thought cells could appear on their own. Later, Robert Remak and Rudolf Virchow proved that new cells come from existing cells. By the 1860s, most scientists accepted all three ideas, which became known as cell theory.

At the same time, natural historians focused on naming and grouping living things. Carl Linnaeus published a basic system in 1735 and gave scientific names to species in the 1750s. Georges-Louis Leclerc, Comte de Buffon, believed species were not fixed and could change, and even suggested that different species might share ancestors.

In 1842, Charles Darwin wrote the first draft of *On the Origin of Species*. Earlier, Jean-Baptiste Lamarck had offered one of the first full theories of evolution. Darwin built a stronger theory of evolution through natural selection. He used ideas from geography, geology, and population studies, along with his own observations. Alfred Russel Wallace reached similar conclusions on his own.

Modern genetics started with Gregor Mendel’s work in 1865. He explained how traits are passed from parents to offspring. People did not fully understand his work until the early 1900s. Then, evolution and genetics were combined into the modern synthesis.

In the 1940s and early 1950s, experiments by Alfred Hershey and Martha Chase showed that DNA is the part of chromosomes that carries genes. Scientists began to study small organisms like viruses and bacteria. In 1953, James Watson and Francis Crick discovered that DNA has a double-helix shape. After this, biology expanded into molecular genetics. Scientists cracked the genetic code, showing how DNA uses codons to build proteins. This work was done by Har Gobind Khorana, Robert W. Holley, and Marshall Warren Nirenberg. In 1990, the Human Genome Project began to map all human genes.

## Chemical basis

### Atoms and molecules

All living things are made of chemical elements. Most of their mass (about 96%) comes from oxygen, carbon, hydrogen, and nitrogen. Other elements like calcium, phosphorus, sulfur, sodium, chlorine, and magnesium make up almost all the rest. These elements can join together to form compounds, such as water, which is essential for life. Biochemistry is the study of chemical processes in living things. Molecular biology is the study of how molecules in cells work, including how they are made, changed, and how they interact with each other.

### Water

Model of hydrogen bonds (1) between molecules of water
Life began in Earth's first oceans about 3.8 billion years ago. Since then, water has been the most common molecule in all living things.

Water is important for life because it is a great solvent. This means it can dissolve many substances, like salt (sodium and chloride ions) and small molecules, to form solutions. When substances are dissolved in water, they can more easily meet and react with each other, which is necessary for life.

#### Structure of water

A water molecule (H₂O) is small and polar. It has:

* 1 oxygen atom and 2 hydrogen atoms
* A bent shape
* Polar covalent bonds, which make:
	+ Oxygen slightly negative
	+ Hydrogen atoms slightly positive

Because of this polarity, water molecules attract each other using hydrogen bonds, which makes water cohesive (they stick together).

#### Properties of water

Water has several properties:

* Surface tension happens because water molecules at the surface stick together strongly.
* Water is adhesive, meaning it can stick to other charged or polar surfaces.
* Water is denser as a liquid than as ice. This makes ice float on water, helping protect the water below from freezing completely.
* Water can absorb a lot of energy before warming up. This gives it a high specific heat capacity, higher than many other liquids like ethanol. A lot of energy is needed to break hydrogen bonds and turn liquid water into vapor.

#### Water molecules in motion

Water molecules are always changing. They constantly break apart into hydrogen ions and hydroxyl ions, then join back together again. In pure water, the number of hydrogen ions and hydroxyl ions is equal, making the pH neutral.

### Organic compounds

Organic compounds such as glucose are vital to organisms.
Organic compounds are molecules that contain carbon bonded to other elements, such as hydrogen. Except for water, almost all the molecules in living things contain carbon.

Carbon is special because it can form up to four covalent bonds with other atoms. This allows it to make a wide variety of large and complex molecules. For example:

* A carbon atom can form four single bonds (like in methane).
* It can form two double bonds (like in carbon dioxide, CO₂).
* It can even form a triple bond (like in carbon monoxide, CO).

Carbon can also create long chains (like in octane) or ring-shaped structures (like in glucose).

The simplest organic molecules are hydrocarbons, which are made only of carbon and hydrogen. These carbon chains can also include other elements such as oxygen, phosphorus, or sulfur, which change the molecule’s chemical properties.

Groups of atoms attached to carbon chains are called functional groups. These groups help determine how a molecule behaves. The six main functional groups in living things are the amino group, carboxyl group, carbonyl group, hydroxyl group, phosphate group, and sulfhydryl group.

In 1953, the Miller–Urey experiment showed that organic molecules could form naturally under conditions like those of early Earth. This suggested that the complex molecules needed for life may have appeared spontaneously in Earth’s early history.

### Macromolecules

Macromolecules are large molecules made from smaller building blocks called monomers. Examples of monomers include sugars, amino acids, and nucleotides.

Carbohydrates are made of sugar monomers or chains of sugars. They can serve as energy sources or provide structure to cells. Lipids are different from other macromolecules because they are not made of polymers. They include fats, phospholipids, and steroids. Lipids do not mix well with water because they are mostly nonpolar and hydrophobic.

Proteins are the most varied type of macromolecule. They can act as enzymes, transport materials, send signals, form structures, or work as antibodies. The building blocks of proteins are amino acids, and living things use twenty different amino acids to make them.

Nucleic acids are made of nucleotides. Their job is to store, pass on, and express genetic information. DNA and RNA are examples of nucleic acids.

## Cells

Cell theory says that cells are the basic building blocks of all living things. Every living thing is made of one or more cells, and new cells come from existing cells by dividing. Most cells are very small—only about 1 to 100 micrometers wide—so you can only see them with a light or electron microscope.

There are two main kinds of cells:

* Eukaryotic cells have a nucleus.
* Prokaryotic cells do not have a nucleus.

Prokaryotes, like bacteria, are always single-celled. Eukaryotes can be either single-celled or made of many cells. In living things made of many cells, all the cells in the body originally come from one cell: the fertilized egg.

### Cell structure

Every cell is surrounded by a cell membrane that separates the inside of the cell, called the cytoplasm, from the outside space. The membrane is made of a double layer of lipids. Cholesterol sits between these lipids and helps the membrane stay flexible at different temperatures. The cell membrane is semipermeable, which means small molecules like oxygen, carbon dioxide, and water can move through it easily, while larger molecules and charged particles such as ions cannot pass through as easily.

The membrane also contains proteins. Some of these proteins go all the way through the membrane and help move substances in and out of the cell. Others are attached loosely to the outside of the membrane and can act like enzymes or help give the cell its shape. The cell membrane has several important roles, such as helping cells stick to each other, storing electrical energy, and sending or receiving signals. It also provides a surface where structures like the cell wall, the cytoskeleton, or the glycocalyx can attach.

Inside the cytoplasm, there are many important molecules, including proteins and nucleic acids. Eukaryotic cells also contain special structures called organelles, many of which are surrounded by their own lipid membranes. The nucleus holds most of the cell’s DNA. Mitochondria produce ATP, which is the energy the cell uses. The endoplasmic reticulum helps make proteins and other molecules, while the Golgi apparatus packages and sends out proteins. Lysosomes can break down proteins and other unwanted materials.

Plant cells have some organelles that animal cells do not. They have a cell wall that gives the cell support and shape. They also have chloroplasts, which use sunlight to make sugar, and vacuoles, which store materials, help support the cell, and can take part in reproduction and seed breakdown.

Eukaryotic cells also have a cytoskeleton that helps the cell keep its shape and allows movement. It is made of microtubules, intermediate filaments, and microfilaments. Microtubules are made from tubulin, intermediate filaments are made from fibrous proteins, and microfilaments are made from actin, which interacts with other proteins. These parts work together to support the cell and move the organelles inside it.

### Metabolism

All cells need energy to carry out their activities. Metabolism is the name for all the chemical reactions that happen in a living organism. Metabolism has three main jobs: turning food into energy for the cell, turning food into small building blocks the body can use, and getting rid of waste made during these processes. These reactions, which are helped by enzymes, make it possible for organisms to grow, reproduce, maintain their structure, and react to their surroundings.

Metabolic reactions can be grouped into two types. Catabolic reactions break down larger molecules into smaller ones, such as when glucose is broken down into pyruvate during cellular respiration. These reactions usually release energy. Anabolic reactions build larger molecules from smaller ones, such as proteins, carbohydrates, lipids, and nucleic acids. These reactions usually require energy.

The chemical reactions in metabolism are arranged in steps called metabolic pathways. In these pathways, one substance is changed step by step into another, and each step is carried out by a specific enzyme. Enzymes are essential to metabolism because they make it possible for organisms to perform useful reactions that need energy and would not happen on their own. They do this by linking those reactions to other reactions that release energy. Enzymes act as catalysts, which means they speed up reactions without being used up. They do this by lowering the activation energy needed to turn reactants into products. Enzymes also help control how fast a metabolic reaction happens, depending on the cell’s environment or signals from other cells.

### Cellular respiration

Cellular respiration is the process cells use to turn the chemical energy in nutrients into ATP, which is the main energy source cells use to function. During this process, waste products are also released. The reactions in respiration are catabolic, meaning they break large molecules into smaller ones and release energy. Respiration is one of the main ways cells get the energy they need. Even though cellular respiration is a type of combustion, it does not look like burning because the energy is released slowly and in a controlled way through several steps.

Glucose, a type of sugar, is the main nutrient that animal and plant cells use in respiration. When oxygen is present, the process is called aerobic respiration, and it happens in four stages: glycolysis, the citric acid cycle (also called the Krebs cycle), the electron transport chain, and oxidative phosphorylation. Glycolysis happens in the cytoplasm and breaks one glucose molecule into two pyruvate molecules, producing two ATP molecules. Each pyruvate is then converted into acetyl-CoA by the pyruvate dehydrogenase complex, which also produces NADH and carbon dioxide. Acetyl-CoA enters the citric acid cycle in the mitochondrial matrix. From one glucose molecule, this cycle produces a total of six NADH, two FADH₂, and two ATP molecules.

The final stage, oxidative phosphorylation, happens in the mitochondrial cristae of eukaryotic cells. It includes the electron transport chain, where four protein complexes pass electrons from one to another. This releases energy from NADH and FADH₂, which is used to pump protons across the inner mitochondrial membrane. This creates a proton gradient called the proton motive force. ATP synthase then uses this energy to make more ATP by adding phosphate to ADP. Oxygen is the final electron acceptor at the end of the electron transport chain.

If oxygen is not available, pyruvate cannot go through cellular respiration. Instead, it stays in the cytoplasm and goes through fermentation. In fermentation, pyruvate is turned into waste products that the cell can remove. This process allows NADH to be converted back into NAD⁺ so glycolysis can continue. Without oxygen, fermentation prevents the buildup of NADH and provides NAD⁺ for glycolysis to keep making ATP.

The waste product of fermentation depends on the organism. In human muscle cells, the waste product is lactic acid, so this process is called lactic acid fermentation. During intense exercise, when the body needs more energy than it can get from oxygen-based respiration, hydrogen atoms carried by NADH join with pyruvate to form lactate. This step is helped by the enzyme lactate dehydrogenase and can be reversed. Lactate can also be used later to help make glycogen in the liver. When oxygen is available again, hydrogen from lactate combines with NAD⁺ to help make ATP.

In yeast, fermentation produces ethanol and carbon dioxide, so this is called alcoholic or ethanol fermentation. The ATP made during fermentation comes from substrate-level phosphorylation, which does not require oxygen.

### Photosynthesis

Photosynthesis is the process that plants and some other organisms use to change light energy into chemical energy. This chemical energy is stored in carbohydrates like sugars, which are made from carbon dioxide and water. Later, the stored energy can be released through cellular respiration to power the organism’s activities. In most cases, oxygen is released as a waste product. Most plants, algae, and cyanobacteria perform photosynthesis, and this process is mainly responsible for maintaining the oxygen in Earth’s atmosphere and providing most of the energy needed for life.

Photosynthesis happens in four stages: light absorption, electron transport, ATP production, and carbon fixation. Light absorption is the first step. During this stage, chlorophyll pigments in the thylakoid membranes absorb light energy. This energy is used to remove electrons from water and pass them to a primary electron acceptor called quinone, or Q.

In the second stage, these electrons move from the quinone through several electron carriers until they reach their final electron acceptor, which is usually NADP⁺. NADP⁺ is reduced to NADPH during this process, which takes place in a protein complex called photosystem I. As electrons are passed along, protons (hydrogen ions) are moved from the stroma into the thylakoid space. This creates a pH difference across the membrane, with more hydrogen ions in the lumen than in the stroma. This is similar to the proton gradient created in mitochondria during aerobic respiration.

In the third stage, the hydrogen ions move back down their concentration gradient from the thylakoid lumen to the stroma through ATP synthase. As they pass through ATP synthase, ATP is produced. The NADPH and ATP made during the second and third stages supply the energy and electrons needed for the final step.

In the fourth stage, carbon fixation, the energy from ATP and the electrons from NADPH are used to make glucose. This happens by adding carbon dioxide from the air to existing carbon compounds like ribulose bisphosphate (RuBP). These reactions do not require light directly and are called the Calvin cycle.

### Cell signaling

Cell signaling, or cell communication, is the way cells receive, process, and send signals to their surroundings and to themselves. Signals can be physical, such as light, electrical impulses, or heat, or they can be chemical. Chemical signals, also called ligands, connect with receptors that may be located on the cell membrane of another cell or inside a cell.

There are four main types of chemical signaling: autocrine, paracrine, juxtacrine, and hormonal. In autocrine signaling, a cell releases a chemical signal that affects itself. For example, some tumor cells divide uncontrollably because they produce signals that trigger their own growth. In paracrine signaling, the chemical signal spreads to nearby cells and affects them. An example is neurons in the brain, which release neurotransmitters that cross the synaptic gap and bind to receptors on nearby neurons or muscle cells. In juxtacrine signaling, the signaling cell and the receiving cell are in direct contact with each other. Hormonal signaling involves signals that travel through the bloodstream in animals or through the vascular system in plants to reach distant target cells.

When a ligand binds to a receptor, it can change the behavior of the target cell, depending on the type of receptor. For example, neurotransmitters that bind to an ionotropic receptor can change how easily a target cell becomes active. Other receptors include protein kinase receptors, like the insulin receptor, and G protein-coupled receptors. When G protein-coupled receptors are activated, they can start a chain of reactions called a second messenger cascade. The whole process by which a physical or chemical signal causes a series of molecular events inside a cell is known as signal transduction.

### Cell cycle

A diagram of the mitotic phases
The cell cycle is the set of events that a cell goes through to divide and form two new daughter cells. During this process, the cell makes a copy of its DNA and some of its organelles. Then the cytoplasm is split into two new cells. This process is called cell division.

In eukaryotes, such as animals, plants, fungi, and protists, there are two types of cell division: mitosis and meiosis. Mitosis is the type that produces two new cells that are genetically identical to the original cell and have the same number of chromosomes. Before mitosis begins, the DNA is copied during the S phase of interphase. Mitosis separates the duplicated chromosomes into two nuclei. It is usually followed by cytokinesis, which divides the cytoplasm, organelles, and cell membrane so that each daughter cell gets a roughly equal share. The stages of mitosis together make up the mitotic phase of the cell cycle. This process is essential for growth, development, and tissue repair. After division, each daughter cell starts a new cycle with interphase.

Meiosis is different from mitosis. It produces four daughter cells, each with half the number of chromosomes (haploid cells). Meiosis involves one round of DNA replication followed by two divisions. In meiosis I, homologous chromosomes are separated. In meiosis II, sister chromatids are separated. Meiosis is used to produce sex cells and is part of sexual reproduction. Both mitosis and meiosis likely existed in the earliest eukaryotic ancestors.

During meiosis, chromosomes are copied and homologous chromosomes exchange genetic information in the first division. The second division creates four haploid gametes.

Prokaryotes, like bacteria and archaea, also divide, but in a simpler way called binary fission. Unlike eukaryotic cells, they do not form a spindle. Before binary fission, the DNA is tightly coiled. Once it uncoils and copies itself, the DNA strands move to opposite ends of the cell as the cell grows. A new cell wall begins to form in the middle of the cell, guided by proteins like FtsZ that form a “Z-ring.” The new wall, called the septum, eventually divides the cell completely. The result is two new daughter cells, each with tightly coiled DNA, ribosomes, and plasmids.

### Sexual reproduction and meiosis

Meiosis is an important part of sexual reproduction in organisms with complex cells. Its main job is to make sure the DNA passed from parents to their children stays correct and complete.

Two parts of sexual reproduction are especially helpful:

* Meiotic recombination helps fix damaged DNA by mixing genetic material.
* Outcrossing (cross-fertilization) combines genes from two different parents. This can hide harmful recessive genes so they do not cause problems.

The benefits of outcrossing are often called hybrid vigor or heterosis. In 1878, Charles Darwin wrote that cross-fertilizing plants was usually helpful, while self-fertilizing them could sometimes be harmful.

Sexual reproduction also creates genetic differences among offspring. These differences can help species that practice outcrossing survive and adapt over long periods of time.

## Genetics

### Inheritance

Genetics is the science that studies how traits are passed from parents to their offspring. Mendelian inheritance explains how genes move from one generation to the next. Genes come in different forms, called alleles, and each parent gives one allele to its offspring. Some alleles are dominant and others are recessive. If an organism has at least one dominant allele, it will show the dominant trait. When gametes, such as sperm or eggs, are formed, the two alleles for a gene separate so that each gamete carries only one allele. If an organism has two different alleles for a gene, it will produce gametes that carry each allele in equal amounts. Another principle, called the law of independent assortment, says that alleles for different traits are usually passed on separately, meaning one trait does not normally affect how another is inherited. However, sex-linked traits can be exceptions to this. To figure out the hidden genetic makeup of an organism that shows a dominant trait, scientists can use a test cross, and a Punnett square can help predict the results. The chromosome theory of inheritance, which states that genes are found on chromosomes, was supported by Thomas Morgan’s experiments with fruit flies, where he showed that eye color was linked to sex.

### Genes and DNA

Gene expression is the process by which the information in an organism’s DNA is used to create traits in its body. This happens through the production of proteins, which affect the organism’s characteristics. The process follows the central dogma of molecular biology, proposed by Francis Crick in 1958, which states that genetic information flows from DNA to RNA and then to protein. Gene expression involves two main steps: transcription, where DNA is copied into RNA, and translation, where RNA is used to build proteins.

### Gene expression

A gene is a unit of heredity and is a section of DNA that carries the instructions for an organism’s form or function. DNA is made of two long chains of nucleotides that twist together to form a double helix. In eukaryotes, DNA is arranged in linear chromosomes, while in prokaryotes it is found in circular chromosomes. All of the chromosomes in a cell together make up its genome. In eukaryotic cells, most DNA is located in the nucleus, whereas in prokaryotic cells, it is found in a region called the nucleoid. The genetic information inside an organism is stored in its genes, and the complete set of this information is called the genotype. When DNA is copied, the process is called semiconservative replication, meaning each original strand is used as a template to make a new one. Mutations are changes in DNA that can be passed on to offspring. They can happen on their own during DNA replication if an error is not corrected, or they can be caused by environmental mutagens such as chemicals or radiation. Mutations can have different effects, such as causing a gene to lose its normal function, gain a new function, or work only under certain conditions. Some mutations are helpful because they create genetic variation that can drive evolution. Others are harmful if they interfere with genes that are needed for survival.

### Gene regulation

Gene expression can be controlled by environmental factors and by the stage of development, and this control can happen at different steps, such as transcription, RNA splicing, translation, and protein modification after translation. Gene expression is regulated by proteins called transcription factors, which can either increase or decrease activity depending on whether they activate or repress a gene. A group of genes that share the same promoter is called an operon, which is mostly found in prokaryotes and some simple eukaryotes. In positive regulation, an activator transcription factor binds to the promoter region to turn on transcription. In negative regulation, a repressor binds to a DNA sequence called an operator to block transcription. Repressors can be turned off by molecules called inducers, allowing transcription to occur. Genes that can be turned on by inducers are called inducible genes, while constitutive genes are almost always active. Structural genes produce proteins that do not control gene activity. In addition to these mechanisms, gene expression can also be regulated by epigenetic changes to chromatin, the combination of DNA and proteins in eukaryotic cells.

### Genes, development and evolution

Development is the process by which a multicellular organism, such as a plant or animal, changes from a single cell into the many specialized cells and forms that make up its body during its life cycle. There are four main processes in development: determination, differentiation, morphogenesis, and growth. Determination sets the future role of a cell, which becomes more limited as development proceeds. Differentiation is when specialized cells form from less specialized cells, like stem cells. Stem cells are undifferentiated or partly specialized cells that can both make more stem cells and turn into different types of cells. Differentiation changes a cell’s size, shape, activity, and how it responds to signals, mainly due to changes in gene expression and epigenetics, without altering the DNA sequence itself. This is why cells can look and act very differently even though they have the same genome. Morphogenesis is the process that shapes the body, guided by differences in gene expression. A small group of genes, called the developmental-genetic toolkit, controls the development of an organism. These toolkit genes are very old and similar across many animal groups. How these genes are used determines the body plan and the arrangement of body parts. Among these, Hox genes are especially important, as they tell the embryo where repeated parts, like the many vertebrae of a snake, should develop.

## Evolution

### Evolutionary processes

Natural selection for darker traits
Evolution is a key idea in biology. It is the change in inherited traits of populations over many generations. In artificial selection, humans breed animals or plants for specific traits. Darwin realized that, in nature, a similar process happens without human intervention. Because traits are inherited and populations have variation, individuals with traits that help them survive are more likely to live longer and have more offspring. Over many generations, these favorable traits become more common, making the population better suited to its environment.

### Speciation

A species is a group of organisms that can mate with each other. Speciation is the process where one group splits into two separate groups that evolve independently. For speciation to happen, the groups must become reproductively isolated, meaning they can no longer successfully interbreed. This isolation can occur because of genetic incompatibilities, as explained by the Bateson–Dobzhansky–Muller model, and it usually increases as the groups become more genetically different. Speciation can also happen when physical barriers, like mountains or rivers, separate a population, a process called allopatric speciation.

### Phylogeny

Phylogeny is the evolutionary history of a group of organisms or their genes. It can be shown with a phylogenetic tree, a diagram that traces lines of descent. Each line represents a lineage of descendants from a species or population, and when a lineage splits into two, it appears as a fork in the tree. Phylogenetic trees help scientists compare and group different species. Species that share traits inherited from a common ancestor are said to have homologous features. Phylogeny also forms the basis of biological classification, which organizes living things into ranks: domain, kingdom, phylum, class, order, family, genus, and species. All organisms belong to one of three domains: Archaea, Bacteria, or Eukarya, which includes fungi, plants, and animals.

### History of life

The history of life on Earth shows how living things have changed and evolved from the earliest forms to the present. Earth formed about 4.5 billion years ago, and all life, living and extinct, comes from a last universal common ancestor that lived around 3.5 billion years ago. Geologists divide Earth’s history using a geologic time scale, which includes four eons: Hadean, Archean, Proterozoic, and Phanerozoic. The first three eons, called the Precambrian, lasted about 4 billion years. The Phanerozoic eon, starting 539 million years ago, is divided into three eras: Paleozoic, Mesozoic, and Cenozoic, which together include eleven periods, from the Cambrian to the Quaternary.

All living species today share similarities that show they evolved from a common ancestor. The universal genetic code is further evidence that bacteria, archaea, and eukaryotes all share this common descent. In the early Archean eon, microbial mats of bacteria and archaea were the dominant life forms, and many key steps in early evolution happened in this environment. The first eukaryotes appeared around 1.85 billion years ago, and their diversity increased when they began using oxygen in metabolism. Multicellular organisms appeared around 1.7 billion years ago, with specialized cells performing different functions.

Land plants similar to algae date back about 1 billion years, though microorganisms may have formed the first terrestrial ecosystems at least 2.7 billion years ago, helping prepare the way for land plants in the Ordovician period. Land plants became so successful that they may have contributed to the Late Devonian extinction. Ediacara biota appeared during the Ediacaran period, and vertebrates along with most modern animal groups originated about 525 million years ago during the Cambrian explosion. During the Permian period, synapsids, including ancestors of mammals, dominated land but were mostly wiped out in the Permian–Triassic extinction 252 million years ago. After this event, archosaurs became the most common land vertebrates, and one group, the dinosaurs, dominated the Jurassic and Cretaceous periods. When the non-avian dinosaurs went extinct 66 million years ago in the Cretaceous–Paleogene extinction, mammals rapidly grew in size and diversity. These mass extinctions may have sped up evolution by creating opportunities for new groups of organisms to flourish.

## Diversity

### Bacteria and Archaea

Bacteria
Archaea: Haloarchaea
Bacteria are tiny single-celled organisms that belong to a large group of prokaryotes (cells without a nucleus). They are usually a few micrometers long and can have different shapes, like spheres, rods, or spirals. Bacteria were among the first forms of life on Earth and live in almost all environments, including soil, water, hot springs, radioactive waste, and deep underground. They can also live in or on plants and animals, sometimes helping them and sometimes harming them. Most bacteria are not well studied, and only about 27% of bacterial groups can be grown in a lab.

Archaea are another group of prokaryotes. They were first thought to be bacteria and were called “archaebacteria,” but that name is no longer used. Archaea have special features that make them different from bacteria and eukaryotes (cells with a nucleus). They are similar in size and shape to bacteria, though some archaea, like *Haloquadratum walsbyi*, have unusual shapes such as flat squares. Even though they look like bacteria, archaea have genes and ways of making proteins more like eukaryotes. They also have unique cell membranes made of special fats called ether lipids. Archaea can get energy from many sources, including sugars, ammonia, metals, hydrogen gas, and sunlight (for salt-loving species called Haloarchaea). Some archaea can take in carbon, but unlike plants, no archaea can do both photosynthesis and carbon fixation. Archaea reproduce asexually through binary fission, splitting, or budding. Unlike bacteria, they do not form spores.

The first archaea discovered were extremophiles, living in extreme places like hot springs and salty lakes where no other life could survive. Later, scientists found archaea in almost every habitat, including soil, oceans, and marshes. They are especially common in oceans, and some plankton archaea may be among the most abundant organisms on Earth.

Archaea are an important part of life on Earth. They live in the microbiomes of humans and other organisms, including the gut, mouth, and skin. Because archaea have different shapes, metabolisms, and habitats, they play many ecological roles, such as fixing carbon, cycling nitrogen, breaking down organic matter, and supporting communities of other microbes.

### Eukaryotes

*Euglena*, a single-celled eukaryote that can both move and photosynthesize
Eukaryotes (living things with complex cells) are thought to have evolved from archaea. Later, these early eukaryotes formed partnerships with bacteria. These bacteria eventually became mitochondria and chloroplasts, which are now permanent parts of eukaryotic cells.

About 1.5 billion years ago, eukaryotes split into several major groups. Scientists group them into eight main clades:

* Alveolates
* Excavates
* Stramenopiles
* Plants
* Rhizarians
* Amoebozoans
* Fungi
* Animals

Five of these groups are often called protists. Protists are mostly tiny eukaryotic organisms that are not plants, animals, or fungi. Most protists are single-celled, and they are sometimes called microbial eukaryotes. The term “protist” is used for convenience, not as an official scientific group, because some protists are more closely related to plants, animals, or fungi than to other protists.

#### Plants

Plants are mostly multicellular and perform photosynthesis. They belong to the kingdom Plantae, which does not include fungi or some algae. Plant cells began when an early eukaryote absorbed a cyanobacterium about 1 billion years ago, leading to the formation of chloroplasts.

Early plant-like organisms were aquatic and are often called algae, though algae are not all closely related. Algae include groups like:

* Glaucophytes – tiny freshwater algae that may look like the earliest plant ancestors
* Red algae – multicellular
* Green algae – multicellular and divided into three groups:
* Chlorophytes
* Coleochaetophytes
* Stoneworts

#### Fungi

Fungi are eukaryotes that digest food outside their bodies. They release enzymes that break food down first, then absorb it. Many fungi feed on dead material and act as decomposers, helping recycle nutrients in ecosystems.

#### Animals

Animals are multicellular eukaryotes. Most:

* Eat other organisms
* Breathe oxygen
* Can move
* Reproduce sexually
* Develop from a ball of cells called a blastula

Scientists have identified over 1.5 million living animal species so far, including about 1 million insects. However, there may be more than 7 million animal species in total. Animals interact with each other and their surroundings, forming complex food webs.

### Viruses

Bacteriophages attached to a bacterial cell wall
Viruses are tiny germs that are too small to see with a regular microscope. They can only make copies of themselves when they are inside the cells of living things. Viruses can infect all kinds of life — animals, plants, bacteria, and even very simple organisms called archaea. Scientists have studied and described more than 6,000 kinds of viruses. Viruses exist almost everywhere on Earth and are the most common type of biological thing.

Scientists are not sure where viruses first came from. Some might have started as bits of DNA that can move between cells (called plasmids), while others may have come from simple bacteria. In evolution, viruses help move genes from one organism to another, which adds variety to life, similar to what happens in sexual reproduction.

Because viruses have some features of living things but not all, people sometimes call them “organisms at the edge of life” or “self-replicators.”

## Ecology

Ecology is the study of the distribution and abundance of life, the interaction between organisms and their environment.

### Ecosystems

An ecosystem is made up of living things (like plants, animals, and microbes) and nonliving things (like water, air, sunlight, temperature, soil, and acidity) that exist together in one place. The living and nonliving parts are connected through the flow of energy and the recycling of nutrients. Energy from the sun enters the ecosystem when plants use photosynthesis to make food. Animals eat plants and other animals, which moves energy and matter through the system. Animals also affect how much plant and microbe life there is. When living things die, decomposers break down their remains. This process releases carbon back into the air and changes the nutrients in dead material into forms that plants and microbes can use again.

### Populations

A population is a group of living things of the same species that live in one area and reproduce over many generations. You can estimate how big a population is by multiplying how many organisms are in a certain space (population density) by the size of the area or volume. An environment’s carrying capacity is the largest number of individuals of a species that the environment can support. This depends on things like food, water, space, and shelter. The carrying capacity can change if the environment changes—for example, if resources become harder to find or more expensive to use. For humans, new technologies—like those from the Green Revolution—have helped increase how many people Earth can support. This growth has proved wrong some older predictions that human populations would soon decline, such as those made by Thomas Malthus in the 1700s.

### Communites

A (a) trophic pyramid and a (b) simplified food web. The trophic pyramid represents the biomass at each level.
A community is a group of different populations of species that live in the same place at the same time. A biological interaction is how two organisms in a community affect one another. These organisms can be from the same species (intraspecific) or from different species (interspecific). Some interactions are short-term, like pollination or hunting. Others are long-term and can strongly affect how species evolve.

A long-term relationship between species is called a symbiosis. Symbiosis can be:

* Mutualism – both species benefit
* Competition – both are harmed

Every species in a community is either a consumer, a resource, or both. These roles form food chains and food webs.

Food webs have different trophic levels (feeding levels):

* Primary producers (autotrophs) are at the lowest level. They include plants and algae that use energy and nonliving materials to make food.
* Heterotrophs are organisms that get energy by breaking down organic material from other organisms. There are different types:
	+ Primary consumers (herbivores) eat plants.
	+ Secondary consumers (carnivores) eat herbivores.
	+ Tertiary consumers eat secondary consumers.
	+ Omnivores can eat at more than one level.
	+ Decomposers eat dead organisms and waste.

Only about 10% of the energy from one trophic level is passed to the next. The remaining 90% is lost as heat or used by decomposers to break down waste and dead material.

### Biosphere

In the world ecosystem (the biosphere), matter is found in different parts that interact with each other. These parts can be living (biotic) or nonliving (abiotic), and they can be easy to use (accessible) or hard to use (inaccessible) depending on where they are and what form they take.

For example:

* The matter in land plants (terrestrial autotrophs) is living and accessible to other organisms.
* The matter in rocks and minerals is nonliving and not accessible.

A biogeochemical cycle is the path that a specific element of matter takes as it moves through the living parts of Earth (the biosphere) and the nonliving parts (the lithosphere, atmosphere, and hydrosphere).

There are biogeochemical cycles for nitrogen, carbon, and water.

### Conservation

Conservation biology is the study of protecting Earth’s biodiversity. Its goal is to save species, their habitats, and ecosystems from disappearing too quickly and to keep natural interactions between living things intact.

It looks at what causes biodiversity to be lost, what helps it survive, and how it can be restored. It also studies how evolution continues to create diversity in genes, populations, species, and ecosystems. Scientists are concerned because estimates suggest that up to half of all species on Earth could disappear in the next 50 years. Losing biodiversity can lead to problems like poverty, hunger, and big changes in how life evolves on Earth. Biodiversity is important because it keeps ecosystems working. Ecosystems provide many services that humans rely on. Conservation biologists study how biodiversity loss and species extinction affect our ability to live well. Governments, organizations, and citizens are taking action through conservation plans. These plans include research, monitoring, and education programs, helping protect biodiversity from local to global levels.

## Branches

* Algalogy
* Anatomy
* Arachnology
* Bacteriology
* Biochemistry
* Biogeography
* Biophysics
* Botany
* Bryology
* Cell biology
* Cytology
* Dendrology
* Developmental biology
* Ecology
* Endocrinology
* Entomology
* Embryology
* Ethology
* Evolution / Evolutionary biology
* Genetics / Genomics
* Herpetology
* Histology
* Human biology / Anthropology / Primatology
* Ichthyology
* Limnology
* Mammalology
* Marine biology
* Microbiology / Bacteriology
* Molecular biology
* Morphology
* Mycology / Lichenology
* Ornithology
* Palaeontology
* Parasitology
* Phycology
* Phylogenetics
* Physiology
* Taxonomy
* Virology
* Zoology