While some scientists and organizations list as few as five traits, many authorities agree on eight characteristics of living things. The eight characteristics of living things are not a hierarchy. One trait does not outrank another.
Since human life began, we have attempted to define what life is. Our current scientific understanding shows eight characteristics of life:
We explained each of them below in alphabetical order to avoid bias.
Successful organisms use adaptation through evolution to survive. Organisms that cannot evolve cannot keep up with changing environments, become extinct. This is what we call natural selection.
All forms of life evolve. They adapt to the external environment, change their heritable traits, and prepare future generations for more efficient life processes. This characteristic has been seen in the laboratory in a fruit fly experiment that showed evolution at work within a span of 60 years.
So far, everything biologists have found to be alive uses cell structure to organize molecules into useful tissues and organs. All life forms use cells, whether they are single-celled organisms or multicellular organisms such as plant cells or animals.
The general structures move along a line from cell to tissue to organ to being, which gives us the word “organism,” a living thing with organ systems. Cellular organization is seen in something as simple as a fungus cell. From simple bacteria up to mammals, life uses cellular organization.
Living things grow. To conserve resources, organisms reproduce with immature and small copies of themselves. Without straining the parent organism, these small copies gather their own resources to grow, enlarge, mature, age, and reproduce themselves. Humans are excellent examples of growth and development. Eventually, organisms die, returning their gathered resources to the earth for reuse by new organisms. The cycles and stages of life are growth and development.
Life transfers characteristics to offspring via deoxyribonucleic acid (DNA) and ribonucleic acid (RNA); these are the building blocks of life. From viruses to humans, traits that benefit the parents are transferred in genes to the offspring. For humans, this genetic material holds our genetic information such as eye color, skin color, and hair type, just to name a few.
Unfortunately, mistakes, mutations, and traits that harm can also be transferred through our genetic material. Whether for good or bad, life exists through heredity.
Maintaining a stable internal environment is called homeostasis. Your body, like that of a cat or a cactus, must maintain a stable environment inside. For every inhalation, for example, you need to exhale. If you take in food, you must eliminate waste. If you lack enough water for cellular functions, you will get thirsty. Body temperature, a homeostatic biological process, has its mechanical equivalent in the thermostats, regulating room temperature in buildings and homes.
Homeostasis is how living organisms maintain their internal systems.
Chemical reactions inside cells, tissues, organs, and living beings perform various actions that keep the organism alive. These reactions break down incoming food, send nutrients to cells, remove waste products, transform energy, and synthesize new chemicals. Together, these processes lead to growth, system repair, and excretion. Photosynthesis in plants is a metabolic process. Together, an organism’s chemical reactions are its metabolism.
Successful organisms reproduce. From fleeting mayflies living one day to Aldabra tortoises reaching 120 years to the ancient Methuselah bristlecone pine (4,800+ years), all life engages in reproduction.
[Insert side-by-side drawings of mayflies, Aldabra tortoise, and bristlecone pine]
Scientists have been able to coax crystal structures to change behavior when exposed to blue light. Though this does not mean the crystals are alive, they exhibit a response to a stimulus, just as you respond to having your name called or your leg pinched. Phototropism is a plant’s response to stimuli (turning toward the light). Everything alive shows a response to stimuli.
Biologists, biology students, and thinking people all ponder the meaning of life. What is life? When is something alive, and when does it only appear to be alive?
The list of characteristics of life is not finished. As humans explore beyond our reach, we find more puzzles.
Extremophiles have been discovered living around hydrothermal vents on the seafloor where no light reaches. How can they, or acid-loving, radiation-resistant organisms like exist?
The basic organizing structure of living things is the cell, a small version of the larger organism’s processes. Individual cells carry on life processes themselves. The characteristics of a cell:
Cells differentiate and build on these basic functions by either not having a nucleus (prokaryotic) or having a nucleus (eukaryotic).
We mentioned that scientists coaxed crystals into showing response to a stimulus. So are crystals alive? What about crystals “growing” in caves, or sugar crystals forming in sugar water to make rock candy?
This is the danger of relying on only one of the eight characteristics of life. For something to be alive, it must exhibit all eight characteristics.
Crystals accrete, which could appear to show growth and development, but they cannot reproduce, cannot metabolize resources, cannot excrete waste, and have no cell structures to store DNA.
Some spiritual adherents view crystals as being “alive” with energy and auras, but biologically, crystals are not alive.
To learn if bacteria are living or nonliving things, let’s go through the list:
Bacteria meet all the requirements for being considered living things. Bacteria are alive.
Viruses are trickier to classify than bacteria. Let’s use our trusty list again:
Viruses have all of the characteristics of living things except cellular organization, homeostasis, metabolism, and response to stimuli.
Currently, most biologists, virologists, doctors, and general scientists say viruses are not alive. That thinking may change as more evidence, such as the giant mimivirus, is revealed.
It may sound silly, but a good test of life's definition is to apply it to things we think of as non-living. How does a computer stack up? Once more, to the list!
Since computers have only evolution, a kind of metabolism, and response to stimuli, they cannot be considered alive. They lack five of the eight characteristics of life.
See how much you know about life. Try these questions and then check our answers against yours.
Behold, the answers as we know them today:
After working your way through this lesson and video, you have learned:
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