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:
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:
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
that combine to form proteins
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.