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Chart of relative sizes of planets
Archaea Bacteria Eukaryota
archaea
bacteria
eukaryota

When people think of living organisms, they typically think of organisms that can be seen in everyday life, such as plants and animals, but many living organisms go undetected by the human eye. Humans, with trillions of cells, have more in common with tiny single-celled bacteria than you might think. Scientists use a set of common characteristics or attributes to define life. The attributes listed below are common to all life as we know it. All living organisms:

Grow
Reproduce
Are composed of chemicals
Respond to their environment
Exhibit organization on many levels

The first cells appeared on Earth about 3.5 billion years ago. Although biologists still argue about how exactly this happened, there are no disputes about the three basic requirements for life on Earth. All life requires:

liquid water
essential chemicals
an energy source

Liquid water is essential because biochemical reactions take place in water. Water is also an excellent solvent that easily dissolves and carries nutrients and other compounds in and out of cells. Life forms are usually made primarily of water. In fact, our human bodies are more than 60% water.
Life as we know it contains specific combinations of elements including carbon, hydrogen, nitrogen, and oxygen that combine to form proteins and nucleic acids which can replicate genetic code. All the basic elements are formed in stars and distributed throughout space as a result of giant explosions called supernovas. Since these essential chemicals are quite common in other places in the Universe we can expect that the development of life somewhere else is also possible.
Organisms require energy to assimilate or put together the chemicals that form an individual. Energy is also required for the organism to grow, reproduce, and respond to the environment. Energy sources may include other organisms, light, or inorganic compounds. The most common source of energy on the Earth is photosynthesis, which transforms sunlight into food. This process will not work very well for the outer Solar System, because not much light reaches such great distances. However, we can look to extremophiles here on Earth for help in figuring out where and what to search for. Extremophiles live in extreme conditions and typically get their energy from a source other than the sun.
Antarctic shrimp

Life was recently found 600 feet under the West Antarctic ice sheet. When water freezes it expands making it less dense than liquid water. This is why ice floats on water. The 3-inch-long critter resembling a shrimp proves that life can exist in extreme cold if liquid water is present.

hydrothermal vent

Giant tube worms live in hydrothermal vents in the ocean where volcanoes release extreme heat and dangerous gases. These unique life forms get their energy from chemicals that would be toxic to most other organisms. Such creatures can help us in the search for life on other planets.

biochemical reaction

The process in which one chemical substance is transformed into another.

periodic table: carbon

carbon

The fourth most abundant element in the universe, carbon is the basis for life on Earth.

periodic table: hydrogen

hydrogen

The most common element in the Universe and the main fuel of stars.

periodic table: nitrogen

Nitrogen

A colorless and odorless gas, and one of the most abundant elements in Earth's atmosphere, and the seventh most abundant element in the Universe.
Nitrogen is a byproduct of the fusion process that occurs in stars.

periodic table: oxygen

Oxygen

The third most abundant element in the universe and a key component of the air we breathe.

Protein

Any of a group of organic compounds, composed of one or more chains of amino acids, which are a component of living tissue.

Nucleic acid

A group of chemical compounds that regulate all cell activites. DNA and RNA are both nucleic acids.

Supernova

An explosion at the end of the life of a large star; it can be up to a billion times brighter than the Sun.

Photosynthesis

The chemical process by which green plants, using sunlight, convert carbon dioxide into organic compounds, generating oxygen as a byproduct.

extremophiles

Organisms that can survive in environments that would be deadly to other life forms.